STATE OF ISRAEL
MASHAV
CINADCO
MINISTRY OF FOREIGN AFFAIRS
MINISTRY OF AGRICULTURE AND RURAL DEVELOPMENT
CENTRE FOR INTERNATIONAL COOPERATION
CENTRE FOR INTERNATIONAL AGRICULTURAL DEVELOPMENT COOPERATION
DRIP IRRIGATION SECOND EDITION By
Moshe Sne Irrigation Consultant and Former Director, Irrigation and Soil Field Service
2005
CONTENTS Chapter Topic
Page
List of Tables
II
List of Figures
III
Foreword to The First Edition
VIII
Foreword to the Second Edition
IX
Acknowledgments
X
Introduction
1
The History of Drip Irrigation
1
Principles of Drip Irrigation
4
Advantages and Limitations
5
3.
The Distribution of Water in The Soil
8
4.
The Drip System
14
5.
Flow Rate – Pressure Relationship
18
6.
Pipes and Tubes for Drip Irrigation
28
7.
Dripper Types, Structure, Function and Properties
32
8.
Accessories
40
9.
Filtration
46
10.
Fertigation
58
11.
Water Quality
64
12.
Monitoring and Control
71
13.
Subsurface Drip Irrigation (SDI)
74
14.
Family Drip Irrigation
80
15.
Water Distribution Uniformity
82
16.
Drip Irrigation of Crops
84
17.
Basics of Drip System Design
93
18.
Drip Irrigation Scheduling
106
19.
Maintenance
112
20.
References and Bibliography
116
Conversion factors
120
1. 2.
I
LIST OF TABLES No.
Page
1.
Pressure Units
18
2.
The Friction Coefficient ( C ) of Pipes
20
3.
The Effect of Dripper Exponent on Head-Loss – FlowRate Relationship
22
4.
Head losses in Acuanet automatic valve
23
5.
Plastro Hydrodrip II Integral Drip Laterals Technical Data
24
6.
PE Pipes for Agriculture
29
7.
Internal Diameter and Wall Thickness of LDPE Pipes
29
8.
Internal Diameter and Wall Thickness of HDPE Pipes
30
9.
PVC Pipes for Agriculture
30
10.
Internal Diameter and Wall Thickness of PVC Pipes
31
11.
Flow-Rate of Spring Actuated Pressure Regulators
42
12.
Characteristics of Water Passageways in Drippers (example)
46
13.
Screen Perforation - examples
47
14.
Sand particle size and mesh equivalent
48
15.
Nominal Filter Capacity – examples
50
16.
Relative Clogging Potential of Irrigation Water in Drip Irrigation Systems
65
17.
Threshold and Slope of Salinity Impact on Yield
67
18.
Yield Increase and Water Saving in Conversion From Surface to Drip Irrigation
84
19.
Manufacturer Data about the Allowed Lateral Length in the Examined Alternatives
96
20
Allowed lateral length of Ram 16 PC drippers
97
21.
Calculation Form: Head losses in pipes
101
22.
Head Loss Calculation Form – Pressure Compensated (PC) Drippers
103
23.
Head Loss Calculation
105
24.
Irrigation Scheduling – Calculation Form (example)
106
25.
Irrigation Scheduling Form for Annuals
109
26.
Operative Irrigation Schedule
111
II
LIST OF FIGURES No.
Page
1.
Clay pot
1
2.
Early patents issued for drip irrigation
2
3.
Wetting pattern of drip irrigation in different soil textures
4
4.
Water distribution in the soil along time
8
5.
Water distribution from a single dripper in loamy and sandy soil
9
6.
Salt distribution in the wetted volume
10
7.
Leaching of salt into the active root-zone by rain
10
8.
Diverse root systems
12
9.
Typical root systems of field crops
13
10.
Root system in drip irrigation vs. root system in sprinkler irrigation
13
11.
Simplified scheme of drip system
14
12.
Typical layout of drip irrigation system
15
13.
Components of drip irrigation system
16
14.
Control Head
17
15.
Relationship between the dripper exponent and lateral length
22
16.
Pressure Compensated dripper flow-pressure relationship
23
17.
Non-pressure compensated flow-pressure relationship
23
18.
Acuanet automatic valve
24
19.
Head loss nomogram, based on Hazen-Williams formula
25
20.
Nomogram for calculation of head losses in HDPE pipes
26
21.
Nomogram for calculation of head losses in LDPE pipes
27
22.
Evolution of the passageway style
32
23.
Turbulent flow
33
24.
Orifice dripper
33
25.
Vortex dripper
33
26.
Labyrinth button dripper
33
27.
Tape dripper lateral: empty and filled with water
33
28.
Point-source and line-source wetting by drippers
34
29.
In-line laminar dripper and turbulent dripper
35
30.
On-line drippers
35
31.
Button drippers insert design
36
III
LIST OF FIGURES (Continued) No.
Page
32.
Adjustable and flag drippers
36
33.
Flexible diaphragm under pressure
36
34.
Button and inline PC drippers
36
35.
ADI PC dripper
37
36.
Change of water passageway length under high pressure
37
37.
Woodpecker drippers
37
38.
Flap equipped dripper
38
39.
Arrow dripper for greenhouses, nurseries and pot plants
38
40.
Six outlets
38
41.
Ultra low flow micro-drippers
39
42.
Integral filters
39
43.
Auto flushing, pressure compensating dripper
39
44.
Plastic and metal pipe and lateral connectors
40
45.
Lateral start, plugs and lateral end
41
46.
Reinforced connectors
41
47.
Drip laterals connectors and splitters
41
48.
Hydraulic valve
42
49.
Spring pressure regulator assemblies
42
50.
Spring actuated pressure regulator
43
51.
Hydraulic pressure regulator
43
52.
Horizontal and angular metering valves
43
53.
Electric valve
44
54.
Air-relief valves
44
55.
Atmospheric vacuum breakers
45
56.
Lateral-end flushing action
45
57.
Screen filter
47
58.
Head losses in clean screen filters
47
59.
Disc filter
48
60.
Media filter
48
61.
Sand separator
49
IV
LIST OF FIGURES (Continued) No.
Page
62.
Hydro-cyclone sand separator – head losses and optimal flow rates
49
63.
Self-flushing screen filter
52
64.
Automatic flushing of disk filters
52
65.
High capacity media filters array
53
66.
Back-flushing of media filters
53
67.
High capcity automatic filter
53
68.
Compact automatic filter
54
69.
Slow sand filter
55
70.
Slow sand filter scheme
56
71.
Treflan impregnated disk stack
57
72.
Fertilizer tank
58
73.
Venturi injector
59
74.
Piston and diaphragm hydraulic pumps
59
75.
No-drain hydraulic pump
59
76.
Mixer
60
77.
Electric pump
60
78.
Check valve
63
79.
Tandem backflow preventer - exploded
63
80.
Tandem backflow preventer
63
81.
Installed backflow preventer
63
82.
Chlorine- distribution below and between drippers
68
83.
Salt level in relation to distance from dripper
68
84.
Water quality for irrigation
68
85.
Tensiometers
71
86.
Soil moisture capacitance sensor
71
87.
Multi-factor simultaneous phytomonitoring
72
88.
Scheme of SDI system
74
89.
Wetting pattern in SDI
77
90.
Burying SDI lateral
78
91.
Three-shank SDI lateral burying machine
79
92.
Bucket kit
80
V
LIST OF FIGURES (Continued) No.
Page
93.
Drum kit
80
94.
"Netafim" Family Drip System (FDS)
81
95.
Components of Family Drip System (FDS)
81
96.
Treadle pump
81
97.
Apple root system in well aerated soil
84
98.
Apple root system in compact soil
84
99.
Drip irrigation Layouts in orchards
85
100.
Drip laterals in vineyard, hung on the trellis wire
85
101.
Dripper layouts in pecan orchard
85
102.
Typical shoot and fruit growth curves for peach and pear
86
103.
Partial Root-zone Drying with two laterals per row
87
104.
Mango grown on nutrition ditches vs. control
87
105.
Mechanized deployment of drip laterals
88
106.
Cotton root development
88
107.
Laterals on top of hillocks in potatoes
89
108.
Lateral between hillocks
89
109.
Potatoes – one lateral per row
89
110.
Wide-scale drip irrigation in greenhouses
91
111.
Drip irrigation of potted plants in greenhouse
92
112.
Roadside drip irrigation
92
113.
Wetted volume in different soil types
94
114.
Apple orchard map
95
115.
Local head losses in accessories
98
116.
Drip system layout scheme
99
117.
Feasible layouts
100
118.
Segmented drawing for head loss calculation
101
119.
The chosen diameter for mainline and manifold
102
120.
One manifold layout
103
121.
Pressure compensated Ram 2.3 l/h dripper, one shift design
104
122.
Melons plot map
104
123.
Melons – In-line non-compensated drippers
105
VI
LIST OF FIGURES (Continued) No.
Page
124.
Schematic wetting pattern in different textured soils
107
125.
Different schedules of drip irrigation operation
108
126.
Layout of drip system for 55 ha. Of cotton
110
127.
Automatic line flushing valve
114
128.
Punch and holder
115
VII
FOREWORD TO THE FIRST EDITION The need for a comprehensive and updated book on Drip Irrigation has long been felt as reflected by the intensive scheduling of international irrigation courses in CINADCO’s yearly training program. The booklet on Drip Irrigation written by Elimelech Sapir, and the late Micha Shani, in 1976 was updated in the early 1990s and is used extensively in CINADCO’s irrigation training courses, in Israel and abroad. However, with the rapid expansion and technological advances of Israeli irrigation equipment, it became apparent that more detailed and systematic literature was needed. Moshe Sne, the former Director of the Irrigation and Soil Field Service of the Israeli Ministry of Agriculture and Rural Development, Extension Service, has been greatly involved in the subject of irrigation systems and techniques in general, and drip irrigation in particular, for many years. He has also served as the chief irrigation course adviser for CINADCO. On the eve of his retirement from government service, he committed himself to the worthy task of preparing a book on Drip Irrigation in Israel. We wish to thank the author for the great amount of work and effort he put into the writing and compilation of the drip irrigation subject matter presented here. He was greatly assisted by the leading irrigation companies in Israel who allowed the use of pictures, charts, diagrams and figures. We wish to thank them and the many professionals who assisted Mr. Sne in this project and are credited throughout the book. We are happy to share the professional material presented here with irrigation experts, agriculturalists and others in the field, in countries throughout the world that participate in Israel’s international cooperation programs. The contents have been formulated particularly for the physical conditions prevailing in Israel. These are recommendations only and should not take the place of local detailed irrigation planning. This is the first edition of Drip Irrigation printed in a limited number of copies. We would appreciate your comments and suggestions for the coming editions.
Abraham Edery, Director of Training, CINADCO Shirley Oren, Publications’ Coordinator, CINADCO May 2004
VIII
FOREWORD TO THE SECOND EDITION A year has passed since the publication of the First Edition of Drip Irrigation written by Moshe Sne. At the time of the first printing, we requested from the irrigation experts, irrigation course participants and others who would be reading the book to give us their comments and suggestions. This was done and the author incorporated the comments and suggestions received, as well as his own changes and corrections into this publication. We are pleased to bring to print in May 2005 the second edition of Drip Irrigation. We are greatly appreciative of the efforts made by Moshe Sne to improve upon and correct the already comprehensive material he compiled previously. As we mentioned in the Foreword to the First Edition, we are happy to share this professional material with irrigation experts, agriculturalists and other interested parties in countries throughout the world that participate in Israel's international agricultural development programs. In order to facilitate this purpose, the book is currently being translated into Spanish and Russian. The content has been formulated particularly for the physical conditions prevailing in Israel. These are recommendations only and should not take the place of local detailed irrigation planning. Abraham Edery, Director of Training, CINADCO Shirley Oren, Publications' Coordinator, CINADCO May 2005
IX
ACKNOWLEGMENTS I would like to thank my colleagues and friends, as well as the Irrigation course 2004 participants for proofreading the preliminary first edition and for the helpful remarks and corrections. Their valuable contribution had been embedded in the current Second Edition of the publication being printed in 2005. I am deeply grateful to the authors of the books and papers cited in the Reference List and the Bibliography. The vast material on drip irrigation inspired me and filled me with admiration for the enthusiastic and hard-working people in the forefront of irrigation technology. I would also like to thank the manufacturers for the wealth of information embodied in their brochures and professional guides. I am particularly grateful to Mr. Nachman Karu and Mr. Dubi Segal for their contribution of impressive and useful graphic material. Last but not least, thanks to Ms. Shirley Oren and Ms. Bernice Keren for their patient editing and elaboration of the Second Edition of Drip Irrigation. Moshe Sne May 2005
AUTHOR'S NOTE In the first version, uploaded to Scribd on September 19, some mishaps occurred during the conversion from the print to the electronic version, mostly in matching between the table of contents, and the actual document layout. These discrepancies had been adjusted. Additionally, replacement of some outdated figures and minor corrections and adjustments had been done in this version of the document. The author November 2009
X
DRIP IRRIGATION
Chapter 1. INTRODUCTION Drip irrigation, by definition, is an irrigation technology. However, during the last four decades, since the start of its world-wide dissemination during the early sixties, it appeared not only as an irrigation technology but as a comprehensive agro technology that changed crop growing practices and widened modern agricultural horizons. Drip irrigation facilitated increased efficiency of water use in irrigation and triggered the introduction and development of fertigation – the integrated application of water and nutrients. It raised the upper threshold of brackish water use in irrigation and simplified the harmonization of irrigation with other farming activities. Drip irrigation facilitated optimal “spoon-feeding” of water and nutrients to crops, attuned to the changing requirements along the growing season. Drip irrigation enabled the accurate supply of water and nutrients to the active root-zone with minimal losses. In protected cropping, it facilitated the combination of the advantages of hydroponics with improved plant support by solid detached media. Drip irrigation has promoted the sophistication of monitoring, automation and control of irrigation, as well as the diversification of filtration technology. Drip irrigation has gained momentum during the last two decades. The world-wide area under drip irrigation is estimated at 3 million ha., out of a total area of 25-30 million ha. irrigated with pressurized irrigation technologies. The area of surface irrigation is estimated at 270-280 million ha. THE HISTORY OF DRIP IRRIGATION From the early days of irrigated agriculture, farmers and irrigation professionals looked after concepts and technologies to improve water utilization in agriculture. One of these concepts was the localized application of water directly to the root zone. Another concept was subsurface water application to avoid evaporation from the soil surface. Such technology was used by the ancient Persians and is still applied in some countries in Asia and Africa. Clay pots made of unglazed indigenous earth-ware have many micro-pores in their walls. These micro-pores do not allow water to flow freely from the pot, but slowly release the water in the direction in which suction develops by the tension gradient. The pots are buried neckdeep into the ground, filled with water and the plants are planted next to them.
Fig. 1. Clay pot
In south-east Asia, bamboo drip irrigation has been in use for more than 200 years. Stream and spring water was tapped into bamboo pipes in order to irrigate plantations. About 18-20 l/min of water that enters the bamboo pipe system flows along several hundred meters and is finally distributed to each plant at a rate of 2080 drops per minute. This traditional system is still in use by tribal farmers to dripirrigate their black pepper plots. The concept of water saving was further elaborated during the nineteenth century. People involved with irrigation were dissatisfied with the wasteful surface irrigation technologies. There is evidence that in 1860, subsurface tile pipes were used experimentally for irrigation in Europe. Patents for water saving irrigation technologies were registered in Europe and the United States. Patent # US146,572 dated January 20, 1874 by Nehemiah Clark of Sacramento, California, describes a
1
DRIP IRRIGATION
pipe with a "non-clogging" leaking connection. In the year 1888, Mr. Haines of Nashville, Iowa, registered a patent of the direct application of water to the root system of orchard trees. In 1917, Dr. Lester Kellar introduced an agricultural drip system in a symposium at Riverside, CA., but further development of drip irrigation in the United States was delayed for another 40 years. Perforated pipes for subsurface irrigation were used experimentally in Germany in 1920 and in the USSR in 1923. In 1926, Mr. Nelson of Tekoa, Washington, had registered a patent for a subsurface irrigation system. Another subsurface irrigation system was examined in 1934 at the New Jersey and Indiana Agricultural Experiment Stations. After WWII, micro-tubes were used for greenhouse irrigation in England and France. In 1954, Mr. Richard Chapin developed in the USA, drippers for irrigation of potted plants in greenhouse. Mr. Hansen, of Denmark, developed a small plastic tube for the irrigation of potted plants in greenhouses.
Fig. 2. Early patents issued for drip irrigation The breakthrough in drip irrigation occurred in the early sixties, firstly in Israel and later in the United States. This initiative is attributed to Mr. Simcha Blass, who invented a dripper with long laminar water flow passageways in the form of a spiral micro-tube. The micro-tube was first wrapped around the feeding lateral, followed by an improved model comprised of a molded coupling with a built-in spiral. Later it was manufactured as a two-piece in-line dripper (US patent 3,420,064). Mr. Blass collaborated with Kibbutz Hazerim to establish "Netafim", a worldwide leading drip irrigation company. At the same era another Israeli inventor, Mr. Ephraim Luz developed a different drip irrigation system, with perforated polyethylene tubes, 4 – 6 mm in diameter. In both technologies the drip laterals were buried 20 – 40 cm below the soil surface. The main flaw with the buried laterals was the clogging of the drippers by soil particles and intruding roots. Mr. Yehuda Zohar, an agricultural fieldadviser demonstrated that on-surface drip irrigation had the same advantages as the subsurface installation but with significantly less clogging hazard. For many years the on-surface pattern was the dominant drip irrigation technology. During the late sixties and early seventies, "Netafim" licensed some foreign factories of irrigation equipment in the USA and South Africa to manufacture its patented drippers. As mentioned before, in 1954, Mr. Richard Chapin of the United States developed a system comprised of small diameters tubes for irrigation of pot plants in
2
DRIP IRRIGATION
greenhouses. In 1964, he invented a drip tape for the irrigation of cantaloupes. In 1974, he developed the bucket kit for irrigation of small family plots in developing countries. That system does not require an external source of energy. In 1962, Mr. S. Davis installed an experimental subsurface drip irrigation system in a lemon orchard in Pomona, California, USA. Only ten years later, during the early seventies, after the problems of root intrusion and soil particle suction had been resolved, did the installation of subsurface drip irrigation (SDI) systems expand on a wide scale in California and other States of the United States. Hawaiian sugar producers were introduced to drip irrigation In 1970, at an agricultural convention in Israel. Returning to Hawaii, they converted a significant portion of sugar cane acreage to drip irrigation, with astounding achievements in both water savings and sugar content. In order to reduce the costs of the drip system, perforated thin-wall tapes were introduced. However the variance in flow-rate and the clogging of the outlets were unacceptable. These problems were solved with the introduction of a twin-walled tape in which an inner conveyance tube bled water into a second outer distribution duct that emitted water from tiny holes onto the ground at low flow rates. A ratio of four outlet holes for every inner hole rendered low-flow rates with acceptable emission uniformity. Corresponding with the expansion of drip irrigation in the early sixties, fertigation technology evolved. Due to the small volume of wetted soil in drip irrigation, an adequate supply of nutrients to the root system requires the synchronization of water and nutrient supply through the drip system. Further steps in the development of drip irrigation technology was the introduction of seep hoses, woodpecker drippers, compensated drippers, non-leaking (no-drain) drippers, anti-siphon mechanisms and techniques that prevent root intrusion. Drip irrigation triggered the development of filtration systems and chemical water treatment technologies that were necessary to protect the narrow dripper water passageways from clogging. Sophisticated control and monitoring instrumentation has been developed to enable the optimal implementation of this technology. Drip irrigation was also adopted by gardeners and landscape architects. It revolutionized the concept of irrigation in gardening, with its capability to irrigate without disturbing visitors. The utilization of reclaimed water with subsurface installation and the convenience of irrigating narrow strips of vegetation without wetting sidewalks, excited leading professionals in this sector. Nowadays there are many countries where sales of drip irrigation equipment for landscaping and gardening applications surpass those of agricultural applications. Mainstream drip irrigation is relatively expensive and is actually unaffordable for low income farmers in developing countries. This impediment has been partially solved by local production of cheap low-quality drip equipment, which compromises on emission uniformity and life expectancy. Another solution was the development of simple drip kits, such as the bucket and drum kits, designed for small family-run agricultural plots.
3
DRIP IRRIGATION
Chapter 2. PRINCIPLES OF DRIP IRRIGATION Drip irrigation, sprinkler irrigation, center pivot and lateral-move are classified as pressurized irrigation technologies. In pressurized irrigation, the driving force of water movement is provided by an external energy source (or a raised reservoir). The water is delivered through a closed pipe system. This differs from surface irrigation technologies – flood, border, furrow and small basin irrigation – in which the driving force of water flow is gravity, and the delivery and application structures – canals, ditches, furrows, small ponds and basins – are open to the atmosphere. Drip irrigation is a section of the micro-irrigation (localized irrigation) sector, which includes also micro-sprayers and mini-sprinklers. The term trickle irrigation is generally used to describe irrigation methods whereby small quantities of water are applied at short intervals directly to the soil, from point source discrete emitters spaced along thin tubes or tapes, line-source densely mounted dripper outlets, or seep-hoses. Water applied from small sprayers, micro-sprinklers and bubblers is transmitted to the soil through the atmosphere. The terms trickle, micro, drip, low volume and localized irrigation are sometimes used interchangeably in the literature, although each one has a slightly different technical meaning. With micro-irrigation, the emitters deliver water through three different types of emitters: drippers, bubblers and sprayers/micro-sprinklers. Drippers apply water as discrete droplets or trickles. With bubblers, water ‘bubbles out’ from the emitters at higher flow rates and the flow appears as a continuous stream. Micro-sprinklers sprinkle, spray or mist water to the atmosphere around the emitters. The uniqueness of drip irrigation is the partial wetting of the soil. Water is applied by many tiny emitters, 5,000 – 300,000 per hectare. In on-surface installation, each emitter moistens the adjacent surface area. The percentage of the wetted surface area and soil volume depends on soil properties, initial moisture level of the soil, the applied water volume and emitter flow rate. In subsurface installation, the soil surface remains dry.
Fig. 3. Wetting pattern of drip irrigation in different soil textures Adapted from: The University of Maine Cooperative Extension Farm Note
The lateral movement of the water beneath the surface of a medium or heavy textured soil is more pronounced than in sandy soils. Whenever the dripper's flow rate exceeds the soil intake (infiltration) rate and its hydraulic conductivity, the water ponds on the soil surface and wets larger soil volume. The vertical cross section of the wetted volume in sandy soils resembles a carrot. In medium textured soil, the dimensions of the wetting depth and wetted diameter are
4
DRIP IRRIGATION
similar, while in heavy soils the horizontal dimension of the wetted volume is greater than that of the wetted depth. Indicative values for the wetted diameter by a single dripper may be 30 cm in a light soil, 60 cm in a medium soil and 120 cm in a fine textured soil. Due to the partial wetting of the soil in drip irrigation, water has to be applied more frequently than with other irrigation methods that wet the entire area such as sprinkler and flood irrigation. The capacity to apply water to each plant separately in small, frequent and accurate dosing enables high application efficiency. Water is delivered from the emitter continuously in drops at one point, infiltrates into the soil and wets the root zone vertically by gravity and horizontally due to capillarity. During the last three decades, subsurface drip irrigation (SDI) has gained momentum. The wetting pattern with SDI is somewhat different from that obtained with on-surface emitters. The localized and limited wetting pattern by drip systems requires the application of fertilizers through the drip system, a technique named fertigation. The great number of water emitters per unit area requires the minimization of the single emitter’s flow-rate (discharge). The customary dripper flow-rate range is 0.1 – 8 liter per hour (l/h). The low emitter flow-rate is achieved by diverse designs: a tiny orifice, large head losses within a long flow path, turbulent or vortex flow. The narrow passageways in the emitters and the low flow rates lead to the accumulation and precipitation of substances that may fully or partially clog the system. Adequate filtration is a prerequisite for the implementation of drip irrigation. Complementary chemical treatments are required when low quality water is used for irrigation.
ADVANTAGES AND LIMITATIONS OF DRIP IRRIGATION Advantages Drip irrigation technology has many advantages over other irrigation technologies. Drip irrigation significantly increases the efficiency of water utilization and improves the growing conditions of the irrigated crops. • Accurate localized water application: Water is applied precisely to a restricted soil volume, corresponding with the distribution of the root system. Appropriate water management can minimize water and nutrient losses beneath the root-zone. • Minimization of evaporation losses: The reduced wetted upper surface area decreases water losses by direct evaporation from soil surface. • Elimination of water losses at the plot's margins: with drip irrigation, water does not flow beyond the limits of the irrigated plot as happens with sprinkler irrigation. The drip system can actually fit any plot, regardless of shape, size or topography. • Decrease in weed infestation: The limited wetted area decreases the germination and development of weeds.
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DRIP IRRIGATION
•
•
•
•
Desirable air-water equilibrium: The soil volume wetted by drip irrigation usually retains more air than a soil that is irrigated by sprinkler or flood irrigation. Simultaneous application of water and nutrients: Application of nutrients together with the irrigation water directly to the wetted soil volume, decreases nutrient losses, improves nutrient availability and saves the labor and/or machinery required for the application of fertilizers. Adjustment of water and nutrient supply to changing crop demand along the growing season: Fertigation technology together with high frequency water and nutrient applications facilitate the tuning of the supply to the dynamic requirements of the crop. Automation: Automatic controllers can easily be incorporated in drip irrigation systems.
•
Adaptability to harsh topographical and soil conditions: Drip irrigation functions successfully on steep slopes, shallow and compacted soils with low water infiltration rate and sandy soils with low water-holding capacity.
•
Irrigation does not interfere with other farming activities: The partial wetting of the soil surface does not interfere with other activities in the plot, such as spraying, fruit thinning and harvesting.
•
Water distribution is not disturbed by wind: Drip irrigation can proceed under windy conditions. Wind does not interfere with drip irrigation, unlike in sprinkler irrigation.
•
Low energy requirements: Due to the low working pressure, energy consumption in drip irrigation is significantly lower than that of other pressurized irrigation technologies such as sprinkler and mechanized irrigation systems.
•
Decrease in fungal leaf and fruit diseases: Drip irrigation does not wet the plant's canopy. This reduces the incidence of leaf and fruit fungal diseases.
•
Avoiding leaf burns: The elimination of foliage wetting reduces leaf burns by salt and fertilizers present in the irrigation water.
•
Allows for extended use of brackish water for irrigation: Frequent watering with drip irrigation allows for the use of irrigation waters containing a relatively high concentration of salt with minor impact on plant development and yield. The frequent applications dilute the salt concentration in the soil solution beneath the emitter and drive the salt to the margins of the wetted soil volume.
Limitations Due to the limited wetted soil volume, the narrow water passageways in the emitters and the vast amount of equipment needed, drip irrigation has some drawbacks. •
Clogging hazard: The narrow passageways in the emitters are susceptible to clogging by solid particles, suspended organic matter and chemical precipitates formed in the water. Clogging may also occur by suction of soil particles and root intrusion into the dripper.
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DRIP IRRIGATION
•
High initial cost: Due to the large amount of laterals and emitters, mobility of drip systems during the cropping season is rarely feasible. Most systems are solid-set arrays, resulting in high cost of equipment per area unit.
•
Salt accumulation on the soil's surface: Upward capillary movement of water from the wetted soil volume and evaporation from the soil's-surface leave behind a high concentration of salts in the upper soil layer. Light rains in the beginning of the rainy season, leach the accumulated salts into the active root zone and may cause salinity damage to the crop.
•
Vulnerability of on-surface laterals and drippers to damage by animals: The laterals, particularly the thin-walled tapes and the tiny drippers are prone to damage by rodents, rats, moles, wild pigs and woodpeckers. Subsurface laterals and drippers may be also damaged by rodents.
• Negligible influence on microclimate: Irrigation is occasionally used to improve local climate conditions – reducing temperature during heat spells and rising the temperature during frost events. With sprinkler and sprayer irrigation, a fraction of the sprinkled water evaporates, releasing energy to the atmosphere in cold weather and absorbing heat in hot weather. Naturally, this does not occur with drip irrigation •
Restricted root volume: The frequent water applications to limited soil volume lead to the development of restricted and sometimes shallow root systems. As a consequence, the crop depends on frequent water applications and increases its susceptibility to water stress during extremely hot weather. High-velocity winds can uproot large trees with shallow root systems.
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DRIP IRRIGATION
Chapter 3. THE DISTRIBUTION OF WATER IN THE SOIL The flow of water and its distribution within the soil by drip irrigation is different from that obtained with other irrigation techniques. Water is applied from a point or line source. Point sources are discrete drippers which each of them wets a discrete volume of soil. Line sources are drip laterals in which the drippers are installed close to each other. The water flows along the lateral so that the wetted volumes formed by adjacent emitters, overlap and create a wetted strip. With on-surface drip irrigation, the wetted soil surface area is a small fraction of the total soil surface area. A small pond is created beneath each emitter. The pond's dimensions depend on the soil type and the emitter's flow rate. In light sandy soil, the pond is tiny and is actually hardly observed. In soils of heavier texture, the pond's diameter is greater. Water distribution within the soil follows a three-dimensional flow pattern, compared with the one-dimensional, vertical percolation pattern typical of flood and sprinkler irrigation that wet the entire soil surface area. With subsurface drip irrigation, the wetting pattern is quite different. Water moves downward, sideways and also upwards.
Fig. 4. Water distribution in the soil along time: (a) on-surface drip irrigation. (b) SDI Two driving forces simultaneously affect the flow of water in the soil: gravity and capillary force. Gravity drives the water downwards. Capillary forces drive the water in all directions. The equilibrium between these two forces determines the distribution pattern of water within the soil. The water distribution pattern affects the spreading of the roots in the soil and also the distribution and accumulation of the dissolved chemicals - nutrients and salts.
Soil Wetting Patterns The main factors affecting the distribution pattern of water and solutes in the wetted soil volume with drip irrigation are listed below:
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DRIP IRRIGATION
Soil Properties Capillary forces are more pronounced in finer textured soils than gravity; hence the horizontal width of the wetted soil volume is greater than the vertical depth. The wetted volume shape resembles the shape of an onion. In medium textured soils, the wetted volume is pear-shaped, and in soils with a coarse texture the vertical water movement is more pronounced than the horizontal one so that the wetting volume resembles a carrot. Soil structure also influences water distribution. Compact layers and horizontal stratification enhance the horizontal flow of water at the expense of vertical percolation. On the other hand, vertical cracking in compacted soils enhances preferential downward flow of water followed by incomplete wetting of the upper soil layers. Lateral Placement
The greatest wetting horizontal diameter by drippers of on-surface drip laterals is near the soil surface, 10 – 30 cm deep. The greatest wetting horizontal diameter by drippers of subsurface drip laterals is at the depth of the lateral. The vertical dimension of wetted soil above the emitter in SDI is about ¼ of the wetted width in sandy soil and about ½ of the wetted width in silty and clayey soils. Emitter Flow Rate For the same application time-length and amount of water applied: •
A lower flow rate renders a narrow and deeper wetting pattern.
•
A higher flow rate renders a wider and shallower wetting pattern.
•
On-surface drippers create wider on-surface ponds and the horizontal wetted diameter is bigger than in lower flow rates.
Emitter Spacing
Fig. 5. Water distribution from a single dripper in loamy and sandy soil. 4 l/h and 16 l/h flow rates, 4, 8, 16 l dose
For the same application After Bressler 1977 time-length and volume of water applied:
Narrow spacing with overlapping renders narrower and deeper wetting pattern. The wetted width by each dripper increases until adjacent circles overlap. After overlapping, most of the flow is directed downwards
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DRIP IRRIGATION
Wide spacing renders wider and shallower wetting pattern. Water Dosage The wetted volume grows wider and deeper as the applied water amount increases. Chemical Composition of the Water Chemical compounds dissolved in the water may change the wetting pattern. Detergents and other surfactants contained in reclaimed and storm waters reduce water's surface tension and decrease the horizontal flow. The lower surface tension increases the affect of gravity at the expense of the capillary forces, resulting in a narrower and deeper wetting pattern.
Salt and Nutrient Distribution Dissolved salts tend to accumulate at the perimeter of the wetted zone, particularly at the soil surface where the water content of the soil is lower. A saline ring develops around the wetted circles on the soil's surface, along with a zone of salt accumulation at a depth which depends on the leaching efficiency. Good drip irrigation management at an appropriate irrigation frequency, replenishes the water removed by the crop, so that the soil water content in the soil remains high enough to maintain a low concentration of soluble salts. The nutrients applied with the irrigation water also follow the same distribution pattern.
Fig. 6. Salt distribution in the wetted volume Adapted from Kremmer & Kenig, 1996
Fig. 7. Leaching of salt into the active rootzone by rain Adapted from Kremmer & Kenig, 1996
Salt accumulation at the soil's surface and in the uppermost soil layer requires implementation of preventive measures with the first rains after a dry season. Irrigation should be applied as long as the rain lasts as to avoid the accumulation of the salts leached from the soil surface into the active root-zone. Soil Properties that affect the Water Distribution Pattern As mentioned before, soil properties affect the flow of water in the soil as well as the pattern of the wetted volume.
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DRIP IRRIGATION
The balance between the vertical and the horizontal movement is determined by soil properties such as infiltration and percolation rates that are dependent on the soil’s hydraulic conductivity. Hydraulic conductivity is expressed in units of velocity (length/time) per unit cross section (m/sec). A given soil does not have a constant value of hydraulic conductivity. In one and the same soil the hydraulic conductivity is higher in saturated soil than in unsaturated state. It also depends on the degree of stratification - the presence of compact soil layers and the moisture content of the soil before irrigation. Though different mathematical models have been developed for the prediction of soil water distribution patterns, the use of empirical field techniques for the estimation of the size and volume of the wetted soil is preferable. While plants are not consuming water, as it happens at night, the volume of the soil that is wetted depends on the volume of water applied by the dripper and the change in water content in the wetted volume. V = L X [100/(Mf-Mi)] Where V = Soil wetted volume, l'. L = Amount of the applied water, l' Mf is the average percentage of water content per unit volume in the wetted zone after irrigation and Mi is the average percentage of soil water content per volume unit before irrigation. For example, if 100 l' of water were applied at night and the soil water content in the wetted volume increased by 10% per volume, then the wetted volume would be 1000 l' (1 m3) of soil. Mf – Mi = 10% V = 100l X (100/10) = 1000l Wetting Width and Depth Selection of the most suitable dripper and determination of the spacing between laterals and between drippers on the lateral, commit a thorough estimation of the wetting pattern of the soil by the drippers. For a simple estimation of the width and depth of soil wetting, it is assumed that the capillary forces drive the flow of water in the soil at the same rate in all directions and gravity drives the water downward. For a given amount of applied water, the balance between these two forces determines the dimensions of the soil wetted volume and the ratio between the vertical and horizontal axis. During the wetting of a dry soil, gravity initially drives the water downwards through the empty, non-capillary voids much faster than the horizontally capillary movement. As the capillary voids are filled with water, the horizontal flow becomes more pronounced. This happens earlier at higher flow rates, therefore the horizontal diameter of the wetted volume by drippers with higher flow rates is larger. The same happens with soils of fine texture. Vertical gravity-driven percolation is slower and the capillary voids are filled earlier with water.
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DRIP IRRIGATION
Schwarzman and Zur developed a semi-empirical formula for estimation of the dimensions of the wetted volume: W = K3 (Z)0.35(q)0.33(Ks)-0.33 When: W = Max width of the wetted volume (not of the wetted area on soil surface) K3 = 0.0094 (empirical coefficient) Z = Desired depth of the wetting front – m (related to depth of the active root system). q = Dripper flow rate l/h Ks = Saturated hydraulic conductivity – m/s (has to be measured in laboratory or taken from a table) The result of using this formula differs in many cases from the empirical measurements in the field, since the hydraulic conductivity is determined in the laboratory on a disturbed soil sample. Whenever possible, it is recommended to determine the wetting pattern in undisturbed soil in the field. The distribution of nutrients applied by fertigation depends significantly on the interaction between the nutrient ions and the soil. Potassium ions are absorbed on the surface of clay minerals so that their transport with irrigation water in fine and medium textured soils is limited and most of the applied potassium remains in the upper soil layers. Phosphorous precipitates from the soil solution as insoluble salts with calcium and magnesium in basic and neutral pH levels and with iron and aluminum in acid soils. In these cases, it remains in the upper soil layer. In SDI, application of phosphorous in deeper soil layers increases its availability and absorption by the root system.
Root System Development under Drip Irrigation It is well known that the water application regime and water distribution pattern in the soil affect the pattern of root system development. Each plant family has a typical root distribution pattern, stemming from the growing conditions in the plant’s site of origin and its adaptation of the plant to the local growing environment.
Fig. 8. Diverse root systems As depicted in the above drawing, root systems can be shallow or deep, dense, branched or sparse, mostly unrelated to the shape of the plant's canopy.
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DRIP IRRIGATION
The root system pattern and soil properties are important factors in determining dripper spacing and the scheduling of the irrigation regime. Shallow and sparse root systems require a close dripper spacing and frequent water applications, while deep and branched root systems allow for wider spacing and larger intervals between irrigations. Frequent and small water applications by drip irrigation lead to the development of shallow and compact root systems. This increases crop sensitivity to heat spells and water stress. Large plants with shallow root systems are prone to uprooting by strong storms. On the other hand, because of the improved aeration and nutrition in the drip irrigated soil volume, the density of the active fine roots is significantly Fig. 9 Typical root systems of field crops higher than the density of root systems that grow under sprinkler irrigation. grow under sprinkler irrigation.
Fig. 10. Root system in drip irrigation (left) vs. root system in sprinkler irrigation (right) Courtesy “Netafim”
The active root system and most root-hairs of drip-irrigated orchard trees, are concentrated in the wetted volume. The highest density of the active roots is in the aerated upper layers, provided there is no accumulation of salts. At the margins of the wetted volume, where salt accumulates, active roots are sparse. Evergreen fruit trees such as avocado and citrus develop shallower root systems under drip irrigation than deciduous orchards and vineyards. This determines the irrigation regime and necessitates the addition of a second drip lateral per row on light textured soil. With SDI, the root distribution pattern is different. Roots are mainly concentrated under and beside the laterals. Very few roots develop above the laterals due to the higher salinity in these soil layers.
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DRIP IRRIGATION
Chapter 4. THE DRIP SYSTEM Although the drippers are the core of the drip irrigation network, the system is made up of many additional components. These components have to be compatible with each other, with the crop demands and with the characteristics of the plot to be irrigated. The components are classified in six principal categories: •
Water source: A pumping system from an on-surface or underground source or a connection to a public, commercial or cooperative supply network
•
Delivery system: Mainline, sub-mains and manifolds (feeder pipes)
•
Drip laterals
•
Control accessories: Valves, water meters (flow-meters), pressure and flow regulators, automation devices, backflow preventers, vacuum and air release valves, etc.
•
Filtration system
•
Equipment for the injection of plant nutrients and water treatment agents
The Water Pumping/Supply Head There are two alternative sources of water supply: a. independent pumping from an onsurface source (such as a lake, river, stream, pond or dam reservoir) or from an underground source (such as a well). b. connection to a commercial, public or cooperative supply network on the other. With independent pumping, the pump is chosen according to the discharge and pressure requirements in the irrigated area. In connection to a water supply network, the diameter of the connection, main Fig. 11. Simplified scheme of drip system valve and the delivering pipeline should correspond with the planned flow-rate and the requested operating pressure, with the smallest possible friction head losses. The Delivery System Mainlines for water delivery and distribution Pipes are made of PVC or polyethylene (PE). PVC pipes are installed underground as usually they have no protection against UV-radiation. PE pipes are installed underground or above ground, as they contain carbon black, which provides UV protection. The pipes’ PN (nominal working pressure) has to be higher than the PN of the drip laterals, particularly if the system has to withstand pressure with closed valves. The most common PN of delivery and distribution lines is 6 – 8 bar (60 – 80 m pressure head).
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DRIP IRRIGATION
Sub-mains The sub-mains are installed under or above ground. Underground installed pipes can be made of PVC or PE, while above-ground installed pipes can only be made of PE. In the case of retrieveable drip systems for the irrigation of annual crops (the system is layed out at the beginning and retrieved at the end of the growing season). Aboveground pipes can be made of P.E., aluminum or vinyl “lay-flat” hose. The lay-flat hose is durable and lays flat when not in use, so mechanic equipment can travel over it. The lay-flat hose, connectors, and feeder tubes are retrieved after the growing season to be used for the irrigation of another plot or stored until the following season. Wide-diameter PE pipes are more rigid, and are not easily rolled up at the end of the season. Manifolds In certain circumstances, when rows are very long or in harsh topographic conditions, sub-division of the plot by sub-mains is insufficient. In these conditions, additional division is accomplished by manifolds.
Fig. 12. Typical layout of drip irrigation system Drip Laterals The drip laterals are connected to the sub-mains or the manifolds. The laterals are made of LDPE (Low Density Polyethylene). There are different types of connectors between the sub-mains/manifolds and the laterals. The connectors have to withstand the working pressure as well as pressure spikes and water hammers. The lateral may be laid on soil surface or underground (SDI). Shallow burying, 5 – 10 cm below soil surface is common in vegetables grown under plastic mulch. Two basic types of drip laterals are used: Thick-walled laterals with on-line or in-line discrete drippers and thin-walled tapes with turbulent flow inherent water passageway molded into the tape during the extrusion process. The tape shrinks
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DRIP IRRIGATION
when it is not under water pressure. Thick-walled laterals have a PN of 1 – 2 bar (10 – 20 m), and tapes have a PN range from 0.4 to 1 bar (4 – 10 m). Control and Monitoring Accessories Valves and Gauges Simultaneous irrigation of several plots, each one with different water requirements from a single water source requires the sub-division of the irrigated area into sectors, each controlled by its assigned valve. These valves can be operated manually or automatically. Water-meters as well as automatic water-metering valves are used to measure and control water supply to the various sectors. Pressure regulators are used to prevent excessive pressure above the working pressure of the system. A backflow prevention/anti-siphon valve is required if the water is supplied from a well or a municipal water source that distribute drinking water, when fertilizers or other chemicals are injected into the irrigation system. Air-release/relief valves have to be installed at the highest topographic points of the system in order to avoid interference with water flow, excessive friction with pipe walls and pipe burst as an outcome of the flow of a high volume of air in the system. Vacuum breakers are used to avoid the collapse of pipes in steep slopes. In SDI systems, they are installed to avoid suction of soil particles into the drippers after shut-down of the water supply.
Fig. 13. Components of drip irrigation system Filtration The narrow passageways of the emitters are susceptible to clogging by suspended matter and chemical precipitates from the irrigation water. Three measures are taken to prevent clogging: •
Preliminary separation of suspended solid particles by settling ponds, settling tanks and sand separators.
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DRIP IRRIGATION
•
Filtration of the irrigation water.
•
Chemical treatments for decomposition of suspended organic matter, blocking the development of slime by microorganisms and prevention of precipitates deposition.
Filtration devices are usually installed at the control head. When the irrigation water is heavily contaminated, a main filtration system is installed at the plot control head and secondary control filters are installed at the sectorial control heads. Filters should be flushed and cleaned routinely. Flushing can be manual or automatic. Automatic back-flushing of media filters is performed with filtered water, hence, the filters are installed in pairs and flush one-another alternately. Chemical Injectors Three types of chemicals are injected into drip irrigation systems: fertilizers, pesticides, and anti-clogging agents. Fertilizers are the most commonly injected substances; the ability to “spoon-feed” nutrients contributes to the increased yields obtained with drip irrigation. Systemic pesticides are injected into drip irrigation systems to control insects and protect plants from certain diseases. Chemicals that clean drippers or prevent dripper clogging are also injected. Chlorine is used to kill algae and microorganisms and for decomposition of organic matter, while acids are used to modify water pH and dissolve precipitates. The different types of injectors are described in the chapter on fertigation.
Fig. 14. Control Head
Courtesy “Netafim”
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DRIP IRRIGATION
Chapter 5. FLOW RATE - PRESSURE RELATIONSHIP Water Pressure Water pressure is a key factor in the performance of pressurized irrigation systems. Pressure can be expressed in different unit systems. Table 1. Pressure units Definition Pressure/Tension Pressure/Tension Pressure/Tension Head
Unit
Sub units
Conversion
Bar Kilopascal (kPa) Atmosphere (Atm) Meter
=100 Centibar = 1000 Pascal ~100 Centibar =100 cm
0.99 Atm. 0.01 Bar=1 Centibar 1.01 Bar 0.1 Atm. ~ 0.1 Bar
For simplicity and convenience in the design of irrigation systems, the preferred unit system is pressure head, expressed in meters (m) height of water column. Pressure is converted to head units by dividing the pressure (weight/area) by the water’s specific weight (weight/volume). Therefore the head units are length (m) units. For example: A pressure of 5 atmospheres (5 kg/cm2) divided by water’s specific weight (1 g/cm3) equals (5000 g/cm2)/(1 g/cm3) = 5000 cm = 50 m. In practice, a column of water with cross section of 1 cm2 and weighing 1 Kg is 10 m high. This unit system enables the concurrent calculation of the effects of topography and friction losses due to the flow of water in the pipes on the pressure head at each point of the irrigation system. Water pressure head can be referred to as the water’s hydraulic potential energy. This potential energy is capable to accomplish work, e.g. to move a certain mass of water along a certain distance. Water Head Components The total water head, measured at a specific point of the irrigation system, is madeup of three components: Elevation Head (z) Elevation head is due to the topographical position, the relative height of a given point above or below a fixed point of reference. For example, if the main valve in the plot lies 5 m above the distal end of the plot, the measured static (elevation) head at the distal end will be 5 m higher than the measured static head at the valve. Static head is the pressure measured in a point in the water system when no water flow is taking place. Pressure Head Water under high pressure has more energy than water under low pressure. Although water is considered incompressible, water under pressure is stressed by the pressure. The resultant stress compresses the water and squeezes the bonds and electric fields in and around the water molecules. The water absorbs the energy that pushes the water molecules back against the surrounding water molecules and the container wall. The energy stored in the water molecules and the bonds between them is available to move the water to lower energy points.
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Velocity Head Flowing water has kinetic energy (velocity energy) represented by V2/2g where V is velocity which is measured in m/sec and g is the gravitational constant 9.81 m/sec2. Squaring V by itself (V x V = V2) results in units of m2/sec2 which divided by g in m /sec2 gives velocity head in m. units.
Conservation of Hydraulic Energy Globally, energy is never perished, it only changes forms. Hydraulic energy may change back and forth between the three forms; elevation energy, pressure energy and velocity energy. Some of it may be lost from the system and dissipated as heat due to friction, but it is still all there. If the sum of the three energy components does not remain constant as water flows through the irrigation system, then energy must either be added by a pump or booster, or be lost by friction. Between any two points, point 1 and point 2, in a closed system, changes in energy are accounted with the following formula: P1 + V12/2g + Z1 + Energy Added (pump head) = P2 + V22/2g + Z2 + Head Losses Initial Hydraulic Energy
Final Hydraulic Energy
Pressure Head @1 + Velocity Head @1 + Elevation Head @1 + Pump Head Added Equals Pressure Head @2 + Velocity Head @2 + Elevation Head @ 2 + Friction Losses The above expression is known as Bernoulli’s Equation which is used to solve hydraulic problems in irrigation systems. The two dynamic components in this expression are the pump’s energy (added) and the friction losses (subtracted). Head losses are the consequence of friction between the pipe's walls and water as it flows through the system and meets obstacles (turns, bends, expansions and contractions) along its way. The degree of head loss is a function of the following variables: a. b. c. d. e.
Pipe length Pipe diameter Pipe wall smoothness Water flow-rate (discharge) Water viscosity
Diverse theoretical and empirical equations have been developed to calculate these losses.
Friction Losses There are two types of friction losses: friction losses in water flow along straight pipes, defined as major losses; and friction losses due to the turbulent flow at bends and transitions, defined as minor (local) losses. If the flow velocities are high and there are many bends and transitions in the system, minor losses can build-up and be quite considerable. The most common equation used to compute friction losses of water flow along a pipe is known as the Hazen-Williams formula. J = 1.135 x 1012 (Q/C)1.852 X D-4.871
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DRIP IRRIGATION
Where: J = head loss (‰ =m/1000 m) D = inner pipe diameter (mm) C = friction coefficient (indicates pipe wall smoothness, the higher the C coefficient, the lower the friction head loss) Q = flow-rate (m3/h) Minor (local) Head Losses Minor head losses are usually defined as equivalent length factors which add a virtual length of straight pipe in the accessory same diameter to the length of the pipe under calculation. Total Dynamic Head
Table 2. The friction coefficient ( C ) of pipes Pipe material C PVC and PE Asbestos-cement New steel 5 year old steel Steel with internal concrete coating Concrete
140-150 130-140 110-120 80-90 110-120 90-100
The total dynamic head created by the pump is the sum of the pumping suction lift (the difference between water surface height at the source and pump height), the requested working pressure in the emitters, and friction losses within the irrigation system. The energy consumed per pumped unit of irrigation water depends on the total dynamic head provided by the pump and the pumping system's efficiency. The total dynamic head depends on: • Vertical distance that the water is lifted • Pressure required in drippers' inlets • Friction losses in the pipeline along the way from the water source through filters, valves, pipelines and manifolds on the way to the emitters Pumping system efficiency depends upon the pump efficiency, its power unit efficiency, and the efficiency of power transmission of power between them. The power output required by the pump is calculated with the formula below: N=
QxH ---------270 x ŋ
Where: N = required input – HP Q = pump discharge – m3/h H = total dynamic head – m η = pump efficiency – decimal fraction Example: Q = 200m3; H = 150 m; η = 0.75. N = 200 X 150/(270 X 0.75) = 148 HP When measuring pressure, it should be remembered that the pressure gauges are calibrated to read 0 (zero) at atmospheric pressure (about 1 bar). It is important to
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DRIP IRRIGATION
remember this fact for the operation of devices such as Venturi suction injectors in drip irrigation. Absolute Pressure Absolute pressure is the formal expression of total force per unit area. It is composed of the pressure of the atmosphere, the pressure due to any external forces applied on the fluid and the pressure resulting from the weight of the fluid itself. Gauge Pressure The gauge pressure is the absolute pressure minus the atmospheric pressure that typically acts in all directions and on all objects in open air. Since atmospheric pressure at sea level height is typically about 1 bar, an absolute pressure of 3 bars would be equivalent to a gauge pressure of 2 bar (~20 m pressure head). Working Pressure The working pressure is the pressure required at the emitters to guarantee effective performance and uniform water distribution. The range of the appropriate working pressure of the emitter is defined and published by the manufacturer in the operating guide. The type of the emitter chosen and its working pressure, have to be taken into account in the design of the irrigation system and in the irrigation scheduling. The distributing pipelines are designed to deliver the water to the emitters with such pressure losses that guarantee the appropriate working pressure in the emitters, so that water will be applied uniformly in the whole irrigated block. Although there are a number of formulae for calculation of head losses, in daily life, tables, nomograms and dedicated software are mostly used. When calculating the head losses in a pipe network, a distinction is made between the flow in pipes with a single outlet at their distal end and distributing pipes with multiple outlets. In a non-distributing pipe, head loss values taken from a table or a nomogram are expressed in % or ‰ units by its length in m. Multiplication by the pipe length in m. length units renders the actual losses in m. head units. Christiansen friction factor (F) is used also to calculate the head losses in pipes with multiple outlets such as drip laterals, This factor accounts for the decrease in flow along the lateral and depends upon the number of outlets or emitters (N) and the exponent (m = 1.852) of (Q) in Hazen-Williams equation. The formula to calculate this factor is as follows F = 1/(m+1) + 1/(2N) +((m-1)0.5/(6N)2) For a lateral with more than 10 emitters, F= 0.35 can be used regardless of which friction loss calculation formula is used. The head loss due to friction in drip laterals is then determined by Hl = F(Hlp), where Hl is the head loss due to friction in the drip lateral and Hlp is the head loss due to friction of the same discharge in a pipe of the same diameter and length but with a single outlet at the end. As mentioned above, Hl = 0.35 Hlp can be used when there are more than 10 outlets on the pipe. Hydraulic Characteristics of the Emitter The flow-rate of emitters in micro-irrigation is affected variably by pressure fluctuations. The performance of a given model depends on its design and construction. The relationship between the emitter operating pressure and flow-rate is calculated with the following equation:
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DRIP IRRIGATION
Q = kPe Where: Q = dripper flow-rate – l/h k = dripper constant – depends on the units of flow rate and pressure head. P = Pressure at the dripper's inlet – m e = dripper discharge exponent (dripper exponent) The dripper exponent indicates the specific relationships between the working pressure and the flow-rate of the emitter. The range of emitter exponents is 0 – 1.0 Drippers with laminar flow pattern have high exponents, in the range of 0.7 – 1.0. Drippers with turbulent flow pattern have exponents between 0.4 and 0.6. Compensating drippers have exponents which approach zero in the regulated flow range. The larger the dripper exponent, the more sensitive the flow-rate is to pressure variations. A value of 1 means that for each percent change in pressure there is an identical percent change in flow rate. On the other side, an exponent value of 0 (zero) means that the emitter flow-rate does not change at all as pressure changes. Table 3. The effect of dripper exponent on pressure – flow-rate relationships % pressure change Exponent ---->
0.4
0.5
0.6
0.7
0.8
10 20 30 40 50
3.9 7.6 11.1 14.4 17.6
4.8 9.5 14.0 18.3 22.5
5.9 11.6 17.1 22.3 27.5
6.9 13.6 20.2 26.6 32.8
7.9 15.7 23.3 30.9 38.3
% flow rate change
Fig. 15. Relationship between the dripper exponent and lateral length Courtesy “Netafim”
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DRIP IRRIGATION
Whenever the laterals are laid out on the soil surface, the ambient temperature affects dripper flow-rate. As water temperature increases, water viscosity decreases and the discharge increases. Lateral heating is more pronounced at the distal end due to decrease in flow velocity. As a result, the emitters in the lateral's end may have a higher flow-rate than the emitters at the beginning of the laterals. In pressure compensating (PC) drippers, pressure fluctuations above the threshold of the regulating pressure do not affect the flow-rate. The regulating pressure is that head range in which regulation of flow-rate takes place. The graphs to the right show that in Ram PC Fig. 16. Pressure Compensated dripper flow-pressure relationship drippers, the regulating pressure threshold is about 4 m. Calculation of the Head Losses As mentioned before, slide rulers, tables, nomograms, hand-held and on-line calculators as well as dedicated software can be used for the calculation of Fig. 17. Non-pressure compensated flow-pressure relationship head losses. Pipe and accessories manufacturers publish tables and nomograms depicting the head losses in their products. Valve producers use the Kv coefficient that designates the discharge of the valve in m3/h at which 10 m head (1 bar) are lost by friction. Table 4. Head losses in Acuanet automatic valve Flow m3/h
3 5 7 10 12 14 16 18 20 24 28 32
Model 1" El/St
1½" El/An
1½" El/St
1½" Hy/St
2" El/St
2" Hy/An
2" Hy/St
2" Hy/An
1.6 2.3 4.7
0.5 0.5 1.3 2.2 3.0 3.5 4.6 5.8 6.6 8.5
0.4 0.4 1.0 1.8 2.2 2.8 3.4 4.3 5.2 6.5
0.3 0.3 0.7 1.3 1.3 2.2 3.0 4.0 4.7 6.5
0.3 0.6 0.9 1.5 1.9 2.4 3.0 3.6 4.2 5.6 7.2 9.6
0.2 0.4 0.7 1.0 1.3 1.7 1.9 2.4 2.9 3.8 4.9 8.8
0.3 0.3 0.5 1.0 1.4 3.4 2.4 3.2 3.8 5.5 7.2 8.5
0.2 0.3 0.8 1.0 1.2 1.4 1.6 2.1 2.6 3.6 4.6 6.4
El=Electric; An=Angular; St=Straight; Hy=Hydraulic Courtesy "Netafim"
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Technical Data Dripper manufacturers provide detailed technical data, in catalogues or on-line, about the flow-rate pressure relations of their products, such as the dripper's coefficient and the dripper exponent. This information should be utilized for the design of lateral length and the pressure required at the lateral's inlet. Low dripper exponents allow higher pressure difference between drippers without deviating from the rule allowing flow-rate difference of 10%. (This rule is dealt with in the topic on system design).
Fig. 18. Acuanet automatic valve
Table 5. "Plastro" Hydrodrip II integral drip laterals technical data EMITTER TYPE
FLOW RATE l/h
DIAMETER mm NOMINAL
12-/26/40 2.1 16/18 1.6 2.2 3.6 16-/25/35 1.7 -40/45 2.3 3.6 20-/24/36/44 1.7 2.3 3.6 25-/17/34 1.7 2.3 3.6
12 16 16 16 16 16 16 20 20 20 25 25 25
DRIPPER CONSTANTS
DH=7.5%
DH=10%
INTERNAL
COEFFICIENT
EXPONENT
Pmin-m Pmax-m Pmin-m Pmax-m
10.4 15.2 15.2 15.2 15.2 15.2 15.2 17.6 17.6 17.6 22.2 22.2 22.2
0.6442 0.5300 0.7260 1.1940 0.5212 0.7646 1.1940 0.5212 0.7646 1.1940 0.5212 0.7646 1.1940
0.506 0.4830 0.4840 0.4792 0.5090 0.4704 0.4792 0.5090 0.4704 0.4792 0.5090 0.4704 0.4792
9.25 9.19 9.19 9.19 9.24 9.17 9.19 9.24 9.17 9.19 9.24 9.17 9.19
10.75 10.81 10.81 10.81 10.76 10.83 10.81 10.76 10.83 10.81 10.76 10.83 10.81
9 8.91 8.91 8.90 8.97 8.88 8.90 8.97 8.88 8.90 8.97 8.88 8.90
11 11.09 11.09 11.10 11.03 11.12 11.10 11.03 11.12 11.10 11.03 11.12 11.10
Adapted from "Plastro" CD-Rom
This example shows that a large difference in pressure head – up to 20%, is tolerated for drippers with a dripper exponent of 0.5 and below. A comprehensive nomogram for the estimation of friction losses in straight pipe sections that can be used with any type of pipe is presented overleaf. The example shows the head loss (in J ‰) for a flow-rate of 200 m3/h through a pipe with an inner diameter of 200 mm. The first step is to draw a straight line from Q=200 on the left scale through the 200 mm point on the D mm scale. The crossing point of the ruler with the axis (the blank line) has to be clearly marked. The second step is to draw a straight line connecting the mark through the relevant friction coefficient on the scale C and to mark the point where this line crosses the scale J0/00. The value of the crossing point is the head loss in 0/00 (m pressure head per 1000 m length of the pipe. The following nomograms are useful for LDPE and HDPE pipes. In each nomogram, the relevant PN values are designated under "Class".
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DRIP IRRIGATION
Fig. 19. Head loss nomogram, based on Hazen-Williams formula
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DRIP IRRIGATION
Fig. 20 Nomogram for calculation of head losses in HDPE pipes Adapted from "Plassim" brochure
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DRIP IRRIGATION
Fig. 21 Nomogram for calculation of head losses in LDPE pipes Adapted from "Plassim" brochure The class designation relates to the working pressure (PN) in atm. 1atm = 10 m ≈ 1 bar
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DRIP IRRIGATION
Chapter 6. PIPES AND TUBES FOR DRIP IRRIGATION The commercial development of drip irrigation is based on the use of plastic materials. Drippers, pipes and most of the other drip system components are made of plastic materials. Plastic solid materials are comprised of one or more polymeric substances that can be shaped by molding or extrusion. Polymers, the basic ingredient of plastic materials, are a broad class of materials that include natural and synthetic substances. In professional terminology, polymers are frequently defined as resins. For example, a polyethylene (PE) pipe compound consists of PE resin combined with colorants, stabilizers, anti-oxidants and other ingredients required to protect and enhance the quality of the material during the fabrication process and operation in the field. Plastic materials are divided into two basic groups: thermoplastics and thermosets, both of which are used for the production of plastic pipes. Thermoplastics include PE, polypropylene, polybutylene and PVC. These materials can be re-melted by heat. The solid state of thermoplastic materials is the result of physical forces that immobilize polymer chains and inhibit them from slipping past each other. When heat is applied, these forces weaken and allow the material to soften or melt. Upon cooling, the molecular chains stop slipping and are held firmly against each other in the solid state. Thermoplastics can be shaped during the molten phase of the resin and therefore can be extruded or molded into a variety of shapes, such as pipes, flanges, valves, drippers and other accessories. Thermoset plastic materials are similar to thermoplastics prior to a chemical reaction (“curing”) by which the polymer chains are chemically bonded to each other by new cross-links. That is usually performed during or right after shaping of the final product. Cross-linking is the random bonding of molecules to each other to form a giant threedimensional association. Thermoset resins form a permanent insoluble and infusible shape after applying heat or a curing agent. They cannot be re-melted after shaping and curing. This is the main difference between thermosets and thermoplastics. As heat is applied to a thermoset component, degradation occurs at a temperature lower than the melting point. Thermosetting resins can be combined with reinforcements to form strong composites. Fiberglass is the most popular reinforcement and fiberglassreinforced pipe (FRP) is a common form of thermoset-type pipes.
Polyethylene Polyethylene (PE) is the most prevalent material in pipes and laterals in drip irrigation systems. There are four types of PE, classified by material density: Type I – Low Density (LDPE), 910 – 925 g/l Type II – Medium Density (MDPE), 920 – 940 g/l Type III – High Density (HDPE), 941 – 959 g/l Type IIII – High Homo-polymer, 960 and above g/l Carbon black 2% is added to reduce pipes’ sensitivity to ultraviolet (UV) sun radiation. PE pipes can be classified according to the working pressure (PN) they can withstand. The common grades of PN used in irrigation are: 2.5, 4, 6, 10, 12.5 and 16
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DRIP IRRIGATION
bars (atm). Some thin-wall laterals withstand much lower PN: 0.5 – 2 bar. The tolerance to working pressure depends on pipe density and wall thickness. The tolerance data given by the producers relates to temperature of 20 C0. In higher temperatures, the tolerance decreases significantly, hence pipes are tested at twice their designated working pressure. Plastic pipes are defined according to their external diameter, in mm. In the USA and some other countries, pipe diameter is defined by imperial inch units (“). 1” = 25.4 mm. Pipe wall thickness is also defined in mm units (in the USA by mil units - 1/1000 of inch). 1 mil = 0.0254 mm. Laterals are commonly made of LDPE (PE grade 32) while delivering and distributing pipes with diameters greater than 32 mm are mostly made of HDPE. HDPE pipes are further classified according the grade of the material: PE-63, PE-80, PE-100. The higher the grade, the higher the pipe quality. Table 6. PE (polyethylene) pipes for agriculture PE type
ND
LDPE
6 mm
LDPE
Applications
PN - m
Hydraulic command tubing
40 – 120
6 – 10 mm
Micro-emitters connection to laterals
40 – 60
LDPE
12 – 25 mm
Thin-wall drip laterals
5 – 20
LDPE
12 – 25 mm
Thick-wall drip laterals
25 – 40
LDPE
16 – 32 mm
Micro and mini emitter laterals
40 – 60
HDPE
32 – 75 mm
Sprinkler laterals
40 – 60
HDPE
40 – 140 mm Main lines and sub-mains
40 – 100
HDPE
75 – 450 mm Water supply networks
60 - 160
Table 7. Internal diameter and wall thickness of LDPE pipes OD/PN
25 m
40 m
60 m
80 m
100 m
mm
ID
Wall thickness
ID
Wall thickness
ID
Wall thickness
ID
Wall thickness
ID
Wall thickness
12
9.8
1.1
9.6
1.2
9.2
1.4
8.6
1.7
8.0
2.0
16
13.2
1.4
12.8
1.6
12.4
1.8
11.6
2.2
10.6
2.7
20
17.0
1.5
16.6
1.7
15.4
2.3
14.4
2.8
13.2
3.4
25
21.8
1.6
21.2
1.9
19.4
2.8
18.0
3.5
16.6
4.2
32
28.8
1.6
27.2
2.4
24.8
3.6
23.2
4.4
21.2
5.4
40
36.2
1.9
34.0
3.0
31.0
4.5
29.0
5.5
26.6
6.7
50
45.2
2.4
42.6
3.7
38.8
5.6
36.2
6.9
33.4
8.3
Adapted form "Plastro" brochure
ND = Nominal Diameter OD = External (Outer) Diameter. In plastic pipes it is mostly equivalent to the ND. ID = internal (inner) Diameter
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DRIP IRRIGATION
Table 8. Internal diameter and wall thickness of HDPE pipes 25 m 40 m 60 m 80 m 100 m 160 m OD/PN Wall Wall Wall Wall Wall Wall mm ID ID ID ID ID ID thickness thickness thickness thickness thickness thickness 12 16 20 25 32 40 50 63 75 90 110 125 140 160 180
46.8 59.8 71.2 85.6 104.6 118.8 133.0 152.0 172.2
28.8 36.8 46.0 58.2 69.2 83.0 101.6 115.4 129.2 147.6 166.2
1.6 1.6 1.9 2.2 2.7 3.1 3.5 4.0 4.4
1.6 1.6 2.0 2.4 2.9 3.5 4.2 4.8 5.4 6.2 6.9
21.8 28.2 35.2 44.0 55.4 66.0 79.2 96.8 110.2 123.4 141.0 158.6
1.6 1.9 2.4 3.0 3.7 4.7 5.5 6.6 8.1 9.2 10.3 11.8
16.8 21.1 27.2 34.0 42.6 53.6 64.0 76.8 93.8 106.6 119.4 136.4 153.4
1.6 1.9 2.4 3.0 3.7 4.7 5.5 6.6 8.1 9.2 10.3 11.8 13.3
12.8 16.2 20.4 26.2 32.6 40.8 51.4 61.4 73.6 90.0 102.2 114.6 130.8 147.2
1.6 1.9 2.3 2.9 3.7 4.6 5.8 6.8 8.2 10.0 11.4 12.7 14.6 16.4
8.6 11.6 15.4 18.0 23.2 29.0 36.2 45.8 54.4 65.4 79.8 90.8 101.6
1.7 2.2 2.8 3.5 4.4 5.5 6.9 8.6 10.3 12.3 15.1 17.1 19.2
Adapted form "Plastro" brochure
PVC Pipes PVC (Polyvinyl Chloride) is a rigid polymer. To soften the material and enable its shaping, it is common to add substantial amounts (up to 50%) of plasticizers. These additives render flexibility to tubes made of soft PVC. PVC pipes are sensitive to UV sun radiation. Soft flexible PVC pipes are used in a limited scale mainly in gardening and landscape. Their life span is short. Rigid PVC pipes are used In agriculture mainly for water delivery and distribution. PVC pipes are installed underground only, to avoid UV radiation damage. In the last decade, UPVC (unplasticized PVC) pipes are preferred because of their improved durability and ability to withstand pressure. PVC pipes appear in discrete 4 – 8 m long segments and have to be jointed in the field. The working pressure of rigid PVC pipes is 6 – 24 bars (60 – 240 m). Table 9. PVC pipes for agriculture PVC type
ND
Applications
PN - m
Hydraulic command tubing
40 – 80
Soft PVC
6 mm
Soft PVC
6 – 10 mm
Micro-emitters connection to laterals
40 – 60
Soft PVC
12 – 25 mm
Thin-wall drip laterals
5 – 20
Rigid UPVC
½” – 4”
Rigid UPVC
63 – 1000 mm
Risers
40 – 100
Supply networks, main lines, submains
40 – 240
When PVC pipes are installed in heavy or stony soil, it is recommended to pad the trench with sand to avoid damage to the pipe wall caused by soil swelling and stone pressure.
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DRIP IRRIGATION
Table 10. Internal diameter and wall thickness of PVC pipes PN------>
60 m
80 m
100 m
OD - mm
ID - mm
Wall thickness mm
ID - mm
Wall thickness mm
ID - mm
Wall thickness mm
63 75 90 110 140 160 225 280 315 355 400 450 500
59.0 70.4 84.4 103.2 131.4 150.2 210.2 262.8 295.6 333.2 375.4 422.4 469.4
2.0 2.3 2.8 3.4 4.3 4.9 6.9 8.6 9.7 10.9 12.3 13.8 15.3
58.2 69.2 83.0 101.6 129.2 147.6 207.8 258.6 290.8 327.8 369.4 415.6 461.8
2.4 2.9 3.5 4.2 5.4 6.2 8.6 10.7 12.1 13.6 15.3 17.2 19.1
57.0 67.8 81.4 99.4 126.6 144.6 203.4 253.2 285.0 321.2 361.8 407.0 452.2
3.0 3.6 4.3 5.3 6.7 7.7 10.8 13.4 15.0 16.9 19.1 21.5 23.9
Lay-Flat Hoses Flexible PVC lay-flat hoses can be used as mainlines and sub-mains. The hose is impregnated with anti-UV radiation protecting agents. When the water is shut-off, the hose lays flat on the ground and can be crossed-over by tractors and other farm machinery. The lay-flat hoses can be laid out on the soil surface or in a shallow trench. These hoses are available in diameters of 75 – 200 mm.
Fiberglass Pipes In addition to UPVC and HDPE pipes, reinforced fiberglass pipes are used to deliver water under high pressure from the water source to the irrigated area, as a substitution for steel and asbestos-cement pipes. GRP (Glass Reinforced Polyester) fiberglass pipes are manufactured in diameters of 300 – 3600 mm and PN grades of 40 – 250 m. They are particularly useful in delivery of reclaimed water. External and Internal Pipe Diameter The internal diameter (ID) of a pipe can be calculated by deducting twice the wall thickness from the external diameter (OD). In most cases, the nominal pipe diameter (ND) is the same as its external diameter. Friction head losses of water flow in the pipe are determined by the internal diameter. When using nomograms, on-line calculators and design software it is important to check whether the designated diameter is nominal (mostly external) or internal.
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DRIP IRRIGATION
Chapter 7. DRIPPER TYPES, STRUCTURE, FUNCTION AND PROPERTIES Introduction The dripper is the core of the drip irrigation system. Drippers are small water emitters, made of plastic materials. The design and production of high quality drippers is a delicate and complicated process. Manufacturing the most effective dripper commits compromising, taking into account diverse and contradicting demands. The most important attribute of a dripper is low flow-rate, in the range of 0.1 - 8 liter per hour (l/h). This low flow-rate can be obtained by different methods. Flow-rate is determined by the pattern and the dimensions of the water passageway, as well as the water pressure at the dripper inlet. The smaller the passageway cross-section, the lower the dripper flow-rate at a given pressure. However, the narrower the passageway the greater the risk of clogging by suspended solid particles and chemical precipitates. Since the water pressure at the dripper's outlet is a key factor in determining flow rate, reducing that pressure may facilitate a low flow rate through a relatively wide water outlet. Pressure reduction is achieved by diverse means. Water passageway pattern: Historically, the initial method for pressure reduction was a long flow passageway along a tiny micro-tube. Water friction against the wall of the micro-tube resulted in substantial head losses. The factors affecting the degree of head loss are: micro-tube length and diameter, micro-tube wall smoothness, flow pattern and velocity. Initially, the micro-tubes were attached to a lateral and delivered water at the desired application point. Later, the micro-tubes were wrapped around the lateral and finally, drippers with internal built-in spiral water passageways were constructed. The laminar passageway was problematic. The long water path and low flow velocity led to the precipitation of chemicals that changed the dripper's flow-rate or plugged it completely. Initial The labyrinth passageway is a laminar more advanced design. The water design flows along a labyrinth wherein the flow direction changes Laminar intermittently and gets a turbulent spiral pattern with high head losses dripper along a significantly shorter path as compared to the spiral dripper, Labyrinth resulting in the manufacture of passageway smaller and cheaper drippers. An additional advantage of this Toothed design is the lower accumulation (zigzag) of dirt - particles and chemical passageway precipitations. Later designs modified the labyrinth Turbonet passageway into a zigzag toothed passageway path with more efficient pressure Fig. 22. Evolution of the passageway style Courtesy “Netafim” dissipation and self-cleaning attributes. An advanced refinement of the toothed passageway is the turbonet (by "Netafim") that enables shortening and widening of the water path even further.
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DRIP IRRIGATION
Turbulent flow has a cleaning effect in the corners of the water passageways. The vortex design is another method for significantly dissipating pressure along a short passageway. The water enters tangentially into the dripper and flows in a spiral whirlpool pattern with high head losses along a short path and improved prevention of precipitates.
Fig. 23. Turbulent flow from "DIS" brochure
Another type of dripper is the orifice dripper. Pressure dissipation occurs at the tiny water inlet into the dripper.
Fig. 24. Orifice dripper Adapted from Karmeli & Keller, 1975
Fig. 26. Labyrinth button dripper (“Netafim”)
The shortening of the labyrinth passageways enables fabrication of small and cheap button labyrinth drippers.
Fig. 25. Vortex dripper
Adapted from
Karmeli & Keller, 1975
Along with the development of the discrete dripper technology, a different concept - the trickling tape was developed. The first product was a perforated plastic tube. In order to obtain low discharge, the water outlets were tiny and highly sensitive to clogging. This flaw was eliminated later by the molding of long labyrinth water passageways into the tube.
Fig. 27 Tape dripper lateral: empty (left) and filled with water (right) Adapted from "T-Tape" brochure Dripper systems are classified according to various parameters:
Lateral Location On-Surface Drip Irrigation This is the prevalent drip technology. It enables convenient monitoring of dripper's clogging and other disturbances during operation. On the other hand, this method is susceptible to mechanical damage and degradation by solar radiation; it may interfere with some farming activities and commits laying out and retrieving the laterals when irrigating annual crops. In vineyards and some other deciduous orchards (of apples and pears) grown at the palmeta pruning shape, laterals are attached to trellises, improving the monitoring of drippers function and decreasing the hazard of mechanical damage.
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DRIP IRRIGATION
Subsurface Drip Irrigation (SDI) This technology has gathered momentum during the last two decades. Although the system imposes extra costs for burying the laterals into the soil, this technology simplifies irrigation operation and minimizes interference with other farm activities. SDI has better water savings and nutrient utilization attributes than on-surface drip irrigation. It also decreases infestation by weeds. Dripper clogging by root intrusion and suction of soil particles may hinder proper function and must be avoided by the proper selection of equipment, competent installation and strict routine maintenance practices.
Layout of Water Outlets along the Lateral There are two typical arrangements of drippers on laterals that affect the pattern of water distribution in the soil. Point Sources Drippers are installed along the lateral with a spacing that creates a discrete wetted soil volume by each emitter, without overlapping by the adjacent drippers. This layout is mostly prevalent with thick wall laterals irrigating orchards and in annuals grown in ample spacing.
Fig. 28. Point-source (left) and line-source (right) wetting by drippers Line Sources In a different layout, the drippers are installed along the lateral closed to each other so that the wetted soil volumes of adjacent drippers along the lateral overlap. This arrangement is typical with tapes and is the preferred alternative for densely grown annual crops.
Lateral Type Thick-Walled Laterals Thick-walled laterals are made of Low Density Polyethylene Pipes (LDPE) with 12 – 25 mm external diameter and 1 – 2 mm wall thickness. The discrete drippers are installed on-line or inline, 10 – 100 cm apart. The working pressure (PN) is 1 – 4 bar (10 – 40 m). Thin-Walled Laterals Thin-walled laterals are also made of LDPE, however, the wall thickness is only 0.1 – 0.5 mm and the PN is 0.1 – 1 bar (1 – 10 m). Laterals may have discrete drippers that are molded or inserted in the lateral. A different design is contiguous pressure dissipation passageways as integral components of a tape.
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DRIP IRRIGATION
Structure and Water Passageway Characteristics As mentioned in the introductory section, the main objectives of the different dripper's designs is the dissipation of pressure that renders low flow-rates, with minimal clogging hazard and a cheap emitter. These objectives are achieved by different methods, as is indicated below. Long Path The water flows through a long and narrow micro-tube. The micro-tube may be a long one (spaghetti) or a built-in spiral in the dripper's body. Water flow is laminar and the friction with the tube walls and the internal friction between water molecules results in pressure dissipation. The discharge (flow-rate) of laminar-flow drippers is highly sensitive to changes in pressure.
Fig. 29 In-line barbed laminar dripper (left, "Netafim") and turbulent in-line integral dripper (right, "Drip-In")
Labyrinth Path The water flows through a labyrinth that abruptly changes its direction, causing turbulence. The frequent change of direction within the labyrinth results in highenergy losses reducing the flow-rate along a relatively wide flow path. This pattern is more effective in pressure dissipation than the laminar passageway. It enables using wider water passages and reduces chemical and particle precipitations. The flow-rate is less affected by changes in pressure in comparison to laminar flow. Zigzag (Toothed) Path The shape of the passageway is similar to the labyrinth path. However, the zigzag flow dissipates more pressure in a shorter path and decreases clogging hazard. Vortex The water enters tangentially into vortex drippers. The water stream hits the walls of the circular chamber, spins and looses energy. This allows for a relatively short flowpath and wide-outlet orifice. Orifice The emitter flow rate is determined by the diameter of the orifice. This requires a tiny aperture that increases clogging hazard.
Location on Lateral Drippers can be installed externally on the lateral or inserted in-line.
Fig. 30. On-line drippers
Courtesy “Netafim”
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DRIP IRRIGATION
On-Line On-line drippers are inserted into the lateral through punched holes. Drippers can be added along time according to changes in crop growth and water requirements. The dripper protrudes from the lateral, making it sensitive to damage in delivery, installation and retrieval (when applicable). The dripper has a barbed or threaded joint that is inserted or screwed into thick-wall laterals. In-Line In-line drippers keep the outer face of the lateral smooth. There are two versions:
Thread Barb Fig. 31. Button drippers conector design
In-line built-in drippers are fused into the lateral during the extrusion process. Barbed In-line drippers are installed by cutting the lateral and inserting the barbs into the cut ends of the lateral.
Distinctive Properties Adjustable Drippers The flow-rate of these drippers can be adjusted according to the changing requirements along the growing season. Flag Emitters These drippers have a twist opening locker that eases the cleaning of fully or partially clogged drippers during irrigation.
Fig. 32. Adjustable dripper (above) and flag dripper (right)
Pressure Compensated (PC) Drippers The flow-rate of compensated emitters remains uniform provided the available pressure is above a given minimum regulation pressure. There are several compensating mechanisms that narrow or lengthen the internal water passageway when the pressure rises, increasing head losses and keeping the flow rate constant. Flexible Membrane above Water Path The compensating mechanism is a flexible diaphragm. As the pressure on top of the diaphragm increases it narrows the water passageway below the diaphragm, increasing the head losses and decreasing the flow-rate.
Fig. 33. Flexible diaphragm under pressure
Fig. 34. Button and inline PC drippers Courtesy "Netafim"
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DRIP IRRIGATION
Changing of Water Flow Path Length In another technique, pressure compensation is applied by varying the effective length of the labyrinth. The higher the pressure the longer the effective passageway. This is accomplished by changing the number of openings between a membrane and the labyrinth. These openings are sequentially closed by an Fig. 35. ADI PC dripper From "Mezerplas" brochure increase in the pressure, maintaining the discharge constant. The shortened water passageway, supported by this technology, decreases clogging hazards and renders an efficient compensating mechanism.
Fig. 36. Change of water passageway length under high pressure - “Mezerplas” ADI PC Dripper Non-Leakage (No Drain) Drippers Drainage of drip laterals after water shutdown promotes accumulation of precipitates at the bottom of the laterals and in the water passageways within the drippers. It lasts some time after the beginning of irrigation until the laterals are full with water and the required working pressure builds-up. During this time interval, the discharge of the drippers in the initial part of the lateral is significantly higher than that of the drippers at the distal end of the lateral. Frequent small water applications, as in vegetables cropping, makes this time segment a significant fraction of the irrigation time length. Bug cover Woodpecker This results in significant difference in water dosage between the initial and the distal ends of the laterals and in the plot as a whole. The non-leakage drippers keep the lateral full of water after shutdown by sealing the dripper's outlet as the pressure drops. It also facilitates fast pressure build-up in the laterals at the start of the next irrigation. Woodpecker Drippers These drippers are designed for use in plots prone to woodpecker’s damage. The woodpeckers drill holes in the LDPE laterals in search of water. Preventive Fig. 37. Woodpecker drippers action is taken by burying the laterals with the woodpecker drippers underground and connecting thin micro-tubes to the dripper outlet. The distal end of the micro-tube is laid on the soil surface. Woodpecker damage can also be reduced by distributing water-filled cans in the plot to satisfy the woodpeckers’ thirst.
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DRIP IRRIGATION
Flap Equipped Drippers Drippers equipped with a flap on the water outlet prevent the suction of small soil particles into the dripper at shutdown as well as the intrusion of roots into drip lateral installed below the soil surface. Treflan Impregnated Drippers Fig. 38 Flap equipped dripper For long-term prevention of root intrusion into subsurface drip laterals, the herbicide Trifluraline (TreflanTM) is impregnated into the drippers during the production process. After installation of subsurface laterals, small amount of the herbicide is released with each water application into the soil around the dripper, sterilizing its immediate vicinity. Drippers containing Trifluraline can substitute routine Treflan application for up to 15 years.
Fig. 39. Arrow dripper for greenhouses, nurseries and pot plants Courtesy "Netafim" Arrow Drippers Arrow dippers are used for the irrigation of potted plants. The stick-like dripper is inserted into the bed inside the pot. A high capacity built-in filter and efficient zigzag turbulent water passageway keep the tiny dripper clean and reliable in long-term use. Multi-Outlet Drippers Each dripper has 2 – 12 outlets onto which small diameter microtubes are connected. These drippers are used mostly in landscaping and potted plants irrigation. Fig. 40 Six outlets
Ultra Low-Volume Drippers
Extremely low water application rates, in the range of 0.1 – 0.3 l/h per dripper, change the water distribution pattern in the soil and the water-to-air ratio in the wetted soil volume. The horizontal movement is more pronounced than with drippers of conventional flow-rate range. Therefore, water can be applied to shallow root systems with minimized drainage beneath the root-zone.
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DRIP IRRIGATION
Due to the extremely low water discharge from the emitters, more air remains in the wetted volume, compared with drippers of conventional flow-rate. Extremely low flow-rate drippers are sensitive to clogging because of the narrow water passageways and low flow velocity. There are two technologies to achieve low flowrate with minimal clogging hazard. One technology is based on conventional button drippers that Fig. 41. Ultra low flow micro-drippers emit water into a secondary small Adapted from "Plastro" brochure diameter with 10 – 30 molded or inserted micro-drippers. In the second technology, conventional drip laterals are used but the water is applied in pulses created by pulsators or by the irrigation controller. Because of the relatively short pulses and long intervals between them, drippers should be of the non-leakage (no drain) type. Integral Filtration in Drippers High quality drippers have built-in small integral filters to reduce the clogging hazard of the water passageways and guarantee proper function of the complicated mechanisms needed for pressure compensation, drainage prevention, etc. The filtering area is increased to ensure long-term performance.
Fig, 42. Integral filters Courtesy "Netafim"
Additional anti-clogging means are dual water inlets and outlets in the single dripper and the barbs in on-line drippers which protrude into the lateral so that the water inlet is kept away from the dirt that accumulates on the lateral's walls. Anti-siphon devices such as the above-mentioned flaps also decrease clogging hazard.
Static state Pressure compensation Fig. 43. Auto flushing, pressure compensating dripper Courtesy "Netafim"
Flushing
Auto Flushing Mechanisms In some of the compensated drippers, a flexible diaphragm is used to release debris that clogs the dripper. When a solid particle blocks the water path, the diaphragm is arched, widens the passageway and releases the clogging object.
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DRIP IRRIGATION
Chapter 8. ACCESSORIES In addition to drippers and pipes, drip irrigation systems are comprised of diverse accessories. Wise selection of these components can guarantee optimal long-term performance of the system. These accessories can be classified in four categories:
Connectors: connecting pipelines and laterals to the regulating and control devices, interconnecting pipes of different types and diameters, laterals to manifolds and drippers to laterals.
Control, monitoring and regulation devices: valves, filters, water meters, pressure gauges, etc.
Chemicals injectors and safety devices.
Soil moisture measuring and monitoring instrumentation.
Connectors Connectors are made of metal or plastic materials. They may be two-sided straightthrough or angular units, T or Y pattern triple outlets, four-sided crosses or multioutlet splitters.
Fig, 44. Plastic and metal pipe and lateral connectors Connectors to control devices are usually threaded or flanged. Connectors between pipes and laterals are mostly barbed or conic. There are simple barb connectors, while more sophisticated connectors have an inner barb and external fastening cap. HDPE pipes may be joined by heat fusion in the field. If done properly, fusion is reliable and durable.
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DRIP IRRIGATION
Fig. 45. Lateral start, plugs and lateral end
Fig. 46. Reinforced connectors
Control Devices Valves are basic control devices. There are different types of valves, each of which performs a different task. Gate valves are used for on-off tasks. They are not suitable for gradual opening and closing tasks. Ball valves are also used for on-off tasks. They have low head losses but are not suitable for flow regulation. Globe valves have higher head losses but they are efficient and precise for flow regulation. Angular and Y shaped valves have lower head losses than globe valves and they can also be used for flow regulation.
Fig. 47. Drip laterals connectors and splitters
Butterfly valves have modest head losses and certain throttling capability. Hydraulic valves appear in most of the before mentioned designs. They have a control chamber in which water pressure from the command line actuates a piston or diaphragm that regulate the flow through the valve by narrowing or expanding the water passageway. Hydraulic valves are of two types: normally open (N.O.) and normally closed (N.C.). Normally open (N.O.) valves remain open until the control chamber is filled with water under the system’s pressure, to close it Normally closed (N.C.) valves remain closed by the water pressure in the main-line. The closure is secured by a spring, in case of a rupture in the command line. In order to open the hydraulic valve, the controller opens a small valve at the top of the control chamber, releasing the pressure exerted on the diaphragm.
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DRIP IRRIGATION
Fig. 48. Hydraulic valve The pressure which the water in the system exerts on the lower face of the diaphragm reopens the valve. Normally closed hydraulic valves have higher head losses, but they are safer to use, as the valve remains closed even if the command tube is torn or plugged. Water meters are essential for accurate water application. The most prevalent are the Woltmann models. Bi-annual recalibration is required. Pressure regulators are used to maintain a constant downstream pressure, independent of upstream fluctuations provided it remains above the regulating pressure.
Pressure regulation is particularly important for drip irrigation. Thin walled laterals have PN of 4 – 15 m, and burst at higher pressures. When using non-compensated drippers, pressure regulators installed on the manifolds or lateral heads can maintain uniform pressure under harsh topographic conditions. There are two types of pressure regulators. Simple mechanical devices regulate the pressure against a spring,
42
Fig. 49. Spring pressure regulator assemblies Courtesy "Netafim"
DRIP IRRIGATION
while in the more sophisticated devices the pressure is controlled hydraulically by a diaphragm or piston.
Fig. 50. Spring actuated pressure regulator
Fig. 51. Hydraulic pressure regulator
Metering Valves The metering valve is a combination of a water meter with a hydraulic valve. The desired volume of water to be applied is dialed in. The valve opens and closes automatically only after the assigned volume has been delivered. Metering valves are used extensively in drip irrigation. They facilitate also the gradual opening and closing of water, which is important to avoid the collapse of thin-walled laterals. They are handily compatible with automation.
Fig. 52. Horizontal and angular metering valves
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The actuator in the metering valve can be a diaphragm or a piston. A diaphragm is less sensitive to dirt in the water, but can be torn in high pressure fluctuations and may wear due to chemical degradation. Electric Valves Electric valves are vastly used in automation. They are operated by a solenoid that converts electric pulse to mechanical move. In a small diameter – up to 1” (25 mm) – the solenoid can serve as direct actuator. In wider diameters, the solenoid commands hydraulic actuators. Energy sources are AC current where applicable, batteries and solar cells. Pressure relief valves have the task to instantly release water under excess pressure in order to protect the system. They can be mechanical, working against a spring (cheaper but less reliable) or hydraulic devices. Air relief valves are used to release air from the pipelines during filling with water and to let air in when pipelines on slopes drain. Plastic pipes, Fig. 53. Electric valve "Bermad" manufactured to withstand a pressure of 6, 10 or more bars, are seriously damaged when the pressure inside the pipe falls below atmospheric pressure. “Double action" air relief valves let air escape even when the floating device is lifted by pressure buildup as the pipeline fills with water. Air Relief Valves and Vacuum Breakers
Fig. 54. Air-relief valves As mentioned before, air relief valves and atmospheric vacuum breakers are essential components of the drip irrigation system. There are three types of air relief valves:
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Automatic valve: releases small volumes of air during ordinary operating conditions. Kinetic valve: releases large volumes of air while the system is filled with water and allows a great volume of air to enter the system at shutdown. Combination valves: Automatic and kinetic valves mounted together in one assembly. Atmospheric vacuum breakers are small devices, ½” – 1” in diameter that break the vacuum at water shutdown and do not allow air to escape from the system when water drains from the irrigation system and the pressure in the pipelines falls below the atmospheric pressure. Air relief valves introduce air into the Fig. 55. Atmospheric vacuum breakers irrigation system when its pressure equals or falls below the atmospheric pressure and function as vacuum breakers. Check-Valves and Backflow Preventers These valves are used to prevent backflow from the irrigation network to the water supply network, when that network supplies potable water to consumers. These devices are described in the chapter on fertigation. Lateral-End Flush Devices In drip irrigation, a vast amount of precipitates is accumulated in the lateral distal end. The automatic lateral end flush device releases water at the start of irrigation, before working pressure builds-up in the system. This performs routine flushing of the laterals, eliminating the need to do it manually.
Fig. 56. Lateral-end flushing action
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Chapter 9. FILTRATION Due to the narrow water passageways and low water-flow velocity in the drippers, drip systems are sensitive to clogging. Clogging prevention requires high-level filtration and complimentary chemical and physical water treatments. Table 12. Characteristics of water passages in drippers (example) NonFlow compensated Rate* drippers l/h Inline 8.0 Button 8.0 Inline 4.1 Button 3.8 Tiran 4.0 Typhoon 2.8 Tiran 2.0 Inline 2.0
Water passageway Length
Width
Depth
mm 220 48 258 50 95 17 135 280
mm 1.95 1.39 1.35 1.15 1.38 0.81 1.00 1.10
mm 1.84 1.45 1.45 1.05 1.38 0.81 1.00 1.18
Cross section .mm2
2.80 2.02 1.95 1.22 1.90 0.65 1.00 1.30
Compensated Flow drippers rate PC button ” “ Ram PC “ “ “ Midi button PC “
L/h 8.0 4.0 2.0 3.5 2.3 1.6 1.2 4.0
Water passageway Length
Width
Depth
mm 13 60 60 15 15 19 19 30
mm 1.39 1.39 1.25 1.22 1.04 1.00 0.91 1.20
mm 1.45 1.49 1.09 1.22 1.04 1.00 0.91 1.25
Cross section mm2
2.00 2.07 1.38 1.46 1.08 1.00 0.83 1.50
Button 2.0 53 0.90 0.80 0.72 2.0 32 0.98 1.00 0.98 Typhoon 1.75 20 0.71 0.71 0.5 * In non-compensated drippers – nominal flow rate at 1 bar (10 m) pressure head. Courtesy “Netafim” Impurities in water can be classified in four categories:
Inorganic suspended solids: sand, silt, clay and gravel.
Dissolved chemicals that precipitate from the water in certain circumstances. The most prevalent chemical precipitates are calcium carbonate, calcium phosphates and calcium sulfate (gypsum). Dissolved iron and hydrogen sulfide enhance development of bacteria population that clogs drippers, filtering media and command micro-tubes.
Live organic material: zooplankton and phytoplankton - algae, protozoa, bacteria and fungi. Live organisms can propagate rapidly in suitable conditions and excrete sticky mucus material with enormous clogging potency.
Organic debris The most contaminated waters are raw sewage and low-quality reclaimed water. Water pumped from ponds, lakes, rivers, streams, canals and dam reservoirs, also contains a high load of impurities. Water pumped from sand aquifers contains a relatively high amount of suspended sand. There are diverse filtering methods using different filtering media: screens, grooved discs, gravel and sand. Sand and silt separation is often performed as a pretreatment in settling ponds and tanks or by means of centrifugal separators. In greenhouses with detached growing media, in which drainage water is recycled for reuse in irrigation, slow sand filter systems are used to eliminate water-borne pathogens.
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Screen (Strainer) Filters There are different types of screen filters. The most important properties of a screen filter are filtration degree, filtration surface area and filtration ratio. Filtration degree is defined with two systems of units: microns and mesh number. The filtration degree in microns designates the diameter of the biggest ball-shaped particle that can pass between the screen wires. The mesh number counts the number of wires along 1" (25.4 mm) length of the screen. The two definition procedures are not fully inter-convertable. Holes width may differ in two screens with the same mesh number due to a difference in wire thickness. Rough conversion from one system to another is made using the rule of thumb: mesh number x microns = 15,000. When selecting the filtration degree, both the dimensions of the water passageways in the dripper and the character of water impurities should be Fig. 57. Screen filter considered. When the impurities are suspended inorganic solids (sand, silt, chemical precipitates), the maximum perforation diameter should be 25%-30% Table 13. Screen Perforation – Examples of the width of the emitter’s narrowest Mesh no. Hole size – Wire thickness water passageway. When the impurities microns - microns are organic and biological materials, the 40 420 250 maximum perforation diameter should be 50 300 188 no more than 10%-20% of the water 80 177 119 passageway width. Screen filters are 100 149 102 most suitable for water with inorganic 120 125 86 impurities, while high loads of organic 155 100 66 and biological impurities may quickly clog 200 74 53 the screen.
Fig. 58. Head losses in clean screen filters Adapted from "Odis" brochure One of the main disadvantages of screen filters is the fast accumulation of dirt on the screen's surface. The accumulated dirt increases the head losses and may cause the
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collapse of the screen. Excess dirt accumulation should be prevented by monitoring the pressure difference between the filter inlet and outlet and cleaning the screen when the difference is greater than 5 m head. Disc Filters Disc filters are the favored choice for filtration of water containing mixed impurities inorganic solid particles and organic debris. The casing is made of metal or plastic materials. The filtering element is made of a stack of grooved rings, tightened firmly by a screwed cap or a spring with a water-piston. Water is filtered as it flows through the grooves. The intersections of the grooves provide in-depth filtering. Coarse particles are trapped on the external surface of the stack. Finer particles and organic debris remain in the inner grooves. The disc filter has a significantly higher dirtretention capacity than screen filters. The definition of the filtration degree is identical to that of screen filters and is mostly indicated by the color of the discs.
Fig. 59. Disc filter Media Filters Media filters are used to protect the drippers when using water with a high organic load from open water bodies or reclaimed water. Widebody (0.5 - 1.25 m in diameter) media containers are made of epoxy-coated carbon steel, stainless steel or fiberglass.
Table 14. Sand particle size and mesh equivalent Sand No. Crushed Silica 12 Standard Sand 6/20 Crushed Silica 16 U.S. Silica 80 Crushed Silica 20
Effective sand size – mm 1.1. – 1.2 0.9 – 1.0 0.6 – 0.7 0.6 – 0.7 0.28
Mesh equivalent range 80 - 130 100 - 140 155 - 200 160 - 200 170 - 230
The filtering media are 1.5 - 4 size mm basalt, gravel, crushed granite particles or fine silica sand. The organic impurities adhere to the surface of the media particles. The accumulated dirt should be back-flushed routinely in order to eliminate excessive head losses. The filtration degree is defined according to that of screen and disc filters. Sand Separators High loads of sand and other solid particles should be removed before reaching the main filtration system.
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Fig. 60. Media filter
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There are two methods of sand separation. The traditional practice is based on sedimentation of solid particles from the water by slowing down its flow in settling tanks or basins. Closed tanks conserve the water head while the use of open settling basins requires repumping of the treated water into the irrigation system. Contemporary technology employs centrifugal sand separators that sediment sand and other suspended particles that are heavier than water by means of the centrifugal force created by the tangential flow of water into a conical container. The sand particles are thrown against the walls of the container by the centrifugal force, settle down and accumulate in a collecting chamber at Fig. 61. Sand separator the bottom. The collector is washed out manually or automatically when full. Clean water exits through an outlet at the top of the separator. Each type of separator (designated by the diameter of the inlet and outlet) has an optimal flow-rate range in which the most of the suspended particles are removed without excessive head-losses. At lower flow rates, more sand remains suspended in the water.
Fig. 62. Hydro-cyclone sand separator – head losses and optimal flow rates From "Odis" brochure Filter Characteristics Reliability Disc filters are regarded as highly reliable. Collapse of the filtration element is rare compared to screen filters. In screen filters, the screens are prone to tear and collapse by corrosion and pressure fluctuations. The screen-supporting skeleton has to withstand pressure surges. Capacity and Head Losses Water loses pressure as it flows through a filter. The extent of the head loss depends on the filter's design, filtering degree, flow rate and the degree of dirt accumulation. Normally, for a specific type and size of filter, the finer the filtration degree, the lower
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is the nominal discharge. This is due to higher head losses and faster dirt accumulation. Key filter properties: Diameter: Designates the water inlet and outlet diameter. Filtration area: The total surface area of the filtration element. The required filtration area for moderately dirty water is 10 - 30 cm2 for each 1 m3/h of discharge for sprinkler irrigation, 25 - 60 cm2 for micro-jets and 60 - 150 cm2 for drip irrigation. Perforation area: The total open area of the perforations. Effective filtration ratio: The ratio between the perforation area and the filtration area. Filter ratio: The ratio between the perforation area and the inlet cross-section area. The higher the above mentioned parameters, the higher the filter capacity. The nominal capacity of other types of filters is defined according to the allowed head losses. Table 15. Nominal filter capacity – examples Make Odis Arkal Arkal Arkal Amiad Amiad Odis Netafim Netafim
Filter type & diameter 2” screen 2” disc 2” disc 2” disc 3” screen 3” disc 4” screen 4” gravel 6” sand separator
Filtration grademicrons 60-400 100-400 75 25 80-300 100-250 60-400 60-200
Capacity m3/h 15-25 25 16 8 50 50 80 60-120 140-230
Nominal filter capacity relates to the flow-rate at a head loss of 2 m (0.2 bar.) in a clean filter. As dirt accumulates, head loss increases. Cleaning of the filter is required when head loss reaches 5 m (0.5 bar). A minimal head loss of 1.5 m (0.15 bar) is required for adequate sand separation by hydro-cyclone sand separators. The recommended head loss range in sand separators is 2.5 - 5 m (0.25 - 0.5 bar). When the water flow-rate changes frequently, it is recommended to install an array of sand separators in parallel, each equipped with a valve and connected to each other. The number of units activated per each operation should be adjusted to the specific discharge. Dirt accumulation capacity is lowest in screen filters, higher in disc filters and highest in media (sand and gravel) filters. Flow Direction The direction of flow through the filtration element is an important feature. In disc filters, water flows from the perimeter inwards. This pattern exposes the greater external surface area of the discs stack that is able to retain a much greater quantity of coarse particles than the smaller inner surface area could possibly do. However, in screen filters, flow from the inside - out, is more suitable for self-cleaning
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mechanisms and is less vulnerable to screen collapse by pressure surges. Some models of screen filters have two filtering elements: the preliminary coarse strainer that traps the coarse particles, and a finer screen for final filtration in the second stage. Some filter designs include nozzles that induce the tangential flow of water which drives the dirt to the distal end of the filter from where it can be flushed-out intermittently or trickle out continuously. Water enters into media filters from the top and exits from the bottom after crossing the filtering media that lies on a perforated plate. Back-flushing is accomplished in the opposite direction – from the bottom upwards. To facilitate proper back-flushing, the media fills no more than 2/3 of the height of the tank, so that it may be lifted and agitated during the back-flushing process. The tangential flow of the water in sand separators invokes centrifugal forces, which deposits the solid particles at the bottom of the separator. Clean water outlet is at the top of the separator. Operation and Maintenance Routine cleaning of the filtration elements is crucial. The best indicator for the need to clean the elements is the pressure differential between the inlet and the outlet of the filter. The pressure differential increases as dirt accumulates on the filtration element. In order to enable measurement of the pressure differential, nipples for pressure measurement are mounted on the filter's inlet and outlet. The pressure is monitored manually with a needle type pressure gauge or by connecting the two nipples to a three-way valve, on which a fixed manometer is mounted. As mentioned before, cleaning is recommended when the pressure differential reaches 0.5 bar (5 m). During manual cleaning, the integrity of the filtration elements should be inspected. Particular attention should be paid to the seals, O-rings and their mounting between filter casing and cover. The grooved rings in disc filters should be flushed with water. The tightening screw should be released to enable the water stream to separate the discs from one another. Care should be taken not to drive the dirt into the water outlet and to tighten the discs properly after cleaning the discs. Automatic Flushing and Cleaning Different mechanisms for automatic cleaning have been developed for the different types of filters. Most of the procedures rely on measuring the pressure differential and performing self-cleaning when a preset head differential has built-up. Another method is timer-controlled intervals between flushings.
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Self-flushing screen filters maintain a flow of filtered water without build-up of head loss. The dirt is continuously removed from the screen by a tangential, spiraling downward water flow, which flushes the debris into a collecting chamber at the distal end of the casing. The accumulated dirt is drained manually, continuously by a bleeder or automatically when the preset water difference between the water inlet and outlet has been built. The cleaning process lasts for a preset time. The Fig. 63. Self-flushing screen filter cleaning and flushing mechanism is powered by the inherent pressure in the system or by an electric motor. Rotating brushes or sucking nipples clean the screen. For relatively coarse screens, above 200-micron, brushes are efficient while for finer screens, under 200 microns, cleaning by rotating suckers is preferred. Automatic flushing of disc filters requires release of the disc stack. The Spin-Klin mechanism combines back-flushing by water counter-flow, release of the stack tightening screw and spinning of the separated discs by the water flow, which flushes the dirt from the grooves, through an automatically opened draining valve.
Fig. 64. Automatic flushing of disk filters Adapted from "Arkal" Brochure Media filters are fllushed automatically by back-flow from the bottom that floats the accumulated dirt out through the drain valve. The reverse flow begins automatically when the preset pressure differential has been attained. Automatic flushing of media and disc filters requires the back-flow of filtered water. To meet this requirement these filters are operated in arrays and the flushing of the filters is sequential, one after another.
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Fig. 65. High capacity media filters array
Fig. 66. Back-flushing of media filters
Fig. 67. High capcity automatic filter Adapted from "Netafim" brochure
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Fig. 68. Compact automatic filter Adapted from "Netafim" brochure
Filter Location When surface water with a high load of impurities is pumped from rivers, streams, canals or other open reservoirs, the pump site and depth of the suction pipe are crucial. The suction port should be as far upwind as possible, since floating debris and vegetation are blown downwind. The optimal pumping depth is beneath the upper layer of floating vegetation (plankton) and other debris, but not too deep, in order to verify an adequate concentration of dissolved oxygen in the pumped water. When applicable, it is recommended to pump from a pumping chamber sheltered by a coarse screen to protect the pump and avoid clogging of the suction inlet. When the pumped water contains sand or other suspended solids, a settling tank should be installed just ahead of the pump, or a sand separator just beyond the pump, in order to prevent solid particles from entering the supply network. In highly contaminated water, multi-stage filtration is required. An automatic screen, disc, media filter or a filtration array should be installed at the pumping site, and backup control screens or disc filters should be installed at the control valve at each of the irrigated sectors. With moderately contaminated deep-well water, one filtration stage at each sectorial valve may be sufficient.
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Slow Sand Filtration The new trend of recycling the drainage water in greenhouses and nurseries for reuse in irrigation, poses the hazard of infestation by soil-borne pathogens such as Pythium and Phytophthora spp. Diverse procedures have been developed to deal with this problem, like chlorination, ozone treatment, heating, ultraviolet (UV) irradiation and others. These technologies are expensive and are economically viable only at low flow rates of 5 – 10 m3/h.
Fig 69. Slow sand filter Adapted from Htinik and Krause (1999). The best cost effective solution is Slow Sand Filtration (SSF), first developed during the late eighties at the State Research Station at Geisenheim, Germany. Today, there are in use filtration systems with capacities as high as 50 m3/h. The effective flow rate is up to 100 – 200 l/h per 1 m2 of bed surface area. The efficiency of SSF depends upon the particle size distribution of the sand, the ratio of the filter’s surface area to its depth and the flow-rate of water through the filter. Fine-grade sand fractions and granulated rock-wool have been shown to be most efficient in controlling diseases such as Phytophthora, Pythium and Fusarium oxysporum, the most widespread greenhouse diseases. The SSF completely eliminates Pythiaceous fungi such as Phytophthora and Pythium. Efficiency against bacteria and fungi with small spores is high; however some propagation spores may pass the filter bed. Viruses and nematodes are not satisfactorily eliminated by slow sand filtration. Filters may be constructed in tanks with non-reactive surfaces such as plastic or fiberglass, metallic galvanized tanks or concrete tanks of various capacities, from 200 liter up to 100,000 liter. Before the water enters the SSF system, silt and organic matter should be removed by sedimentation or centrifugation and chlorination. The pre-treated water is filtered
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very slowly through a deep bed of sand. The maximum flow rate depends upon the size of the microorganisms that have to be removed. A slow flow rate of 100 l/h per m2 of surface area has been found to be preferable in high risk situations such as the control of Fusarium or viruses in tomatoes. Rates of 200 or 300 l/h per m2 are recommended for control of Pythium and Phytophthora when the risk of infestation is low to medium. The grain size of the sand is 0.15 – 0.30 mm. (100 – 50 mesh equivalent). One to 4 weeks after the beginning of filter operation, a layer of brown material ("schmutzdecke") develops in the upper 2 cm layer of the bed. Pseudomonas, Trichoderma, and other microorganisms known as bio-control agents in compost-amended container media seem to slowly destroy the Fig. 70. Slow Sand Filter scheme pathogens that were immobilized through filtration in this layer. The filters are not effective until this layer develops. The "schmutzdecke" layer has to remain intact. A layer of water one meter high is always maintained on top of the filter's bed in order to avoid destruction of the schmutzdeke – the upper active bed layer. Water can be added to the reservoir by sprinkling to minimize turbulence. The depth of the sand bed is 1 – 1.2 m. This depth allows periodic cleaning of the filter by removal of the "schmutzdecke", when flow rates drop due to clogging. Underneath the sand there are several layers of gravel and a drainage system. The water layer above the filter bed provides the head to drive the water downwards through the filter bed. It is used as a water storage zone and temperature buffer to stabilize the filter and protect the biological activity in the top layers. The top layer will never be allowed to dry. Continuous filtering supports the development and maintenance of a healthy system Some growers use rock-wool as a substitute for sand. Rock-wool is more expensive, but with rock-wool as a filtering media, gravel is not needed for drainage at the bottom of the filter. The filtered water flows by gravity into a sealed concrete reservoir at the lowest point of the filtration system. The filtered water is then pumped out of this reservoir to a storage facility or directly into the irrigation system. When the flow rate through the filter slows, due to increasing head losses, the water layer above the sand bed should be drained and the top layer of sand with the "schmutzdecke" scraped off. This layer should contain most of the organic matter and silt that were slowing the water flow. Only the 1 – 2 cm upper "schmutzdecke" layer will be scrapped for minimal removal of biomass.
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Filter for SDI "Netafim" supplies filters with disks that are impregnated with the herbicide TrIfluraline. The disks slowly release a low concentration of the herbicide into the irrigation water, preventing intrusion of roots into the drippers. Effective release lasts for 2-3 years, after which the disk stack should be replaced. Replacement disks are packed in opaque bags, as Treflan (Trifluraline) decomposes quickly when exposed to light.
Fig. 71. Treflan impregnated disk stack Courtesy "Netafim"
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Chapter 10. FERTIGATION Because of the limited soil volume that is wetted by drip irrigation frequent water applications are required. Plants absorb nutrients only from the wetted soil volume. Hence, in order to avoid the loss of nutrients by deep percolation, fertilizers have to be applied only to the wetted soil volume. The most efficient and convenient way to accomplish this is to combine water and nutrient application in the same system. This logic led to the development of fertigation technology. The combined application of water and fertilizers through the irrigation system increases yields and minimizes the hazard of environmental pollution by excess fertilization. Chronologically, the development of the fertigation technology followed the introduction and the development of drip irrigation in the early sixties. There are several methods for injecting fertilizers into the irrigation system.
Fertilizer Tank A pressure differential is created by partial throttling of water flow with a valve and diverting a fraction of the flow through a tank that contains the fertilizer solution. A gradient of 0.1 – 0.2 bar (1 – 2 m) is required to divert an adequate stream of water through a 9 – 12 mm diameter tube. The tank, made of corrosion resistant enamelcoated or galvanized cast iron, stainless steel or fiberglass, has to withstand the working pressure of the network. Solid soluble fertilizers or liquid fertilizers are mixed with the flowing water. When using solid fertilizers, the nutrient concentration remains more or less constant, as long as a portion of the solid fertilizer remains in the tank. After the full dissolution of the solid fertilizer, the concentration of the injected solution decreases gradually due to continuous dilution by the water. The system is relatively simple and cheap. There is no need for an external energy source and a wide dilution ratio can be obtained. However, there are some drawbacks. The fertilizer injection rate and nutrient concentration in the irrigation water cannot be precisely regulated. The tank has to be refilled with fertilizer before each application. Valve throttling generates pressure losses and the system is not easily compatible with automation.
Fig. 72 Fertilizer tank From "Odis" brochure
Venturi When water flows through a constricted passageway, suction is created. The high flow velocity of water in the constriction reduces the water pressure below the atmospheric pressure so that fertilizer solution from an open tank is suctioned into the constriction through a small diameter tube.
Fig. 73. Venturi injector
Venturi devices are made of corrosion-resistant materials, such as copper, brass, plastic and stainless steel. The injection rate of the
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Venturi device depends upon the pressure loss, which ranges from 10% - 75% of the system's pressure and is determined by the injector type and operating conditions. Operation of Venturi devices requires excess pressure to allow for the necessary pressure loss. Maintaining a constant pressure in the irrigation system guarantees uniform nutrient concentration over time. Common head losses are above 33% of the inlet pressure. Use of double stage Venturi injectors decreases the pressure loss to 10%. The suction rate depends on the inlet pressure, pressure loss and diameter of the water pipe. It may be adjusted by valves and regulators. The suction rate varies from 0.1 l/h to 2000 l/h. Venturi injectors are installed in-line or on a by-pass. In greenhouses, the water flow in the bypass can be boosted by an auxiliary pump. The advantages of a Venturi system are: no external energy source is required; cheap open tanks may be used; wide range of suction rates; simple operation and low wear; easy installation and mobility; compatibility with automation; uniform nutrient concentration; corrosion resistance. Limitations of the system are: significant pressure losses; injection rates affected by pressure fluctuations. Injection Pumps
Fig. 74. Piston (left) and diaphragm (right) hydraulic pumps From "Amiad" Brochure
Fig. 75. No-drain hydraulic pump From "Dosatron" brichure
Fertilizer injection pumps can be driven by electricity, internal combustion engines, tractor PTO or hydraulically by the irrigation system’s inherent water pressure. Hydraulic pumps are versatile, reliable and have low operation and maintenance costs. Diaphragm and piston hydraulic pumps are driven by the pressure of the irrigation system. Some types cast a fraction of the propelling water after its energy has been dissipated. Centrifugal pumps are used when high capacity is needed or the fertilizer solution is turbid. Roller pumps are used for the precise injection of small amounts of nutrient solution but have a short life span due to corrosion by the chemicals. Water driven diaphragm and piston pumps combine precision, reliability and low maintenance costs. Pumps used in fertigation are mostly automatically controlled. A pulse transmitter is mounted on the pump. The movement of the piston or the diaphragm sends electrical signals to the controller and indicate amount of water delivered. Measurement of the injected solution volume can also be performed by small fertilizer-meters installed on the injection tube. The controller allocates the volume of fertilizer solution according to preset program. Dosing is proportional or quantitative. With proportional dosing,
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fertilizer is injected into the irrigation water at a constant ratio. In the quantitative mode, a preset amount of fertilizer solution is injected in short pulses during the irrigation period. In glasshouses, simultaneous application of a multi-nutrient solution is routine. When the distinct chemical compounds in the fertilizers are incompatible and cannot be mixed together in a concentrated solution, due to risk of decomposition or precipitation, two or three injectors are installed in-line in the control head. The application ratio between the different injectors is coordinated and monitored by the irrigation controller.
Fig. 76. Mixer
More sophisticated device used in glasshouses of high-income crops grown on detached media and irrigated by circulating drainage water. In these circumstances, the irrigation water is mixed with fertilizers in a mixing chamber (mixer). The mixture is pumped into the irrigation system as it. Electric Pumps Electric pumps are inexpensive and reliable, operation costs are low and they are readily integrated into automatic systems. A wide range of pumps is available, from small, low-capacity pumps to massive, high-capacity pumps. Some pumps are based on an alternating displacement diaphragm. Others employ a positive-displacement piston unit, with a single-phase AC motor as the primary power source. The working pressure is 1 - 10 bars. As a standard fitting, diaphragm pumps have a separation chamber and seal that Fig. 77. Electric pump prevent the nutrient solution from flooding the motor and the electromagnetic drive if the diaphragm ruptures. Electric piston pumps are highly precise. They are suitable for accurate mixing in constant proportions of different stock solutions. Variable speed motors as well as variable stroke length allow a wide range of dosing. Capacities are 0.5 to 300 L/h and working pressure is 2 - 10 bars. Fertigation Management The timing of fertilizer application should be attuned to the irrigation schedule. Dosing is determined according to experimental and analytical results. The inherent concentration of nutrients in the irrigation water should be taken into account. Injection Site The fertilizer solution can be injected into the irrigation system at the control head of each plot. Such an assembly requires an injection device for each plot, and the total cost may be higher than that for a single central injection site. Another option for fertilizer injection is at the head of a sub-main and this is a common practice for field crops. The most convenient, and in many cases the cheapest alternative is fertilizer
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injection at the main control head. This saves labor and is more compatible with automation. Control and Automation With quantitative dosing, a measured amount of fertilizer is injected into the irrigation system during each water application. Injection may be initiated and controlled automatically or manually. Proportional dosing is based on a predetermined ratio between the irrigation water and the fertilizer solution. This is common with soil-less cultures. It can be applied in a pulsating pattern. Pulses are regulated by synchronization of signals delivered by pulse transmitters mounted on the injectors those mounted on the metering valve. The fertilizer solution meter is a combination of a small metering chamber and a magnetic affinity interrupter. Constant proportional fertilization is essential with sandy soils and soil-less cultures. Injection Timing Fertigation may be applied during only one segment of an irrigation shift. In this case, fertilizer is not applied at the beginning nor towards the end of the shift. This procedure ensures build-up of the proper pressure before irrigation commences, and flushing out of nutrient residues from the irrigation system towards the end of irrigation. Fertilizer is injected quantitatively or proportionally. Automatic Control Automation facilitates the implementation of diverse fertigation regimes in one and the same system without manual intervention. The main components of automation hardware are: (i) Solenoid: a command valve that converts electric pulses sent from an irrigation controller or a field unit into mechanical motion. The mechanical motion activates hydraulic valves. Some solenoids are AC and others DC. Common solenoids have 2 wires while “latching” solenoids may have 2 or 3 wires. (ii) Controller: the controller unit coordinates and controls the fertigation process. For proportional injection, the fertilizer solution is separated into small portions that are injected at a predetermined ratio in coordination with the pulses sent from the water meter. The controllers can be operated as stand-alone units or connected to a central computer. (iii) Normally closed hydraulic valve: a corrosion-resistant valve that controls the flow of the fertilizer solution into the irrigation system. The valve should be of the normally closed type in order to cut-off the fertilizer solution flow instantly if the control tube gets damaged. Avoiding Corrosion Damage Most fertilizer solutions are corrosive and may seriously damage metallic components. Those accessories that are exposed to the injected solution should be made of corrosion-resistant materials. Furthermore, the injection device and the irrigation system should be thoroughly flushed after each fertilizer injection. Backflow Prevention Whenever the irrigation network is connected to a potable water supply, strict precautions should be taken to avoid backflow of irrigation water containing fertilizers into the potable water network. Backflow occurs when the water supply fails. There
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are two principal methods of preventing backflow caused by back-siphonage or backpressure. Back-siphonage occurs when low pressure in the supply line is created by an excessive hydraulic gradient in undersized pipes in the supply line; a break in the supply line; pump or power failure. Back-pressure occurs when the pressure in the irrigation system is higher than in the water supply pipeline. This happens when booster pumps are used for irrigation or when the irrigated area lies topographically higher than the domestic supply source. Physically isolating the potable water network from the irrigation system can prevent backflow. Some backflow preventers protect against back-siphonage only. Others protect against both back-siphonage and back-pressure. For public safety, in many cases a double check valve assembly is required. In other cases a reduced pressure back-flow check valve is sufficient. An atmospheric vacuum breaker installed beyond the last valve, allows air to enter downstream when pressure falls. A pressure vacuum breaker has an atmospheric vent valve that is internally loaded by a spring. This valve is not suitable for fertigation systems operated by an external source of energy. Vacuum breakers are effective against back-siphonage only and cannot be used to prevent back-pressure. A double check valve assembly has two check valves in tandem, loaded by a spring or weight and installed as a unit between two valves that when closed allow the maintenance of the units. The device is effective against backflow caused by backpressure or back-siphonage. It is installed upstream of the injection system. A reduced pressure backflow preventer consists of two internally loaded check valves, separated by a reduced pressure zone - a chamber between both check valves with a third valve that opens when pressure downstream is greater than the pressure upstream – so that water is released to the atmosphere and does not flow backwards. In this reduced pressure zone the pressure is normally lower than the pressure at the inlet and higher than the pressure at the outlet. Whenever the outlet pressure level approaches the inlet pressure level, both valves close and backflow is prevented.
Fig. 78. Check valve
Fig. 79. Tandem backflow preventer - exploded
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Installation of Backflow Preventers The backflow preventer should be installed upstream, before the fertilizer injection system. It should be accessible to Fig. 80. Tandem backflow preventer inspection and at least 30 cm above ground.
Fig. 81. Installed backflow preventer
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Chapter 11. WATER QUALITY Irrigation water quality is defined by its physical, chemical and biological characteristics. The narrow water passageways in drippers are particularly sensitive to water quality. On the other hand, due to the frequent applications of water that dilute salt concentration in the active root-zone, many crop species can tolerate higher irrigation-water and soil-solution salinity within drip irrigation than in other irrigation technologies. Physical quality parameters Suspended solid mineral particles, organic matter and live zooplankton content. Chemical quality parameters Nutrition elements, precipitate-forming ions, salt content and pH level. Causes of Emitter Clogging Emitter clogging is caused by particulate matter; biological living organisms their excretions and their decomposed debris; chemical precipitates and the combined action of these factors in irrigation water. Poor system design and management increase the risk of dripper clogging. Particulate Matter Emitters can be clogged by particles of sand, limestone or other debris that are too large to pass through the narrow water passageways. Clogging can also occur when small particles stick together to form larger aggregates. Even tiny particles such as suspended clay, which would not cause problems as discrete particles, can initiate clogging if they flocculate to form larger aggregates. Biological Substances Emitters are susceptible to clogging by large particles of organic materials that block the water passageways if they are not removed by filtering, decomposition or sedimentation. Clogging can be caused by secretions from microscopic organisms such as algae and bacteria. Many types of algae are small enough to pass through filters and emitter passageways as discrete particles, but may flocculate in pipelines to form aggregates large enough to clog the emitters. Bacteria are small enough not to cause clogging; however, they can precipitate compounds of iron, sulfur and other elements that clog emitters. Some bacteria secrete slime that acts as an adhesive platform for the buildup of clay, algae and other small particle aggregates. Iron and sulfur bacterial slime is a common problem. Iron-precipitating bacteria grow in the presence of dissolved ferrous iron in irrigation water. These bacteria attach to the surface area of suspended soil particles and oxidize the dissolved iron. The oxidized iron precipitates as insoluble ferric iron. In this process, a slime called ochre is created, which combines with other substances in pipelines and clogs the emitters. Specific bacteria that oxidize hydrogen sulfide and convert it to insoluble elemental sulfur create sulfur slime. This slime, which is a white or yellow stringy deposit, is formed by oxidation of hydrogen sulfide that is present mainly in shallow wells. The slime clogs emitters either directly or by acting as an adhesive agent for other small particles.
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Chemical Precipitates Chemical clogging of emitters frequently results from precipitation of one or more of the following minerals: calcium, iron, magnesium and manganese. These minerals may precipitate from solution and form scales that partially or fully clog emitters. Precipitation can be triggered by changes in pH, temperature, pressure and reaction with ions injected into the irrigation water by fertigation, as well as from exposure to atmospheric oxygen. Table 16. Relative clogging potential of drip irrigation systems by contaminants in water Water characteristic Suspended solids (ppm) pH Total dissolved solids (ppm) Manganese (ppm) Iron (ppm) Hydrogen sulfide (ppm) Bacteria population (per ml)
Minor <50 <7.0 <500 <0.1 <0.2 <0.2 <10,000
Moderate 50-100 7.0-8.0 500-2000 0.1-1.5 0.2-1.5 0.2-2.0 10,000-50,000
Severe >100 >8.0 >2000 >1.5 >1.5 >2.0 >50,000
After Blaine Hanson. 1997
Determination of Water Quality An accurate determination of water quality is essential before selecting the filtration system. Physical Properties Dedicated instrumentation has been developed for estimating the "dirt load" in the water. A tiny hydro-cyclone sand separator is connected to the supply system in order to measure soil-particle content in water. The amount of accumulated soil particles in the separation cone indicates the degree of soil content in the water. A more sophisticated device is the Clogging Potential Meter developed by the Israeli Water Works Association. This device measures the time required to create a head loss of 3 m. (0.3 bar) due to the accumulation of particles on its screen while maintaining a constant flow of water. The time as measured is a comparative index – Clogging Potential Time – that can be used as an indicator of filtration requirements. Chemical Properties The chemical nature of water can be analyzed in a chemical laboratory or with field test kits. It is particularly important to determine the residual chlorine for efficient chlorinating. Salt content can be expressed directly as Total Dissolved Solids (TDS) in mg/l or indirectly by measuring the Electrical Conductivity (EC). An approximate conversion from EC into TDS is made with the formula: TDS (mg/l) ≅ EC (dS/m) X 640 The EC units are deciSiemens/meter (dS/m). TDS units are milligrams/liter (mg/l). Water Hardness Water containing substantial concentrations of Ca++, Mg++ and Fe++ is regarded as “hard water”. Hard water is prone to the precipitation of their carbonates as salts of low-solubility in the irrigation system that may clog drippers and filtering systems.
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Water “hardness” is expressed as calcium carbonate concentration equivalent in mg/l units. Hardness is calculated by measuring the concentrations of the above mentioned cations, summing up their concentrations expressed in meq/l and multiplying by 50 (the equivalent weight of calcium carbonate). There are additional units of hardness, such as British, German or French degrees of hardness. The most prevalent precipitate from hard water is calcium carbonate. However when fertigating with fertilizers that contain phosphorous and sulfur, calcium phosphate and calcium sulfate (gypsum) may also precipitate. Water hardness is defined by three parameters: Total hardness: the sum of the calcium, magnesium and bi-valent iron, expressed as calcium carbonate equivalent in mg/l units Transient hardness in mg/l units: the bi-carbonate (HCO3-) meq/l X 50. Permanent hardness = total hardness minus the transient hardness. When water containing calcium bi-carbonate [Ca(HCO3)2] is heated, the following chemical reaction occurs: Ca(HCO3)2 → CaCO3 ↓ + H2O + CO2↑ Soluble calcium bi-carbonate loses hydrogen and precipitates as calcium carbonate. Similar reactions occur also with magnesium bi-carbonate. Calcium carbonate deposits in the drip laterals, inside the drippers and on the filtering elements. Calcium carbonate sediments may also be created when fertilizers with alkaline reaction are injected into irrigation water containing a high concentration of calcium and bi-carbonate. The chemical reaction is: Ca++ + HCO3- + OH- → CaCO3 ↓ + H2O Transient hardness can be neutralized by adding acid to either the nutrient solution or the irrigation water. The neutralization reaction occurs as follows: Ca(HCO3)2 + H+ → Ca++ + H2O + CO2↑ In this process the bi-carbonate ion is decomposed and CO2 is released to the atmosphere. Attention has to be taken to leave 1 meq/l (61 mg/l) of bi-carbonate in the water, otherwise the buffer system of the water is impaired and the pH level may rapidly drop below 4.5 and damage the root system. One meq/l of bi-carbonate in the water maintains a pH of 6, which is favorable for most agricultural crops. By the neutralization process, transient hardness is eliminated. However, the permanent hardness is increased by the same degree. The total hardness remains constant. Total hardness can be calculated from measured concentrations of calcium and magnesium cations with the formula: Total hardness = 2.5 [Ca] + 4.1 [Mg]. Where, Total hardness is expressed in mg/l as CaCO3. Ca = calcium concentration (mg/l). Mg = magnesium concentration (mg/l). Example: Total hardness of a water sample that contains 120 mg/l Ca and 45 mg/l Mg. Total hardness = 2.5 x 120 + 4.1 x 40 = 464 mg/l as calcium carbonate
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Chloride The chloride anion Cl- is one of the most detrimental ions when its concentration in irrigation water is above 150 mg/l, for sensitive crops, and up to 400 mg/l for tolerant crops. Over 400 mg/l, the water is regarded as brackish, which decreases the yields of most agricultural crops. Sodium High sodium concentrations in irrigation water have a destructive effect on soil structure as it disperses soil aggregates. Calcium and magnesium cations in the cation exchange complex moderate the destructive capacity of sodium. Hence, the destructive nature is determined by the Exchangeable Sodium Percentage (ESP) the ratio between Sodium and the sum of the four cation species: Ca++, Mg++, Na+, K+. ESP = % [Na]/([Ca]+[Mg]+[Na]+[K]) in meq/l units There is a similar relation: SAR – Sodium Absorption Ratio. SAR =
[Na] meq/l [Ca + Mg] 2
ESP and SAR numerical values are close but not the same. When the value is below 4, agricultural crops are not damaged. In the range between 4 and 8, moderatly tolerant species are grown, and above 8 only tolerant crops can be profitably grown. Electrical Conductivity (EC) Crop sensitivity to total dissolved salts expressed in EC values was defined by Maas & Hoffman by two parameters: the threshold level above which the yield decreases, and the slope (%) of yield decrease for increase of one dS/m unit. The data in table 17 was derived from experimental work with diverse irrigation technologies but not with drip irrigation. Later it was found that salinity has a lower impact on yield with drip irrigation than depicted in the table. Many drip-irrigated crops can tolerate higher salinity with smaller impact on yield.
Table 17. Threshold and slope of salinity impact on yield Crop Threshold Slope dS/m %/dS/m Bean 1.0 19.0 Corn 1.7 12.0 Soybean 5.0 20.0 Red beet 4.0 9.0 Strawberry 1.0 33.0 Eggplant 1.1 6.9 Pepper 1.5 14.0 Potato 1.7 12.0 Cucumber 2.5 13.0 Tomato 2.5 9.9
The explanation for this phenomenon is that when the same volume of water that is applied over the full-area by conventional irrigation technologies is applies with drip irrigation, in a smaller wetted volume, the salt is leached more effectively, because of the relatively high amount of water per volume unit. The frequent wetting events maintain the EC of the soil solution closer to that of the irrigation water.
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The drawing below demonstrates the variance in salt concentration and the leaching efficiency of relatively small volume of water applied by drip irrigation.
Fig. 82. Cl- distribution below and between drippers (in ppm)
Fig. 83. Salt level in relation to distance from dripper*
Adapted from Hoffman et al. 1980
Adapted from Bresler, 1975
The salt concentration values in Fig. 83. are relative according to the formula (Cfin – C0 )/Cini , when: Cini = Initial salt concentration in soil solution. C0 – Salt concentration in irrigation water. Cfin = Salt concentration in soil solution after application of 12 l water from the dripper. The Salinity Laboratory in Riverside, California published a nomogram in which salinity and alkalinity hazards to crops are shown as a function of the total salt level, expressed as EC values in the irrigation water, and the sodium level, expressed as SAR values, also in the irrigation water. This nomogram is extensively used worldwide, but it does not relate to the different response of crops irrigated by drip irrigation, which can tolerate higher concentrations of salt. Sodium affects soil structure and in this aspect, drip irrigation has a lower impact on damage elimination. Boron A small amount of boron is essential for plant growth, however, a concentration only
Fig. 84. Water quality for irrigation Adapted from US salinity lab guide
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slightly above the optimum in the soil solution is toxic to plants. Some plants are more sensitive to boron excess than others. Iron and Manganese As mentioned before, iron is another cause of precipitates that can clog emitters. Iron is often dissolved in groundwater as ferrous bi-carbonate. When exposed to air, the iron oxidizes and precipitates. Manganese is sometimes also present in irrigation water, but at lower concentrations. Biochemical Oxygen Demand (BOD) Organic matter in the water is decomposed by microorganisms that consume oxygen in this process. The amount of oxygen consumed by these organisms in breaking down the waste is known as the Biochemical Oxygen Demand or BOD. BOD is a good indicator for dripper clogging hazard by organic matter contained in the water. A BOD test measures the amount of oxygen consumed by organisms along a specified period of time (usually 5 days at 20o C). The rate of oxygen consumption is affected by a number of variables: temperature, pH, the presence of certain species of microorganisms, and the type of organic and inorganic materials in the water. BOD directly reflects the amount of dissolved oxygen in the water. The greater the BOD, the higher the organic material content and its clogging potential. BOD analysis requires taking two samples simultaneously at each site. One sample is tested immediately for dissolved oxygen content, and the second is incubated in the dark at 20oC for 5 days and then tested for the remaining amount of dissolved oxygen. The difference in oxygen levels between the first test and the second test, in milligrams per liter (mg/l), is the BOD value. This represents the amount of oxygen required by microorganisms to break down the organic matter present in the sample bottle during the incubation period. Because of the 5-day incubation, the tests are conducted in a laboratory. An innovative procedure that shortened the BOD analysis to 24 hours was developed and approved by the American Association of Official Agricultural Chemists (AOAC).
Complementary Water Treatments In order to prevent clogging of drip irrigation systems, complementary chemical treatments should be performed on irrigation water, In addition to filtration. Oxidation and acidification are the most prevalent complementary treatments. Oxidation is used for the decomposition of organic matter and the prevention of slime formation by sulfur and iron bacteria, as well as for the elimination of pathogens infestation. Chlorination The common oxidizing agent is chlorine. Chlorine can be added to the water as solid tablets containing 90% chlorine, as liquid sodium hypochlorite (NaOCI) containing 10% chlorine or as gaseous chlorine. The gaseous form is cheap and efficient but is hazardous in application. When ferrous iron is present in the water, one ppm (part per million) of chlorine is required per each ppm of iron to kill the iron bacteria and precipitate the iron from the water. When hydrogen sulfide is present, 9 ppm of chlorine are needed per each ppm of sulfur to kill the sulfur bacteria, prevent slime growth and precipitate the sulfur. The precipitates have to be retained by the filtration system in order to prevent the clogging of the emitters by these particles.
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The main chlorinating agent is liquid sodium hypochlorite. When sodium hypochlorite is injected into the irrigation water it dissociates into hypochlorous acid and sodium hydroxide: NaOCl + H2O → HOCl + NaOH Equilibrium between the hypochlorous acid HOCl and the hypochlorite ion OCL- is maintained in the water: HOCl ⇆ H+ + OCl - . The hypochlorous acid is 80 times more potent than the hypochlorite ion as an oxidizing agent. Effective chlorinating decomposes organic material and blocks the development of algae and plankton present in the laterals and the emitters. 1 - 2 ppm of residual chlorine detected at the distal ends of the laterals guarantee adequate chlorination. To maintain these residual levels, chlorine concentrations at the point of injection should be in the range of 3 – 15 ppm, depending on the load of impurities and the duration of injection. Levels higher than 15 ppm are not recommended since they can harm the diaphragms in pressure-compensated drippers and hydraulic valves. Since the injected chlorine concentrations are very low, the use of metering pumps is preferred. When applied in extremely small amount, the chlorine should be diluted to facilitate precise and even application by the pump.
Acidification Acidification of water is required when "hard" water containing a high concentration of bi-carbonates is used for irrigation. The common acidifying agents are sulfuric, nitric and phosphoric acids. Chlorination of acidified water is considerably more effective than chlorination of alkaline water, and reduces the chlorine requirement. As mentioned before, after injecting chlorine into the irrigation water, equilibrium is created between the active form HOCl and the less active form OCl-: HOCl ⇆ H+ + OCl-. At low pH values, the active form percentage is higher: pH = 6.5; 50% at pH = 7.5 and only 20% at pH = 8.0.
90% at
It is highly recommended to implement the chemical treatments upstream from the filtration system, in order to reduce the load of impurities and trap the decomposed material in the filters. The narrower the water passages in the emitters, the greater the requirement for chemical treatments. Acidification should be performed before chlorination. Mixing both chemical agents of the two processes together results in a dangerous chemical reaction. Acid is injected to fulfill two objectives. For the continuous neutralization of the transient hardness, the concentration of acid in the irrigation water is calculated according to the bi-carbonate concentration. One meq/l (61 mg/) of bi-carbonate should be left in the acidified water. The second objective is to remove calcium carbonate precipitates. For this purpose, the final concentration of 33 % hydrochloric acid, 60% nitric acid and 85% phosphoric acid in the irrigation water should be 0.6% – 0.8%. For moderate clogging, an application time of 10 minutes is recommended, and for severe cases – 20 minutes. Acid can be applied with regular fertigation equipment or by a dedicated metering pump.
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Chapter 12. MONITORING AND CONTROL There are different levels of water and fertilizer management and control for drip irrigation systems. At the lowest level, the decisions on water and fertilizer application are based on personal experience, guessing and intuition, without performing actual measurements. More advanced management is based on soil testing with the “feel and see” method. At the third level, irrigation and fertilization are based on general practice and recommendations with a preplanned schedule for the irrigation season. At the fourth level, soil moisture and nutrient content are monitored and water and nutrients are replenished to desired thresholds. The tensiometer is the simplest and most valuable aid for monitoring the performance of drip irrigation systems. In on-surface drip irrigation, two units are installed at each control point. The ceramic tip of one tensiometer is installed within in the upper layer, 15 – 30 cm deep, in the active aerated root-zone. This tensiometer is used for taking decision about the irrigation timing. The second tensiometer is inserted into the lower limit of the active root zone or the desired wetting depth. This tensiometer indicates, 12 – 24 hours after water application, whether the amount of water did indeed replenish the water in the entire rootzone. The chemical composition of the soil solution can also be monitored using a soilmoisture extractor – a modified tensiometer. The soil solution is extracted by exerting suction with the aid of a syringe.
Fig. 85. Tensiometers
A more recent soil-moisture monitoring technology is based on measuring soil capacitance between electrodes. The capacitance increases with the increase of soil moisture. In greenhouses, drainage water is collected from the beds, and analyzed in order to compare its composition with that of the irrigation water carrying the injected Fig. 86. Soil moisture capacitance sensor nutrients. The difference between the two solutions indicates whether nutrient supply is adequate or not. At the fifth level, the aforementioned operations are followed by monitoring crop nutritional state (tissue analysis) and water status (midday leaf or stem water
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tension). Water budgeting depends on soil moisture, the water status of the plant and prevailing climate. Phyto-monitoring At the highest level of management, Soil moisture, ambient temperature and evapotranspiration, plant water status, nutrient and salt content in irrigation water, water and soil reaction are monitored simultaneously. The monitoring instruments are connected to controllers that are activated according to pre-set programs. Phyto-monitoring is the comprehensive monitoring of soil, plant and climate parameters. Large-scale field tests using the phyto-monitoring method and instrumentation were carried out from the year 1998. Experimental phytomonitoring systems were installed in apple, plum, grapes, peach, kiwi, mango, citrus, avocado and persimmon orchards. The parameters of interest are environmental factors (temperature, radiation, relative humidity); plant factors (daily trunk, shoot and fruit growth rates) and soil factors (moisture content, EC and pH). Three indicators were used for analysis: diameter maximum daily trend (DMT), daily contraction amplitude (DCA) and the midday stem water potential (WP). The DCA is defined as the difference between predawn maximum and daytime minimum trunk diameters. Fruit tree's trunk, shoot and fruit were found to be highly sensitive to soil water deficit. This makes them good indicators for irrigation scheduling With an optimal irrigation regime, there is a good correlation between stem WP, trunk/shoot DCA and vapor pressure deficit (VPD). There is no good correlation however under deficit irrigation or wherever nighttime air relative humidity is low, since Fig. 87. Multi-factor simultaneous phytomonitoring the plants’ water reserves are not completely replenished at night, resulting in a depression of the predawn maximum. When low humidity prevails over night, the DCA is calculated with a potential maximum diameter baseline (PMDB) instead of actual predawn maximum. The ratio of the modified DCA and VPD (DCA/VPD) is a good indicator of plant water status, both in properly watered and in deficit irrigated crops.
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The DMT is a good indicator of soil water availability because of its close relationship to predawn water potential.
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Chapter 13. SUBSURFACE DRIP IRRIGATION (SDI) Drip irrigation was originally conceived as subsurface irrigation. This concept was abandoned in the early sixties due to root intrusion, emitter clogging, and because it was impossible to monitor visually the wetting pattern as well as the clogging of emitters and damage to the buried laterals. Since then, most of the crucial problems have been solved and subsurface drip irrigation has gained momentum with diverse combinations of crops and soils. In subsurface drip irrigation (SDI), drip laterals are buried 10 – 50 cm below the soil surface. Advantages Avoiding Interference Farming Activities
with
Farming operations are free of the impediments due to the presence of above-ground pressurized irrigation systems. Free movement of agricultural equipment in the field, such as spraying and harvesting machines, is essential. Field operations result in less soil compaction, and irrigation Fig. 88. Scheme of SDI system induced soil crusting is reduced significantly. In annual field crops and vegetables, drip laterals do not need to be laid out and retrieved every season. Deep burying enables soil tilling whenever necessary. Protection of the Laterals Burying the laterals protects them from damage by birds, humans and machinery. Avoiding of exposure to UV radiation and extreme changes in climate conditions prolongs the laterals’ life expectancy. Water Conservation Water is applied directly in the root zone. Evaporation from the soil surface and runoff are eliminated. Since upward movement of water is driven by capillary forces against gravity, its range is limited, and with appropriate management, the soil surface remains dry. Saturated conditions due to water ponding on the soil surface with onsurface drip irrigation are also eliminated. A dry soil surface does not allow germination of annual weeds. This further contributes to water conservation and downgrades the use of herbicides. Improved Fertilization Efficiency Uptake of nutrients, particularly phosphorous, is improved, due to its direct application into the active root zone. Decreased Incidence of Moisture-Triggered Diseases The elimination of soil-surface wetting decreases occurrence of moisture-triggered leaf and fruit diseases such as Alternaria, Mildew, Botrytis and Pythium.
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Flexible Application Regime Scheduling irrigation independently from other farming operations enables the optimization of the application regime like the frequent application of small water amounts when needed in extreme weather events such as heat spells. Applicable for Advanced Water Saving Irrigation Regimes Ideal for vineyards and orchards – advanced irrigation methods, such as Regulated Deficit Irrigation (RDI) and Partial Root-zone Drying (PRD) are feasible (These technologies are dealt with in the chapter on crop irrigation). Improved Double-Cropping Opportunities Crops may be planted with optimum timing since the irrigation system need not to be removed at harvest and reinstalled prior to planting of the second crop. Extension of Low-Quality Water Utilization Wastewater can be used safely without workers or the crop coming in contact with it. Increased Yield and Improved Quality High irrigation and fertilization efficiency increases the yield and improves its quality. Disadvantages Difficulties in the Monitoring of Water Application Since water is applied underground, it is difficult to monitor and evaluate the system’s performance and application uniformity. Impaired uniformity can lead to underirrigation, resulting in reduced yield and quality or over-irrigation, resulting in poor soil aeration and losses of water and nutrients due to deep percolation. Dripper Clogging and Change of Flow Rate Within SDI, drippers may be clogged by root intrusion and by soil particles being sucked into the drippers by the vacuum created in the system when it drains after water shutdown. In heavy soils, the emitter flow-rate may exceed the soil capability to redistribute the water in a normal pattern. In such cases, water pressure around the dripper exceeds atmospheric pressure. The counter-pressure may reduce the emitter flow-rate. Incompatibility with Cracking Soils In cracking soils, water applied from drippers underground may have preferential flow to depth in the cracks and lost without notice, leaving considerable soil volume dry. Water “Surfacing” Water may inadvertently “surface” (directing a part of the emitter flow to the soil surface), causing undesired wetting of the soil surface. Due to “surfacing” events, fine soil particles may be carried upward with the water, causing a “chimney effect,” that creates a preferential flow path. The chimney may be difficult to obliterate permanently, since a portion of the chimney remains above the drip line even after tillage.
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Difficulties with the Germination of Annual Crops Using the SDI system for germination may be impossible in certain circumstances, depending on installation depth and soil characteristics. In such cases, supplementary irrigation by sprinklers or lateral move has to be given. Salt Accumulation in the Soil above the Drippers Salinity may increase above the drip line, increasing the salinity hazard for emerging seedlings and small transplants. Leaching of these accumulated salts is problematic. Limits on Soil Tillage Tillage operations are limited by the depth at which SDI laterals are buried. Restricted Root Development Certain soil conditions restrict the development of root systems with SDI more than with on-surface drip irrigation. The smaller root-zone may be insufficient to avoid diurnal crop water stress, even when the root zone is well watered. In such cases, frequent applications of nutrients may be also required, as the smaller root-zone quickly becomes depleted of nutrients. Crop Rotation Difficulties Since SDI systems are fixed spatially, it may be difficult to grow crops with different row spacing. Some crops might require exceptionally close drip lines spacing, which may not be economically viable. Particular care should be taken when planting annual row-crops, ensuring that crop orientation and spacing are appropriately matched to the location of the drip lines. Incompatible Crops Some crops may not develop properly under SDI in certain soil types and climates. Peanuts may not peg properly into dry soil. Some tree crops benefit from a larger wetting volume than is attained with SDI. High Initial Investment SDI requires a high initial investment compared with alternative irrigation systems. In certain circumstances, SDI systems have a shorter life span than other irrigation systems. Water Quality Limits For long-term SDI systems, strict water-quality management is required, due to the impossibility to identify and flush clogged drippers underground. Rodent Damage In certain regions, there is a high population of rodents in the soil that damage the tubes. In such cases, extermination of these rodents is a prerequisite for SDI installation. Expensive Maintenance Maintenance of SDI systems is more complicated and expensive than that of onsurface drip systems. Leaks caused by chewing rodents are more difficult to locate and repair, particularly with deep SDI systems. The drip laterals should be monitored for root intrusion. Roots of some perennial crops may pinch thin-wall drip tapes,
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reducing or cutting-off water flow. The drip laterals should be flushed periodically to remove silt and precipitates. Since there are only few visual indicators of system performance and its application uniformity, frequent monitoring of the system’s water-meters and pressure gauges is required to find out whether the system is operating properly. SDI Design Design of an efficient SDI system requires careful consideration, since once the system is installed underground and covered with soil, major modification and repairs are not economically feasible. When used for irrigating annual crops in rotation, compromises are required regarding dripper and lateral spacing, as well as the spacing between rows and plants in the rotating crops. Spacing Lateral spacing in heavy soils can be wider than in sandy soils. The common range of lateral spacing is from 1 m in sandy soil to 2 m in heavy soil. Spacing between emitters on the lateral depends mainly on the crop’s plant density, and vary from 10 to 100 cm. Lateral and Dripper Type The drippers of choice are those that are less sensitive to clogging and root intrusion. Compensated drippers are less prone to flow-rate decrease by build-up of counterpressure around the dripper. Trifluraline (Treflan) impregnated drippers and those equipped with a flap that closes over the outlet after water shutdown, prevent root intrusion. Drippers with anti-siphonage capabilities are less prone to suction of soil particles. Laterals may be made of thick or thin walled PE tubing. In the past, after installation of thin-wall tapes, the loosened upper soil layer sometimes dried around the flat tape after water drain and prevented inflation of the tape in the next irrigation. This problem has been solved by installation of vacuum breakers and the anti-siphonage capabilities of the new types of drippers. Lateral Depth Deep installation, 50 – 60 cm, below soil surface, enables regular soil tillage. This is common in crops with deep active root systems, such as cotton. Irrigation for germination should be applied by other irrigation systems, such as mechanized or hand-move sprinkler laterals. Shallow installation, 5 – 15 cm deep, is
Sand
Clay
Fig. 89. Wetting pattern in SDI
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practiced with shallow-rooted crops, such as strawberry and potatoes. The system can be used for germinating when the dripper's spacing along the lateral is between 10 and 30 cm. Whenever germination irrigations are not required, the installation depth range is 20 – 45 cm. In sandy soils, installation should be shallower than in heavy soils, since the capillary water ascent is limited. For perennial crops, installation can be shallow as there is no interference with soil tillage provided the soil surface is kept dry. Recent Developments During the last two decades, most of the problems that plagued SDI systems and their maintenance had been solved. Suctioning of soil particles into the drippers at system shutdown is eliminated by installing air-relief valves at the highest topographic points, and vacuum breakers on the laterals. Root intrusion is eliminated by treatment with the herbicide Trifluraline (Treflan). This herbicide, when applied through the drip system, sterilizes the soil in the vicinity of the dripper, in a radius of some 2 – 3 cm around the water outlet. The herbicide does not kill the roots, however it stops their elongation. It remains active for 3 – 6 months after application. One kg/ha/season, divided into 2 – 4 applications along the irrigation season, eliminates root intrusion into the drippers. Some manufacturers have developed Treflan-impregnated drippers that continuously release a small amount of Treflan. This technology is based on the principle of longterm controlled-release by means of a Polymeric Carrier Delivery (PCD) system. The PCD system is the chemical’s reservoir, protecting it from degradation. The bioactive chemical is released slowly to the soil next to the dripper. The declared life-span of the impregnated herbicide is 10 – 20 years. As mentioned before, "Netafim" offers disc filters in which the disc stack is impregnated with Treflan. The anti-intrusion lifespan of the discs is 2 – 3 years. Another technical innovation of SDI installations is a flush-line manifold connected to the lateral's distal ends that is used for routine flushing of the drip system. Occasionally, the flush-line is used as an additional distributing line that allows the installation of longer laterals that are supplied with water from both ends of the lateral. When flushing is required, the water supply to the flush line is closed and the drainage valves are opened. Machinery for mechanized installation of SDI systems has been developed. These machines facilitate fast and relatively cheap installation. After burying the laterals in the soil, the system should be tested under pressure. Leakages and collapse of laterals are easily identified and can be repaired with minimal disturbance.
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Fig. 90. Burying SDI lateral
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Fig. 91. Three-shank SDI lateral burying machine
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Chapter 14. FAMILY DRIP IRRIGATION In developing countries, irrigation is practiced on a wide-scale for growing fruit and vegetables in family-owned gardens, some of which are not larger than 20 – 500 m2. These gardens are mostly furrow-irrigated with water drawn from shallow wells, rivers, lakes and reservoirs; either by hand, animals or by small motor-driven pumps. Experimentally, productivity has been impressively improved by replacing furrow irrigation with drip irrigation. Conventional drip technology is not suitable for these small gardens. It is expensive and out of the reach of small producers. Low-pressure drip irrigation systems for these small holders were developed by Mr. Chapin of "Watermatics", USA, Mr. Rosenberg of "Ein Tal" and by "Netafim", Israel. Mr. Chapin invented the Bucket-kit technique. The kit is composed of an ordinary bucket, a filter, delivery tubes and two 15-m long drip laterals. The bucket is hung one meter above ground, and the two laterals deliver water by gravity to the vegetable garden. Two 40-l buckets of water meet the daily consumption of a family garden. The bucket-kits cost six dollars each to non-profit organizations. They are distributed by NGOs in over 100 countries. A larger capacity bucket kit was developed later on. The Super Bucket Kit provides drip irrigation for 10 rows, 10 meters long each, covering 100 m2. This area can be watered from a 250-l container filled once a day.
Fig. 92. Bucket kit
Fig. 93. Drum kit
A modification of the bucket kit technology was developed by IDE: A five-dollar starter kit irrigates 25 m2 of vegetables. The same system with twice as many laterals irrigates 50 m2. A $25 drum kit utilizes a 200 - 300-l drum tank for the irrigation of 125 m2 and a simplified irrigation system for irrigating 0.4 ha. was also developed Shift able drip systems reduce capital cost by using more labor. Each lateral is capable of irrigating up to ten rows. Water drips out of baffled holes or curled microtubes, instead of expensive emitters. Larger low-cost drip systems for irrigation of 1000 to 10,000 m2 for crops like cotton cost $625/ha. One lateral can irrigate four rows with attached micro-tubes.
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"Netafim International" developed two models of gravity-pressurized, drip systems for irrigating 400 m2 of vegetables. Both types include a tank, filter, valves, main line, manifold and laterals. The first model has a 50 m main line and fifty 7.5 m lateral tubes. It is suitable for greenhouse or low-tunnel cropping. The second model has a 9 m main line and nine 20 m lateral tubes on both sides. The tubes are heavyduty, durable, small-diameter of 5–9 mm OD. The system costs $150 per 1,000 m2. Complementary to these systems, a human-powered treadle pump that costs only $30 (compared to $300 for a diesel pump) was developed. There are two treadle pump models: one is powered by walking on two bamboo treadles, while the other is comprised of steel treadles connected to concrete platforms and tubes. The treadles activate two steel pistons that may be manufactured in any village workshop
Fig. 94. "Netafim" Family Drip System (FDS)
. Fig. 95. Components of Family Drip System (FDS)
Fig. 96. Treadle pump
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Chapter 15. WATER DISTRIBUTION UNIFORMITY One of the parameters for the evaluation of irrigation excellence is the Irrigation Efficiency (IE), which is defined as: water beneficially used total applied water Water beneficially used includes replenishment of evapo-transpiration consumption, frost protection, pesticide and fertilizer applications, salt leaching, and crop cooling. IE =
Drip irrigation facilitates the application of the same volume of water to every single plant in the irrigated plot. This requires the suitable spacing between laterals and drippers as well as the appropriate pressure regime. Application uniformity can be expressed with various indices. A uniformity of 100% means that each point within the plot receives exactly the same amount of irrigation water. When uniformity is low, some sections of the plot receive less water than others. In order that those areas that obtain less water will receive a sufficient amount of water, additional water has to be applied to the plot as a whole. Application uniformity is particularly important with drip irrigation systems, due to the cumulative nature of non-uniformity. A common index of application uniformity is Distribution Uniformity (DU). To calculate this value, the flow-rate of a representative sample (40 - 100 emitters randomly selected in different sections of the irrigated plot) is measured.
Where: Q25% is the average flow rate of 25% of the emitters with the lowest flow rate, and Qn is the average flow rate of all the sampled emitters. DU values above 87% indicate excellent distribution uniformity; 75% - 87% - good uniformity; 62% - 75% acceptable and below 62% the uniformity is unacceptable. Variation in the flow rate depends on the pressure regime, partial emitter clogging and the manufacturing variance of the drippers. In addition to variation of the flow-rate between emitters due to pressure differences, flow rate variation also occurs because of manufacturing variation. No two emitters can be identically manufactured; there will be some variation among emitters. The flow rate uniformity of new drippers is evaluated with the Manufacturing Coefficient of Variation (Cvm). The flow-rate variation due to manufacturing variance is determined statistically. Randomly-selected emitter samples or a lateral section are tested under constant pressure. The Cvm is defined as the standard deviation divided by the average flow rate of a sample of emitters. It is expressed as a decimal fraction or percentage. 0.01 = 1%. According to the formula: Cvm = Sdm/Xm (as decimal fraction). Multiplication by 100 gives the % expression. Where:
Cvm = manufacturing coefficient of variation, Sdm = standard deviation,
Xm = mean flow rate. A Cvm of 0.1 (10%) means, assuming a normal distribution (a “bell shaped” curve), where 68 %of all dripper flow rates are plus or minus within 10% of the mean flow
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rate. The emitter's design, material used in its production, and manufacturing precision determines the variation for any particular emitter type. A Cvm of 0.05 or less is considered excellent, 0.05 - 0.10 is good, 0.10 - 0.15 is marginal, and higher than 0.15 is poor. With the recent improvements in manufacturing tolerances, most emitters have Cvm < 0.10. Pressure compensating drippers have a somewhat greater Cvm than non-compensating labyrinth path emitters. Another expression used for the evaluation and design of drip systems is the emitter flow variation in the lateral. This compares maximum and minimum emitter flow rates. qvar=(qmax - qmin)/qmax or qvar= 1-(qmin/qmax) Where qmax is the maximum emitter flow rate, qmin is the minimum emitter flow rate, and qvar is the emitter flow rate variation. It is assumed that the manufacturer's emitter flow variation follows a normal distribution so that the mean value plus two standard deviations is considered as the maximum flow rate and the mean value minus two standard deviations is considered as the minimum emitter flow rate. This range covers over 95% of the emitter flow-rates measured in the tests. Relating test results to the manufacturers Cvm indicates that with a manufacturing Cvm of 0.05 = 5%, the difference between maximum and minimum flow rates may reach 15%. Emission Uniformity (EU) combines variation due to emitter-manufacturer variance and variation due to pressure. This is a design parameter. In new installations or when there is no emitter clogging, EU (design) is approximately equal to DU. EU is given as: EU = [(1 -1.27(Cvm)) X (Qmin/Qavg) X100] Where, Cvm = manufacturer’s coefficient of variation Qmin = minimum emitter pressure dependent discharge. Qavg = mean emitter pressure dependent discharge. The actual uniformity of discharge from drippers decreases along time due to partial clogging of the water passageways, deformation of drippers and compensating membranes, as well as mechanical damage to laterals. Routine periodical inspection and corrective measures are requested to guarantee uniform water distribution within an irrigated plot for the long run.
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Chapter 16. DRIP IRRIGATION OF CROPS Drip irrigation is suitable for most agricultural crops as well as for many gardening and landscaping applications. Use of detached media for hydroponics led to the expansion of drip irrigation to the protected cropping industry.
Table 18. Yield increase and water savings in conversion from surface to drip irrigation
Impressive water saving and yield increase have been achieved all over the world after converting from traditional surface irrigation to drip irrigation. Table 17 presents data from the state of Maharashtra in India. The figures relate to the third year after conversion from surface to drip irrigation.
Banana Grapes Sweet lime Pomegranate Sugarcane Tomato Watermelon Cotton Cabbage Papaya Radish Beet Chillies Sweet potato
Crops respond in different ways to drip irrigation. Perennial crops growers benefit from the localized water application that reduces interference of irrigation with other farming activities, as well as from the frequent application of water and nutrients and the decreased hazard of salinity damage in the root zone.
Crop
Yield increase
Water saving
% 52 23 50 98 33 50 88 27 2 75 2 7 44 39
% 45 48 61 45 56 39 36 53 60 68 77 79 62 60
After Desai & Sudhakar, 1993
Annual crops benefit from substantial water saving during the early stages of crop development as well as from the frequent application of water and nutrients. Another benefit is the decreased incidence of plant diseases enhanced by high ambient humidity.
Drip Irrigation in Orchards
Fig. 97. Apple root system in well aerated soil The introduction of drip irrigation to orchards, significantly changed the water and nutrient application regime. Prior to drip irrigation, the irrigation policy in orchards was to stretch the intervals between irrigations as much as possible. The prevalent concept was that stretched intervals induce the development of a deep root-system and the trees would better withstand water Fig. 98. Apple root system in compacted soil stress without damage to yield and fruit Adapted from Tamasi 1986 quality. Intervals of 20-30 days between irrigations were common and large volume of water had been applied at each application. After the introduction of drip irrigation, it was found that most fruit trees respond positively to frequent applications of small amounts of water. Root exposure
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revealed that contradicting the traditional perception, in certain soil conditions, like shallow topsoil, stratified soil, poor soil aeration and high water tables the crop benefits from frequent irrigations because of the shallow root system is shallow. There are different types of drippers' layout in orchards. For heavy and medium textured soils, one drip lateral along the row is usually sufficient. On sandy and shallow soils, two laterals, 20 – 60 cm apart on each side of the row, perform better. There are additional layouts such as loops and half circles around the trunk, star layout, meander, “snake” and fishbone layouts as shown in the figure. The most pronounced water savings in orchards with drip irrigation is during the early years, prior to fruit bearing. Some types of laterals allow for opening Fig. 99. Drip irrigation Layouts in orchards only the water emitters adjacent to the tree in the early years of the orchard establishment. The plugged outlets between the trees are gradually unplugged, matched to the expansion of the root system. Some fruit crops particularly benefit from drip irrigation, while others are better compatible with spray and microsprinkler irrigation. The shift from surface to drip irrigation of bananas led to water savings in the order of 50% - 70%. One lateral per row is sufficient. Drip irrigation and fertigation of table and wine grapes improved yield and quality, followed by impressive water savings. To avoid the damage to on- Fig. 100. Drip laterals in vineyard, hung on the trellis wire surface laterals, it is common to hang the laterals 30 cm above ground by fastening them with a clip to the lowest wire on the trellis.
Fig. 101. Dripper layouts in pecan orchard
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The introduction of mechanized harvesting and pruning machinery triggered the shift to SDI. Burying the laterals avoids damage by the machinary. For wide-spaced orchards, such as pecan plantations that are planted 10 by 10 to 15 by 15 m spacing, one lateral per row is mostly insufficient to satisfy the tree's water requirements. Two laterals per row or loops around the trunks are the recommended layouts. When considering implementation of drip irrigation in citrus and other evergreen subtropical crops such as avocado and mango, it should be taken into account that drip irrigation is not suitable to modify the micro-climate in the plot while sprinkler and sprayer systems can reduce or avoid the effect of extreme weather conditions, such as heat spells or frost by decreasing or increasing the ambient temperature. In deciduous fruits orchards, such as peaches, prunes, apples, pears and vineyards, drip irrigation facilitates implementation of the innovative RDI and PRD water-saving regimes. RDI (Regulated Deficit Irrigation) is based on the fact that some of the phenological growth stages during the plant’s life cycle are less sensitive to stress due to water deficit.
Fig. 102. Typical shoot and fruit growth curves for peach (left) and pear (right) For example, fruit growth of peaches is slow during the pit hardening stage that lasts 45 – 60 days. Full deficit replenishment of the transpired water during this period only slightly affects the final fruit size at harvest and does not reduce yield. Water saving of up to 20% of the annual application may be obtained. The less insensitive stage of pears is shorter and water saving of some 10% - 15% of the annual consumption is common with RDI. Similar results were achieved in prunes, table and wine grapes and almonds. Water deficit is deliberately created in wine grapes during the post-set period of berry development, to restrict vegetative growth. This practice has resulted in significant improvement of wine quality. RDI can be applied also with other micro-irrigation methods, such as sprayers and micro-sprinklers; however water savings will be less significant.
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PRD (Partial Root-zone Drying) was developed in Australia. The concept is to alternately wet one side of the root system using two laterals per row. This allows maintaining the vines at a low level of stress, with only half or slightly more of the volume of water applied when wetting the entire rootzone.
Fig. 103. Partial Root-zone Drying with two laterals per row
PRD is based on the manipulation of transpiration control mechanisms. Approximately one half of the root system is maintained in a dry or drying state, while the remainder of the root-zone is kept wet. For wine grapes, the wetted and dried sides are alternated in a 10 – 14 days cycle. PRD saves more water than RDI – up to 40% in wine and table grapes – and further improves wine quality of certain varieties. Nutrition Ditches Another contribution of drip irrigation to overcome orchard management problems due to harsh soil conditions such as compactness and alkalinity is the implementation of drip irrigation in nutrition ditches. This technology implies excavating, one or two ditches, 30 – 50 cm deep and 20 cm wide at some distance from the row of trees. The sides of the ditches are padded with a Fig. 104. Mango grown on nutrition ditches (right) vs. control (left) geo-chemical fabric, in order to protect them from destruction. The ditches are filled with aggregates of volcanic tuff, pumice, gravel or perlite. A drip lateral is laid over each ditch. During the irrigation season, water and nutrients are applied in pulses, several times a day with a regime similar to that of greenhouse irrigation. The most impressive results were achieved in mango plantations. Yields increased from 5 – 10 t/ha. to 40 – 50 t/ha. Peaches, nectarines and table grapes also responded positively but in a lower extent.
Drip Irrigation in Field and Fodder Crops Drip irrigation does not compete with mechanized irrigation, which is cheaper when applied to large, rectangular and flat plots of field crops. Drip irrigation is the recommended technology for small plots, plots of irregular form or where topographic conditions are particularly harsh.
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SDI is widely used in field-crop irrigation when mechanized irrigation is not applicable. The required precision in sowing and planting in the same rows every year is supported by the use of GPS instrumentation on the farm machinery. For on-surface drip irrigation, drip laterals are laid out at the beginning of the irrigation season and retrieved pre-harvest to avoid damage to the equipment. The spacing between rows and between plants in the row is not the same for all field crops. Lateral and dripper spacing should conform to the crop spacing. Row spacing is modified for some crops in order to reduce the number of laterals. For example, instead of a uniform spacing of 1 m between rows, the rows are paired at 80 cm with 1.20 m between adjacent row pairs. This allows for the installation of one lateral between each pair, (at 40 cm from each row), instead of one lateral per row. This layout decreases total lateral length in the plot by 50%. With SDI, there are always dilemmas regarding the spacing and installation depth of the laterals. For deep-rooted crops such as cotton Fig. 105. Mechanized deployment of drip laterals growing on heavy soil, the spacing From "Naan" brochure between laterals can be twice the spacing between rows, approximately 2 m. This requires germinating the crop with a separate irrigation system, such as self-propelled irrigation machines. For crops with shallow root systems, the greatest spacing between laterals is 1 m and installation depth is 30 – 40 cm. This limits tillage options.
Drip Irrigation of Cotton Drip irrigation in cotton is applied mainly on shallow soils, small or irregular plots and on steep slopes. A substantial part of drip irrigated cotton employs on-surface retrievable laterals that are laid out after planting and collected pre-harvest. Machinery developed for this technology enables laying out up to 8 rows at one Fig. 106. Cotton root development pass. SDI systems are used mainly on heavy compacted soils, in order to eliminate traffic of laying/retrieving machinery on wet soil. Since cotton is a non-edible crop and is relatively resistant to salinity, vast area of cotton is irrigated with low-quality reclaimed and brackish water. Reclaimed water requires the use of high-quality filtration systems.
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Drip Irrigation of Tomatoes for the Processing Industry Drip irrigation has many advantages in irrigation of tomatoes for processing. The capacity to optimize water and nutrient supply according to climatic conditions, phenological stages, yield potential and planned harvest date, increases yield and maximizes the fruit’s dry matter and sugar content. Tomatoes for processing are sown at spacing of 1.5 – 2 m between rows, with one or two laterals per row, depending on soil texture, depth and stratification, SDI installation eliminates the annoying task of laterals retrieval under the sprawling plants. Improved quality was also reported as a result of better nutrient utilization and elimination of soil-surface wetting.
Drip Irrigation of Potatoes
Fig. 107. Laterals on top of hillocks in potatoes Fig. 108. Lateral between hillocks After Kremmer & Kenig Drip irrigation of potatoes was controversial for many years. Growers argued that drip irrigation triggered the development of malformed bulbs. Shallow burying of the lateral, 5 – 15 cm deep with small spacing, 10 – 20 cm between drippers along the lateral, eliminates this problem by wetting a continuous strip along the row. Installing one lateral on each raised bed is essential for potatoes on sandy and medium textured soils. When the lateral is laid on the raised bed, it should be placed in a shallow groove to maintain Fig. 109. Potatoes – one lateral per row Courtesy "Netafim" its position on the top and to decrease runoff along the bed's slope. On heavy soils, the lateral can be laid between two beds so that the root-zone remains well aerated.
Drip Irrigation of Corn Corn appears to be one of the most responsive crops to drip irrigation. The capacity of managing the optimal supply of nutrients contributes to the development of bigger cobs that increases yield. Laterals are laid one per row or between paired rows, depending on soil type.
Drip Irrigation of Alfalfa Alfalfa was one of the last crops that were adapted to drip irrigation. Due to the high plant density, it was regarded as not justified economically. The use of reclaimed water for the irrigation of alfalfa obliged the installation of SDI in order to avoid
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contamination of the crop with pathogens. The deep root system of alfalfa allows for a spacing of 1 – 1.2 m between laterals, without any decrease in yield.
Drip Irrigation of Vegetables Most vegetables grown both in the open field and within protecting structures respond positively to drip irrigation and fertigation. Consumption curves of water and nutrients were elaborated for many species. Drip irrigation facilitates supply of water and nutrient following these curves. The predominant technology for open field culture is on-surface seasonally-retrievable drip irrigation. SDI is seldom installed. In California, the pioneer of SDI for vegetables, there has been recently return to traditional drip irrigation with the drip laterals on the soil surface; due to problems during the germination and emergence of the crops grown with SDI. Yield decrease in long-term SDI irrigation in some vegetable species had been reported.
Drip Irrigation of Open-Field Tomatoes, Pepper and Eggplants Drip irrigation of the main species of the Solanaceae family (in addition to the before mentioned potatoes and tomatoes for processing), has greatly increased worldwide during the last three decades. Water saving and decrease in fungal disease occurrence, compared with sprinkler irrigation; improved nutrient supply, compared with furrow irrigation; and increased yields, convinced farmers to expand the drip irrigated area of these crops. The prevalent layout is one lateral per row. The soil type determines the spacing between drippers on the lateral, ranging from 10 cm with thin-wall tapes and in sandy soils to 50 cm on heavy soils.
Drip Irrigation of Strawberry Strawberries are usually grown on four-row raised beds with plastic mulch in open fields. The spacing between the rows is 20 – 30, and between plants in the rows 15 – 30 cm. The common layout is one lateral between each pair of rows and spacing of 10 – 30 cm between drippers along the lateral. The laterals are installed beneath the plastic mulch in order to decrease incidence of Botrytis, which is enhanced by direct contact of the berries with wet soil.
Drip Irrigation of Cucumbers, Melons and Watermelons The wide spacing between rows (1 – 2 m) of crops of the Cucurbitaceae family, results in enormous water saving during the early growth stages before the foliage fully covers the soil surface between rows. The common layout of one lateral per row renders a relatively cheap system.
Drip Irrigation of Celery Celery is grown on 4 row beds, 1.5 – 2 wide. Laterals are laid in the middle of each pair of rows. Drippers are spaced 20 – 30 cm apart along the lateral.
Drip Irrigation of Cabbage and Lettuce Cabbage and lettuce are grown on 4 row beds, 1.5 – 2 m wide. Laterals are laid in the middle of each row pair with drippers spaced 20 cm along the lateral.
Drip Irrigation of Cauliflower Cauliflower is grown on a double-row bed, 1.2 -1.8 wide. On heavy and medium textured soils, the layout is one lateral per bed, in the middle, between the two rows.
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On sandy soils, one lateral per row is the preferred layout. Dripper spacing along the lateral is 20 – 30 cm.
Drip Irrigation in Protected Crops Protected crops are grown at diverse levels of environmental protection. The highest level is a glasshouse structure, with full environmental, irrigation and nutritional regulation. The second degree of protection consists of plastic covered greenhouses and walk-in tunnels. The greenhouse sector can be divided into those growing the crop on the native soil and those using diverse types of detached media. In some of these structures, full environmental control is also maintained. However, in most of them, only irrigation and nutrition are automatically controlled. A lower degree of control is maintained in low tunnels that provide only partial environmental control, however irrigation and plant nutrition may be fully controlled. The lowest degree of protection is plastic mulch that covers the soil to preserve water, reduce temperature fluctuations within the root-zone and eliminate direct contact of the fruit and foliage with the soil and with the irrigation water.
Fig, 110. Wide-scale drip irrigation in greenhouses Courtesy "Netafim" Apart from specific circumstances in which the relative humidity within the protected structure must be increased by spray, fogger or sprinkling emitters, most of the protected cropping area is irrigated by drip irrigation. For crops grown on native soil, the drip system layout is similar to that implemented in the open field. The only difference is that protected crops are grown mostly on coarse textured soils, that may be imported from an external source if the local native soil is of fine texture. Coarser soils require narrower spacing between laterals and drippers as well as shorter intervals between irrigations. Most of the detached beds have a low water-retention capacity requiring frequent water applications and denser layout of laterals and drippers. In pot plants, multioutlet drippers are used, as well as dedicated drippers such as the arrow dripper. Many of the detached beds are fully or partially inert materials, therefore complete fertilization is required, including all the 15 plant nutrition elements. Some of these elements cannot be mixed together in their concentrated forms, and 2 – 4 separate fertilizer tanks are required, each with its own injector. More sophisticated systems employ the mixing tank in which 2 – 4 different nutrient solutions are mixed and
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injected into the irrigation system. In some mixers the nutrient mixture is diluted with water up to the desirable final nutrient concentration and pumped into the irrigation system as it. Environmentally controlled greenhouses are expensive. Therefore, in order to maximize income, the available space is filled to the maximum: potted plants, propagation beds, grafts and trays for germinating transplants are arranged in several horizontal layers, on separate stories, one above another. Multi-outlet drippers are the most economical irrigation solution for this arrangement.
Fig. 111. Drip irrigation of potted plants in Greenhouses that recycle drainage greenhouse Courtesy "Netafim" water for reuse in irrigation require a sterilization system like ultra violet (UV) irradiation; heating the recycled water to high temperature or slow sand filters (SSF). These means are required to prevent infestation by pests such as fungi, bacteria, nematodes and viruses that may exist in the recycled drainage water. All these systems are monitored with and controlled by diverse sensors and computerized controllers.
Drip Irrigation in Landscaping Drip irrigation has been extended to the irrigation of private and public landscaping. Small-scale private landscape installations may use adjustable drippers to facilitate concurrent irrigation of plants with different water requirements. Adjustable drippers are also useful when water requirements of the plants change during the irrigation season.
Fig. 112. Roadside drip irrigation Courtesy "Netafim" SDI is used extensively on turf and golf courses as well as in sports facilities such as football fields and tennis courts. Dripper density in turf grounds is much higher than for agriculture. Spacing of 40 – 50 cm between laterals is prevalent in sandy turf and sport grounds with coarse aggregates – volcanic tuff, pumice, gravel and perlite infrastructure. Drip irrigation provides the optimal solution for roadside irrigation, along pavements and at interchanges. In addition to substantial saving in irrigation water, it eliminates the danger to traffic stemming from wet roads and walking lanes. Frequently, the water supply to gardens is connected to the drinking-water supply system. This obliges the installation of backflow preventers. In many countries, installation and management of backflow preventers are enforced by state or local authority regulations.
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Chapter 17. BASICS OF DRIP SYSTEM DESIGN When designing new drip irrigation system, a number of parameters should be taken into account in order to engineer an optimal and durable system. The first step should compare the crop water requirements with the available annual water supply capacity. The system's discharge capacity should be compared to peak season demand. The pre-design data can be divided into a number of categories: climate, cropping technologies, soil properties, topography, water-supply capacity and quality, available equipment. Climate and Crop Data 1. Peak season maximum daily evaporation (mm/d). Measured with a class A pan or calculated from climatic data. 2. Net daily crop water requirement (mm/d) = daily evaporation X crop coefficient – mm (multiplied by 10 = net daily crop water requirement m3/ha/d) 3. Gross daily water requirement (mm/d) = Net daily crop water requirement divided by the application efficiency (expressed as percentage or decimal fraction) 4. Net daily water requirement per irrigated area (mm/d or m3/d) = Net daily requirement X irrigated area (ha.). 5. Gross hourly water requirement = gross daily requirement divided by the number of water supply hours. Supply hours never exceed 20 hours per day. The extra hours are set aside for maintenance. Example: Max daily evaporation during the irrigation season: 8 mm Crop coefficient: 0.7 (70%) The daily irrigated area: 30 ha Application efficiency: 80% Available water supply hours per day: 14h Net daily crop water requirement: 8 mm/d X 0.7 = 5.6 mm/d (56 m3/ha/d) Gross daily crop water requirement: 56 m3/ha/d /80% = 70 m3/ha/d Gross daily water requirement per irrigated area: 70 m3/ha/d X 30 ha = 2100 m3/d Hourly water demand: 2100 m3/d /14 h/d = 150 m3/h Cropping Data 1. Crop rotation data 2. Length of growing season 3. Spacing between rows and between plants along the row 4. Depth of root-zone 5. Water and nutrient consumption curves, (required for the design of the fertigation system)
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Soil Properties 1. Soil depth 2. Soil texture and structure 3. Specific gravity 4. Bulk density 5. Saturation Percentage, Field Capacity, Wilting Point 6. Presence of stratified layers 7. Infiltration rate and hydraulic conductivity data, if available 8. Soil salinity Topography Topographic maps Water Supply Capacity 1. Water source properties (river, dam, pond, well, public/commercial supply) 2. Hours of supply (if by external supplier or limitations in the electricity supply) 3. Maximum hourly flow-rate (discharge) 4. Pressure at supply connection (if by external supplier) 5. Water quality (physical contamination, salinity) Existing Equipment Existence of pumping equipment, delivery and distribution pipelines, emitters, accessories, etc. Preliminary Considerations Preliminary considerations include the selection of dripper type and flow-rate and the recommended working pressure. This has to conform to the crop's spacing and with soil properties. The pattern of the wetted soil volume by a single dripper is an important factor in these considerations. The wetting pattern is determined by the dripper discharge, infiltration rate of the soil (expressed in mm/h) and its hydraulic conductivity (expressed as mm/s). The difficulty with the last two parameters is that the first decreases along time during the irrigation and the second is measured in saturated soil, while in drip irrigation, there is water movement also in unsaturated soil.
Fig. 113. Wetted volume in different soil types
Models for estimating the wetting pattern were developed by Schwarzman and Zur (1986) and Shani (1987). In the first model, the wetted-volume diameter depends on the dripper flow rate.
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D = K X 1.32z0.35 X q0.33 X Ks-0.33 When: K = Empirical coefficient (29.2) D = wetted diameter, m z = desired wetting depth, m q = dripper flow rate, l/h Ks = saturated hydraulic conductivity, mm/s If the irrigated plot is uniform – one and the same crop at the same phenological stage – water distribution should be as uniform as possible. For drip-irrigation system design, the accepted maximum allowed difference in the flow-rate between emitters in the plot is 10%, namely 5% above and 5% below the average. Calculation of head losses when water flows in pipes is the primary step in the design of the system. There are different formulae for calculating head losses. Designers routinely use tables, nomograms, on-line calculators and dedicated software for irrigation design. When designing drip systems, it is recommended to analyze several alternatives, comparing initial investment cost, labor and energy expenses. Example Design of drip system in apple orchard: Crop Data Crop: Apple Variety: Golden Delicious Area: 10.4 ha. Spacing 4 X 4 m Irrigation season: Apr - Oct Harvest: Sept – Oct Active root depth: 80 cm Maximum allowed water depletion: 40% Highest crop coefficient: 0.9 a
Soil data Texture: Loamy clay Depth: 1.20 – 1.50 m Bulk density: 1.4 Fig. 114. Apple orchard – 7.68 Ha Field capacity: 32% V/V . Wilting point: 15% V/V Available water: 17% V/V Wetted soil volume diameter by a single dripper: 80 cm (on the spot test) Climatic data Peak season daily class A pan evaporation: 8 mm Water supply data Maximum supply hours: 14 hours a day Maximum available hourly discharge: 100 m3/h EC water: 1.2 dS/m Chloride content: 150 mg/l
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Calculation of Peak Season Water Demand 8 mm/d X 7.68 ha. = 61.44 mm/d = 614 m3/d (1 mm = 10 m3/ha.) Average hourly demand: 614/14 = 43.9 m3/h. The demand complies with supply limits. The initial examined alternatives are "Netafim" in-line non-compensated 122 dripper (OD = 12 mm) with flow-rate of 2.1 l/h at 10 m head and 2.6 l/h at 15 m head, spaced 1 m apart on the lateral. A second alternative is Ram 16 compensated dripper (OD = 16 mm) with flow-rate of 1.2 l/h, spaced 50 cm on the lateral. Dripper manufacturers publish data about the maximum allowed length for drip laterals on flat land, keeping drippers' flow-rate variation due to friction in laterals within 10% (+/- 5% of the average). Table 19. Manufacturer data about the allowed lateral length in the examined alternatives "Netafim" In-line drippers Maximum lateral length (m) on leveled ground at 10% flow-rate variation* Lateral Flow-rate Dripper Spacing - m Ǿ Type mm l/h 0.30 0.40 0.50 0.60 0.80 1.00 1.25 121 12** 1.0 47 60 73 84 106 127 150 122 2.0 30 38 46 54 68 87 96 124 4.0 19 25 31 36 45 54 63 161 16*** 1.0 83 104 124 142 176 207 242 170 1.5 63 79 95 108 164 158 185 162 2.0 53 66 79 91 112 133 156 164 4.0 35 44 52 60 75 88 103 168 8.0 22 27 33 37 47 55 65 * Distal pressure=10 m. ** O.D.=12.5 mm / I.D.=10.1 mm. *** O.D.=15.8 mm / I.D.=13.2 mm.
1.50 171 111 73 274 210 177 117 73
Drippers and laterals are chosen according to the manufacturer’s data. The table presenting in-line dripper hydraulic data shows that for laterals of Type 122 with drippers spaced at 1 m., a length of 80 m is marginal. To be on the safe side, it is recommended to choose the type 162 dripper lateral with a 16 mm OD tube instead of a 12 mm OD that has the same flow rate as the 122 dripper. The maximum allowed length of these laterals at 1 m spacing is 133 m instead of 81 m. The design process includes two phases. In the first phase, head loss is calculated from the tail to the head of the plot, using average values of head and flow rate. In the second phase, the design is checked, going from the head to the distal end. At this stage, the calculation relates to precise actual data. The map of the plot is divided into sectors and detailed calculations are performed on each pipe segment. The data have to be registered in the design form. Head losses in accessories are calculated using the data of equivalent length indicating head losses in a virtual pipe of the same diameter as that of the accessory. Most manufacturers provide tables and nomograms of head losses in their products.
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DRIP IRRIGATION
Table 20. Allowed lateral length of Ram 16 PC drippers Ram 16 PC (OD 16 mm), Flow-rate 2.3 l/h, allowed lateral length (m), distal head = 10 m Head in Dripper spacing - m Inlet (m) 0.15 0.20 0.25 0.30 0.40 0.50 0.60 0.80 1.00 15 38 50 61 72 93 113 132 168 201 20 44 57 70 83 107 130 152 193 231 25 48 63 77 91 118 143 168 214 255 30 52 68 83 99 128 155 181 230 276 35 55 72 89 105 136 165 193 246 294 40 58 76 94 111 143 174 203 259 310
97
1.2 235 275 305 325 351 376
DRIP IRRIGATION
Fig. 115. Local head losses in accessories
98
DRIP IRRIGATION
System Layout
Fig. 116. Drip system layout scheme Adapted from Watermatics brochure The drip irrigation layout presents the different components. Each of them creates head losses when the water flows through it. In the absence of detailed data about head losses in these accessories, a total head loss in the control head of 5 – 10 m is assumed. Pressure regulators are essential components of drip irrigation systems. Whenever non-compensated drippers are in use, pressure regulators can be installed on sub-mains and manifolds in order to control the pressure in each sub-plot. Using one lateral per row (with row spacing of 4 m) and drippers spaced 1 m apart along the lateral indicates spacing of 4 m2 per dripper. 7.68 Ha. = 76800 m2 divided by this spacing gives: 76800/4=19200. There are 19200 drippers in the plot. More accurate calculation can be done by multiplying row length: 4 X 80 m = 320 m, by the number of rows 320 m X 61 rows = 19520 m = 19520 drippers. Since the drippers are not compensated, a higher flow rate than the nominal flow rate with 10 m head should be taken into account for calculation. Flow rate of 122/162 drippers: 2.1 l/h at 10 m, 2.57 l/h at 15 m and 3 l/h at 20 m head. To be on the safe side, the highest flow rate is used in the calculation and multiplied by the number of drippers: 3 l/h X 19520 = 58.5 m3/h.
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DRIP IRRIGATION
Since the total flow-rate in the plot is lower than the available maximum hourly supply, the whole plot could be irrigated in one shift. Since the spacing between the trees is square (4 X 4 m), two lateral orientation can be checked. The above drawing presents four different layouts. Various considerations determine selection of the best layout. Since the plot is divided vertically, on the map, into four blocks, horizontal orientation of on-surface laterals is to be preferred. Therefore the upper left alternative is rejected. Of the other three alternatives, the lower left one allows irrigation of the plot in 4 separate shifts. This allows reducing the diameter of the distributing pipe and provides more operation flexibility. The entire plot can be irrigated in one shift by selecting a mainline with a wider diameter than that required for separate four shifts.
Fig. 117. Feasible layouts
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DRIP IRRIGATION
The design starts with the most problematic segments, where the water flows uphill: D-E, E-F, G-H and H-I segments (in Fig. 118). The head losses in each of the four segments are checked. The alternatives that will enable irrigation in one shift are examined.
Fig. 118. Segmented drawing for head loss calculation Table 21. Calculation Form: Head losses in pipes: a. for 4 shifts Flow Length Fric. Segment Description Topographic height initial – rate Head m distal - m
B-G B-G B-D D-E D-E E-F G-H
50 mm HDPE PN 6 63 mm HDPE PN 6 63 mm HDPE PN 6 50 mm HDPE PN 4 63 mm HDPE PN 4 162 drip lateral 50 mm HDPE PN 4
Loss %
Fric. Head Loss m
Topo head change m
Total head loss m
58.5-57.5
15
240
7
16.8
-1
15.8
58.5-57.5
15
240
3.6
8.6
-1
7.6
58.5
15
80
3.6
2.9
-
2.9
58.5-60
15
120
8X0.35*
3.4
+1.5
4.9
58.5-60
15
120
3X0.35*
1.3
+1.5
2.8
60-60.5
0.24
80
3X0.35
1
+0.5
1.5
57.5-58
15
120
8X0.35
3.4
+0.5
3.9
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DRIP IRRIGATION
G-H H-I b. for one shift B-D D-G B-D D-G B-D D-G
63 mm HDPE PN 4 162 drip lateral 63 mm HDPE PN 6 63 mm HDPE PN 6 75 mm HDPE PN 6 75 mm HDPE PN 6 90 mm HDPE PN 6 90 mm HDPE PN 6
57.5-58
15
120
3X0.35
1.3
+0.5
1.8
58-59
0.24
80
3X0.35
1
+1
2
58.5
60
80
30
24
-
24
58.5-57.5
30
160
10
12
-1
11
58.5
60
80
13
10.4
-
10.4
58.5-57.5
30
160
5
8
-1
7
58.5
60
80
6
4.8
-
4.8
58.5-57.5
30
160
2.4
3.8
-1
7
* 0.35 is the head loss factor for distributing pipes with more than 10 outlets Conclusions drawn from the above table (The acceptable alternatives appear in bold type): a. The main line should have a minimum diameter of 63/6 mm (diameter in mm and nominal pressure - PN in bar). Head losses in 50/6 mm pipe are too high. b. In the manifolds - distributing pipes (D-E/G-H segments), the head loss along a 50/4 pipe is also too high (beyond the acceptable max 10% - 15% difference in manifolds). Hence the distributing pipes have to be also of 63/4 diameter. c. Because of the topographic height differences, Installation of pressure regulators at the inlets to the manifolds does not equalize adequately the head at the inlets to the laterals. Appropriate equalization will be achieved only with pressure regulators mounted on each outlet from the manifold. d. That fact shows the difficulties in using non-compensated drippers in slopes. e. If the plot is irrigated in one shift, the mainline should be 90/6 mm. These conclusions justify choosing the second alternative: 2.3 l/h Ram pressure compensated drippers. Spacing is the same as with the in-line drippers, namely, 4 m between laterals and 1 m between drippers on the lateral. The design with compensated drippers is simpler. The pressure head at the inlet to each dripper
Fig. 119. The chosen diameter for mainline and manifold (in bold)
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DRIP IRRIGATION
inlet has to be higher than the regulating head that activates the compensating mechanism, which in the case of the Ram dripper is 4-5 m. Basic calculation: 320 m row length X 61 rows = 19520 m = 19520 drippers Total flow rate for a single shift: 19520 drippers X 2.3 l/h = 44.9 m3/h Since with a 20 m head at the lateral's inlet, the maximum allowed lateral length (on flat terrain) is 231 m, the manifold is positioned along the N-S midline of the plot. The plot will be irrigated in one shift, Fig. 120. One manifold layout but two valves will be installed on the manifold to enable irrigation in two shifts. As demonstrated in the form below, the maximum friction head loss (A-B + C-d) is 11.7 m, plus 2.5 m topographic elevation. In order to maintain an adequate head at the last and highest dripper's inlet, the head at the outlet from control head should be at least 20 + 14.2 m = 34.2 m. Table 22. Head loss calculation form – Pressure Compensated (PC) drippers Friction Topographic Total Segment Description Topographic Flow Length Friction height initial rate Head Head head head m – distal - m
A-B B-C C-D
90 mm 58.5-58 HDPE PN 6 63 mm 58-59 HDPE PN 6 16 mm 59-60.5 lateral
Loss %
Loss m
change m
loss m
45
160
4
6.4
-0.5
5.9
22.5
120
6X0.35
2.5
+1
3.5
0.37
160
6X0.35
3.3
+1.5
4.8
103
DRIP IRRIGATION
Fig. 121. Pressure compensated Ram 2.3 l/h dripper, one shift design Irrigation Design for Field Crops and Vegetables in the Open Field For annual field crops, the recommended method is retrievable drip irrigation. The laterals are laid out on-surface at the beginning of the irrigation season and collected before harvest. Distribution and retrieval of the laterals are mechanized and the level of damage to drippers and laterals is significantly reduced by new technologies. Example Crop: melon Area: 1.08 ha 120 X 90 m Topography: flat Spacing: 1.80 m, between rows 30 cm between plants in the row Peak season demand: 8 mm/day Soil: sandy-loam Bulk density: 1.6 Field capacity: 20% v/v Wilting point: 9% v/v Allowable deficit 40% Active root system depth: 60 cm Water supply limit 30 m3/h Supply hours limit: 14 h a day
Fig. 122. 1.08 Ha. Of melons
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DRIP IRRIGATION
Fig. 123. 10.08 Ha. Melons – In-line non-compensated drippers. Calculation: Daily peak demand: 8 mm = 80 m3/Ha. = 86.5 m3/day for the whole plot Daily supply capacity: 30 m3 X 14 h = 420 m3 per day In this case, non-compensated drippers can be used, "Netafim" 162 inline dripper lateral with drippers spaced at 30 cm on the lateral is limited to a length of 53 m. Since the length of the row is 120 m and with the manifold at the middle of the plot a dripper with lower flow rate should be selected. It is recommended to choose type 161 with an allowed lateral length of 83 m. Table 23. Head loss calculation Segment Description Topographic
Flow rate
Length m
Fric. Head Loss %
Fric. Head Loss m
Topo head change m
Total head loss m
A-B
30
90
4X0.35
1.4
-
1.4
0.3
60
4X0.35
0.84
-
0.84
height initial – distal - m
B-C
75 mm HDPE PN 4 16 mm lateral
In this case, the non-compensated drippers are suitable (table 22). On flat land, head differences in the irrigated plot are kept as low as 2.25 m with a 75/4 mm manifold and the low-flow 161 dripper. This is 10% of the operating pressure, far below the allowed upper threshold of 20% difference.
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DRIP IRRIGATION
Chapter 18. DRIP IRRIGATION SCHEDULING Drip irrigation is characterized by precise water application pattern on the one hand and a limited wetted soil volume on the other. This requires accurate scheduling and the implementation of a strict irrigation regime. The recommended irrigationscheduling method is a step-by-step process relating to all the relevant data available. Table 24. Irrigation scheduling – calculation form (example)
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DRIP IRRIGATION
Remarks A (General): The irrigation-scheduling process is purely mathematical. Some of the data are not measured but estimated, such as dripper wetted volume, readily available water percentage and crop coefficients. Hence the result of the calculations is an estimate and should be validated in the field. Validation aids are soil moisture and soil tension measurements, as well as physiological indicators such as shoot growth rate, fruit growth rate, trunk expansion and midday water tension of the shoot. C: The lower the rainfall the less its efficiency in replenishing soil moisture. Rainfall of less than 10 mm per event is not taken into account. F: The monthly precipitation deficit is the monthly evaporation (ET0) minus the effective rainfall. G, I, K: For simplification, data is given in volume per volume values. If soil moisture data is available as weight per weight value, it is converted to volume per volume value by multiplication by the bulk density value. L: Readily available water is the percentage of the available water that can be depleted without damaging the crop's yield or development. Clay Loam Sand N: There are a number of ways to estimate wetted soil volume Fig. 124. Schematic wetting pattern in different textured soils by a dripper. Semi-empirical formulae were applied by Schwarzmann & Zur and Shani. When the active root-zone depth and the maximum wetting diameter are known, a good approximation can be achieved, by relating to the wetted volume as an ellipsoid and calculating its volume with the formula:
Vw = 4/3 (π πr1r2r3) Where:
r1 = half of the root-zone depth r2 = half of the maximum wetted volume diameter r3 = the average of the two above values
T: The crop coefficient, as well as the readily available water percentage, change along the growing season according to the crop’s sensitivity to water stress at the different phenological stages. X: Irrigation efficiency of drip irrigation is in the range of 80% - 90%. Irrigation Shifts and Timetable Once the net and the gross water applications have been calculated, a detailed operative schedule should be elaborated. The application per area unit (e.g. m3/ha.) should be multiplied by the area of the plot and compared with the water supply capacity. For example: in the above table, the calculated water applications are in the range of 50 – 100 m3/ha.
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DRIP IRRIGATION
Fig. 125. Different schedules of drip irrigation operation Adapted from Benami & Ofen, 1993 Assuming a plot area of 10 ha. And a water supply limited to 30 m3/h, 14 hours/day. The drippers are Ram pressure compensated drippers 2.3 l/h, spaced 4 X 1 m. Number of drippers per ha,: 10000 m2 / 4 m2 = 2500 drippers/ha Application rate: 2500 drippers/ha X 2.3 l/h/dripper = 5.75 m3/h/ha Maximum concurrently irrigated area = 30 m3/h / 5.75 m3/h/ha. = 5.2 ha. The 10 ha. area should be irrigated in two shifts. The maximum volume that can be applied during the 14 hours of water supply is: 5.75 m3/h/ha X 14/h = 80.5 m3/ha This result means that during the months of June and November not enough time is available to complete the irrigation application during a single day, and irrigation should be continued on the following day or the irrigation interval should be reduced. Irrigation Scheduling for Annuals In annual crops, irrigation scheduling is related to phenological establishment, vegetative development, flowering, fruit growth and harvest.
stages:
During the first stage of establishment, only a small fraction of the surface area is covered by foliage and the water demand is very small. On the other hand, the root system at this stage is shallow and application frequency has to be high. During the vegetative development stage, controlled water stress is applied to some crops to restrict the vegetative growth in order to achieve a favorable ratio between vegetative and reproductive development. The flowering stage is highly sensitive to water stress. The allowed water depletion percentage in this stage is lower than at the vegetative growth stage. The fruit development stage is less sensitive than the flowering stage to water stress, but an adequate water regime has to be maintained as to guarantee optimal fruit development and avoid physiological disorders.
108
DRIP IRRIGATION
Table 25. Irrigation scheduling form for annuals (example)
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DRIP IRRIGATION
The fraction of the soil surface area covered by the foliage of annual crops during the establishment stage is very small. Hence, drip irrigation contributes to substantial water savings. Operation Timetable After the scheduling of the irrigation intervals and water applications, a timetable should be prepared. Water supply limitations and the topography should be considered when elaborating this timetable. Example
Fig. 126. Layout of drip system for 55 ha. Of cotton A 55 ha. plot of cotton should be irrigated according to the following data: Maximum daily water requirement: 5 mm/day Interval: 7 days Emitters: Ram PC 1.2 l/h Spacing: 1 X 0.5 m = 0.5 m2 Irrigation efficiency: 90% Net weekly requirement: 7 X 5 mm = 35 mm = 350 m3/ha Gross weekly requirement: 350/90% = 389 m3/ha Number of drippers per ha. = 10000 m2 /0.5 m2 = 20,000 Application rate: 1.2 l/h X 20,000 = 24,000 l/h/ha = 24 m3/h per ha Max irrigation hours: 350/24 = 14.5 h Nineteen controlled hydraulic valves divide the area to 19 sub-plots. Since the irrigation cycle is 7 days and two spare days are reserved for maintenance and emergency events during each cycle, 4 sub-plots must be irrigated simultaneously.
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DRIP IRRIGATION
To reduce friction head-losses, each block will be irrigated in two shifts. Those subplots irrigating simultaneously will be distributed along the mainline in order to decrease head-losses. The maximum flow rate per shift is 242 m3/h. The mainline should be of 200 mm diameter/ 60 m PN. Due to the 7 m descending slope, the friction head loss in the main pipeline is compensated by the topographic difference gain. The direction of the rows and the laterals was adjusted according to the topography for convenience of mechanized harvest.
Table 26. Operative Irrigation Schedule
Shift
Valves
1
1 5 11 17 2 6 12 18 3 7 9 13 4 8 10 14 15 16 19
2
3
4
5
111
Flow rate m3 64 50 49 66 64 50 49 66 60 60 72 50 60 60 72 50 61 61 48
Flow rate per shift
229
229
242
242
170
DRIP IRRIGATION
Chapter 19. MAINTENANCE Drip irrigation commits careful maintenance. Particular attention should be given to the weak points of the system: •
The narrow water passageways in the drippers that are prone to clogging.
•
The low working pressure of the piping system renders it highly sensitive to pressure spikes.
•
The filtering systems are sensitive to clogging by excessive dirt load, which increases head losses and may decrease the filtration system performance.
•
Sediments accumulate at the distal ends of the manifolds and laterals. Routine flushing is required.
The best maintenance policy is to inspect the whole system periodically and systematically. The time intervals between inspections depend on the water quality and the attributes of the system's components. Inspections may be weekly, monthly, or twice a year in favorable conditions. Monitoring drip irrigation performance is not an easy task. The low-flow emitters do not facilitate visual observation of application uniformity, particularly of SDI systems, where laterals are buried underground. Nevertheless, there are some procedures that can be implemented to roughly evaluate performance. The first step is to check the hourly flow rate at the main flow-meter (water meter) and compare it with the designed flow-rate (the number of emitters multiplied by the dripper’s nominal flow-rate). Significant deviation from the designed flow rate is an indication that there are problems in the system. A flow-rate that is lower than the calculated value indicates possible clogging. A flow-rate that is higher than the calculated value is an indicator of a possible rupture of the main lines or manifolds as well as torn or punctured laterals. The second step is to check all the pressure gauges installed in the plot and compare the measured values to the designed pressure for each set. If a high flow variance is suspected, on-farm inspection of dripper flow-rate uniformity should be performed. The minimum number of drippers in a sample is 20. The recommended number is 40-50. Once measured, the DU can be calculated. If the DU is unacceptable, the drippers should be cleaned with acid, flushed with pressurized air or replaced. The system should be checked again after the treatment. Visual indicators of inadequate system performance are random stressed plants, surface runoff, “surfacing” in SDI and white salt spots on the soil surface. Protective measures are filtering, chlorination, acidification and flushing. If the system includes a pumping unit, this is subject to wear and requires periodic lubrication. Periodic pump testing, every five years (or more often if the water carries sand) guarantees long-lasting performance. Pressure regulators are based on spring resistance or hydraulic equilibrium maintenance. Springs are weakened after prolonged operation and should be inspected and calibrated once every two years.
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DRIP IRRIGATION
Vacuum-relief valves perform an important function in drip systems. When the irrigation is turned-off, the water that remains in the system flows downhill to the lowest outlets. The water vacating the high points creates a vacuum, which causes the emitters in this section of the plot to suck in air and dirt. In extreme cases, PVC mainlines and thin-wall laterals may collapse. Vacuum-relief valves, installed at the high points of the system, are prone to clogging and need periodic inspection to guarantee that no solid objects are caught inside and that they are not stuck in the open or shut position. Air release valves also require similar periodic examination. The filtration system should be thoroughly inspected. In many filter types, the steel body is coated with epoxy paint to protect it from corrosion. The intactness of the epoxy paint should be checked routinely. Cracks in the coating endanger the endurance of the entire body. The collectors of sand separators, should be purged periodically, otherwise the excess of accumulated sand reduces separation efficiency. Screen filters should be opened and the screens visually inspected for wear, tear and blockage by organic matter, silt and precipitates. The same is relevant for disk-type filters. If the filter is of the manual cleaning type, the filter element should be cleaned carefully when the pressure difference between its inlet and outlet exceeds 5 m. Automatic back-flushing filters require periodic visual inspection of the filtering elements for wear and presence of persistent contaminants. Back-flushing filter components: hydraulic valves, solenoids and rotating brushes or vacuum devices, may require periodic servicing and lubrication. Many of them include a secondary small water filter to protect from blockage of the solenoid ports and valve control chambers. This filter needs periodic manual cleaning. Automatic back-flushing media-filters fluidize and resettle the filtering bed with every flush cycle and require special attention. The discharge of media filters should be within the specified range of each model. For a typical 48" diameter tank, the range is 50 - 70 m3/h. Below the lower margin, contaminants tend to infiltrate deeper into the media bed. Flow rates higher than the recommended upper threshold can lead to coning and channelization of the filtering media. Coning in sand media filters is created when an excessive downwards flow striking the diffusion plate is deflected towards the inner walls of the tank and scours the media sand away from the walls, depositing it towards the center of the tank, underneath the diffusion plate, creating a cone of sand. With channelization, some areas of the tank base or the under-drain are exposed, allowing some water to pass through the tank without contacting the filtration media, evading filtration and potentially plugging the under-drain and the drippers on the laterals. This can be caused by excessive downward flow rate due to coning, or by excessive upward flow, when the tank is being back-flushed. Channelization and coning are indicators of poor filtration. Installation of a pressure-sustaining valve may prevent these problems. To effectively back-flush a filter, adequate back-flush flow rate is critical, particularly for sand filtering media. It should be large enough to fluidize and lift the filtering media, while passing just a minor amount of sand out through the flushing discharge manifold. The recommended back-flush flow rate for a 48" tank is 800 - 1000 l/min.
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DRIP IRRIGATION
The water flushing out from the filters should be inspected visually and by touch while a tank is back-flushing, using a 100-mesh nylon screen or by hand, to validate that only a very small amount of media sand is being discharged from the tank. Sometimes channelization creates sectors in the tank that are not being properly fluidized during back flushing. Clay balls or silt stratifications are the indicators of such a condition. In water containing organic matter, iron, sulfur and manganese bacteria, routine oxidation by chlorine is essential. Chlorination can be accomplished continuously with 2 – 5 ppm of active chlorine or intermittently as a “shock treatment” when there is a high build-up of slime in the system. A “shock treatment” with 15 – 30 ppm chlorine is given for 20 – 30 minutes. As mentioned before, some dripper manufacturers suggest 15 ppm as upper threshold. Higher concentrations may damage the diaphragms in pressure compensating drippers and hydraulic valves. Periodic flushing of the mainline, manifolds and drip laterals is an essential maintenance practice. The best form of manually flushing is to release the lateral end stoppers one after another and let the dirty water exit until clean water appears. Automatic line flushing valves can be installed at lateral ends. These valves automatically flush the laterals at the beginning of each irrigation cycle.
Fig. 127. Automatic line flushing valve Courtesy "Netafim" Flushing also removes air that may accumulate and become trapped, especially in laterals, due to slight undulations in the lateral. Flushing velocities should be at least 0.5 to 0.6 m/s to effectively remove air from the laterals. Fertigation system performance should also be checked. Excessive fertilization can induce salinity damage as well as antagonistic interference between nutrition elements. There are four ways to check precision of nutrient application:
Collecting water samples from the dripper laterals beyond the injection point and comparing the sample analysis with the desired concentration.
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DRIP IRRIGATION
Analyzing extracted soil solution by soil solution extractors
Analyzing the nutrient content of soil samples
In detached beds, collecting drainage samples and comparing them with samples of water collected from drippers. If the nutrient concentration of the drainage is significantly lower than that of the dripper emission, the rate of the injected nutrients should be increased. The likelihood of leaching of nutrients by excess water should also be examined. If nutrients level in drainage is higher than that of the dripper emission, there is a possibility of excess nutrients injection or deficit water application.
Installation Aids There are diverse tools to ease installation and maintenance. Among them are the punch and holder that enable precise punching of the lateral as well as fast and easy insertion of connectors, splitters and drippers into the drilled holes.
Fig. 128. Punch (left) and holder (right) Courtesy "Netafim"
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DRIP IRRIGATION
Chapter 20. REFERENCES AND BIBLIOGRAPHY Ascough, G. W., and G. A. Kiker (2002). The Effect of Irrigation Uniformity on Irrigation Water Requirements. Agricultural Research Council - Institute for Agricultural Engineering, PO Box 2252, Dennesig 7601, South Africa School of Bio-resources Engineering and Environmental Hydrology, University of Natal, Private Bag X01, Scottsville 3209, South Africa. Attanayake, M. A. M. S. L and J. P. Padmasiri. (1994). An Appropriate Iron Removal Technology. 20th WEDC Conference: Colombo, Sri Lanka. Avidan A., D. Yolles and M. Sne. (2004). Fertilizers Characteristics. Irrigation & Soil Field Service, Extension Service, Ministry of Agriculture, Bet Dagan 50250, Israel. (in Hebrew). Ayers, R. S. and D. W. Westcot. (1985). Water Quality for Agriculture. FAO Irrigation and Drainage paper 29, FAO Rome. Barber, S. A., A. Katupitiya and M. Hickey. (2002). Effects of Long-Term Subsurface Drip Irrigation on Soil Structure. Charles Stuart University, School of Agriculture, Wagga Wagga, NSW. Barth, G. (2004), Slow Flow Sand Filtration (SSF) for Water Treatment in Nurseries and Greenhouses. The Nursery Papers, South Australian Research and Development Corporation, Adelaide. Bassoi, L. H. et al. (2003). Grapevine Root Distribution in Drip and Micro-sprinkler Irrigation. Scientia Agricola, v.60, n.2, p.377-387, Apr./Jun. Benami, A. and A. Ofen. (1993). Irrigation Engineering. Agripro, Kfar Galim 30865, Israel. Boman, B. J., P. C. Wilson, and E. A. Ontermaa (2002). Understanding Water Quality Parameters for Citrus Irrigation and Drainage Systems. Circular 1406, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Bresler, E. (1978). Analysis of Trickle Irrigation with Application to Design Problems. Irrig. Sci. 1: 3-17. Broner I. And M. Alam. (2003). Subsurface Drip Irrigation (SDI). Colorado State University Cooperative Extension. www.ext.colostate.edu. Burke, K. And Parlevliet G. (2002). Irrigation of Native Cut Flowers in Western Australia. Department of Agriculture, Western Australia. Burt C. M., J. T. Barreras (2001). Evaluation of Retrievable Drip Tape Irrigation Systems. ITRC California Polytechnic State University, San Luis Obispo 93407. Burt, C. M. and S. W. Styles. (1999). Drip and Micro Irrigation for Trees, Vines and Row Crops.. ITRC, BioResource and Agricultural Engineering Dept., California Polytechnic State University, San Luis Obispo, 93407. Chapin R. D. (2000). A Worldwide Problem -" Drip Irrigation vs. Relief Food" An update. Chapin Living Waters Foundation. 364 N. Colorado Ave. Watertown, NY 13601 2000. Dvir, Y. (1997). Flow Control Devices. Control Appliances Books. Lehavot Habashan 12125, Israel. Gerstl, Z. (1998). A Study to Compare the Release of Trifluraline into Irrigation Systems for the Purpose of Root Intrusion Prevention. Institute of Soil and Water, ARO, Volcani Center, Bet Dagan, Israel. Gleick, P. Et al. (2002). The World’s Water 2002 - 2003. Pacific Institute for Studies in Development, Environment and Security. Oakland, California. Hagin, J., M. Sneh and A. Lowengart-Aycicegi (2002). Fertigation – Fertilization through Irrigation,
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IPI Research Topics No. 23. International Potash Institute, P.O.Box 1609 CH-4001 Basel, Switzerland. Haman, D. Z., A. G. Smajstrla and F. S. Zazueta. (1989). Screen Filters in Trickle Irrigation Systems. University of Florida, Florida Cooperative Extension Service. Haman, D. Z., A. G. Smajstrla and F. S. Zazueta. (1989). Settling Basins for Trickle Irrigation in Florida University of Florida, Florida Cooperative Extension Service. Haman, D. Z., A. G. Smasjstrla and F.S. Zazueta (1994). Chemical Injection Methods for Irrigation. Florida Cooperative Extension Service. Hammami, M. et al. (2002). Approach for Predicting the Wetting Front Depth beneath a Surface Point Source: Theory and Numerical Aspects. Irrig. and Drain. 51: 347–360 (2002). Hanson, B. and D. May. (1998). Drip Irrigation Increases Tomato Yields in Salt-Affected Soil of San Joaquin Valley. UC Cooperative Extension. California Agriculture, Volume 57, No 4. Hanson, B. R., G. Fipps, E. C. Martin (2002). Drip Irrigation of Row Crops: What is the State of the Art? Kansas state University. Hanson, B. R., D. M. May and L.J. Schwankl. (2003). Effect of Irrigation Frequency on Subsurface Drip-Irrigated Vegetables. Hort Technology January-March 2003. Hartz, T .K. (1996). Drip Irrigation Improves N Efficiency. University of California, Davis. Hla, A. K. and T. F. Scherer (2003). Introduction to Micro-irrigation. North Dakota State University Fargo, North Dakota 58105 Hoitink, A. J. and M. S. Krause. (1999). New Approaches to Control of Plant Pathogens in Irrigation Water. Special Circular 173-00 Ohio State University Extension Service. Intermediate Technology Consultants (ITC). (2003). Low Cost Micro-Irrigation Technologies for the Poor – Final Report, Oct 2003. Jayalath, J., J. Padmasiri, S. Kulasooriya, B. Jayawardena, W. Fonseka, and L. Wijesinghe. (1994). Algae Removal by Roughing Filter. 20th WEDC Conference: Colombo, Sri Lanka. Kemble, J. K. and D. C. Sanders. (2000). Basics of Vegetable Crop Irrigation. Department of Horticulture, Auburn University. Kidder, J. And E. A. Hanlon. (1998). Neutralizing Excess Bicarbonates From Irrigation Water. Soil and Water Science Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Kremmer, S. and E. Kenig. (1996). Principles of Drip Irrigation. Irrigation & Soil Field Service, Extension Service, Ministry of Agriculture, Israel (in Hebrew). Laemmlen, F. (1998). Proper Use of Drip Tape and Fertigation will Maximize Celery Yields. 624 West Foster Road, Suite A, Santa Maria, CA 93455. Lamm, F. R. (1998). Advantages and Disadvantages of Subsurface Drip Irrigation. Northwest Research-Extension Center, Kansas State University, Colby, Kansas.
[email protected]. Luke, G. And T. Calder. (2000). Blockages in Irrigation Lines. Division of Resource Management, South Perth, Department of Agriculture, Western Australia. Maas, E. V. (1984). Salt Tolerance in Plants. In: The Handbook of Plant Science in Agriculture. B.R. Christie (ed.). CRC Press, Boca Raton, Florida. Mahbub A., P. T. Todd, F. R. Lamm and D. H. Rogers. (1992). Filtration and Maintenance Considerations for Subsurface Drip Irrigation (SDI) Systems. Kansas State University
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Agricultural Experiment Station and Cooperative Extension Service Manhattan, Kansas. Peacock, B. (2000). Amending Soil and Water Chemistry in Drip Irrigated Table Grape Vineyards. University of California, Tulare County, Cooperative Extension. Phene, C. J. (1999). Subsurface Drip Irrigation Part I: Why and How?. Irrigation Journal, April 1999. Pitts, D. (1996). Field Evaluation of Micro Irrigation System Performance. SWFREC Report No. IMM-96-OO. Southwest Florida Research and Education Center, University of Florida, Immokalee, FL. Pitts, D. J., D. Z. Haman and A. G. Smajstrla. (1990). Causes and Prevention of Emitter Plugging In Micro-irrigation Systems.. Soil and Water Science Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Poffley, M. (1997). Growing Vegetables Using Black Plastic Mulch and Trickle Irrigation. Agnote 367, Horticulture Division, Darwin. Reed, D.W. ed (1996). A Growers Guide to Water, Media and Nutrition for Greenhouse Crops. Ball Publishing, Batavia, Illinois. Russo, D, J. Zaidel, A. Laufer and Z. Gerstl (2001). Numerical Analysis of Transport of Trifluralin From a Subsurface Dripper. Soil Science Society of America Journal 65:1648-1658. Rust, M. And K. McArthur. (1998). Slow Sand Filtration. Water Treatment Primer. Environmental
Information Management, Civil Engineering Dept. Virginia Tech. Salgado, E. A. and M. A. Toro. (1995). Spatial Distribution of Avocado Roots under Drip and MicroSprinkler Irrigation. Proc. World Avocado Congress III, 1995 206 – 208 Sanders, D. C. (2001). Drip or Trickle Irrigation Systems: An Operations and Troubleshooting Checklist. North Carolina State University. Sanders, D. C. (2001) Drip or Trickle Irrigation Systems: An Outline of Components. Department of Horticultural Science, College of Agriculture & Life Sciences North Carolina State University. Sanjines, A. and R. Ruskin R. (1991). Root Intrusion Protection for Subsurface Drip Emitters. ASAE paper No. 91-2047. Shock, C. (2003). Efficient Irrigation Scheduling. Malheur Experiment Station, Oregon State University 595 Onion Avenue Ontario, OR 97914. Shock, C. C., E. Feibert, and L. Saunders (2002). Irrigation Frequency, Drip Tape Flow Rate, and Onion Performance. Malheur Experiment Station, Oregon State University Ontario, OR 97914. Smajstrla, A. G., and D. S. Harrison. (1998) Tensiometers for Soil Moisture Measurement and Irrigation Scheduling. Circular 487, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Smajstrla, A. G., B. F. Castro, G. A. Clark. (1999). Energy Requirements for Drip Irrigation of Tomatoes in North Florida. Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, 32611. http://edis.ifas.ufl.edu . Solomon, K. S. and G. Jorgensen. (1993). Subsurface Drip Irrigation, California State University Fresno, CATI. Solomon, K. S. (1992). Subsurface Drip Irrigation: Product Selection and Performance. Subsurface Drip Irrigation Theory, Practices and Applications. California State University Fresno, CATI Publication Number 92-1001 pp 3-25. Tamasi, J. (1986). Root Location of Fruit-Trees and its Agrotechnical Consequences. Akademiai
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Kiado, Budapest, Hungary. Van Voris, P., D. A. Cataldo and R. Ruskin (1988). Protection of Buried Drip Irrigation Devices from Root Intrusion through Slow-Release Herbicides. Proceedings, 4 Intl Micro-Irrigation Congress, Albury-Wadonga, Australia, October 23-28, 1988. Wickham, D.L. (2000) A Brief Look at the Early Days of Modern Irrigation.. Landscape & Irrigation Magazine P.O. Box 361400 Strongville, OH. 44136. Wilcox, L. V. and C. C. Magistad. (1943). Interpretation of Analyses of Irrigation Waters and the Relative Tolerance of Crop Plants. Regional Salinity Laboratory, Bureau of Plant Industry, Soils and Agr. Engineering, Agricultural Research Administration, U. S. Department of Agriculture, Riverside, California. Wu, I-pai, H. M. Gitlin. (1979). The Manufacturer’s Coefficient of Variation Emitter Flow for Drip Irrigation. Cooperative Extension Service College of Tropical Agriculture and Human Resources University of Hawaii at Manoa Zoldoske, D., R. K. Striegler, G. T. Berg, G. Jorgenson, C. B. Lake, S. G. Graves, and D. M. Burnett (1998). Evaluation of Trellis System and Subsurface Drip Irrigation for Wine Grape Production: A Progress Report. CATI Publication #980401.
Commercial Brochures and CD-Roms Amiad Arad Ari Arkal Audis Bermad Chapin-Watermatics Dorot Ein Tal Eldar-Shani Galcon Haifa IDE Mezerplas NaanDan Netafim Plassim Plastro Queen Gil Soil Moisture T-Tape Wade Rain
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CONVERSION FACTORS Col.1 Col.2 multiply by
Column 1 SI Unit
Column 2 non-SI Unit
kilometer, km (103 m) meter, m meter, m micrometer, µm (10-6 m) millimeter, mm (10-3 m) millimeter, mm (10-3 m) nanometer, nm (10-9 m)
mile, mi yard, yd foot, ft micron, µ inch, in mil (1/1000 inch) Angstrom, A
Col.2 Col.1 multiply by
Length 0.621 1.094 3.28 1.0 3.94 x 10-2 39.4 10
1.609 0.914 0.304 1.0 25.4 0.0254 0.1
Area 2.47 247 0.386 2.47 x 10-4 10.76 1.55 X 10-3
hectare, ha acre square kilometer, km2 (103 m)2 acre square kilometer, km2 (103 m)2 square mile, mi2 square meter, m2 acre 2 square meter, m square foot, ft2 2 -3 2 square millimeter, mm (10 m) square inch, in2
0.405 4.05 x 10-3 2.590 4.05 x 103 9.29 x 10-2 645
cubic meter, m3 cubic meter, m3 cubic meter, m3 liter, L (10-3 m3) liter, L (10-3 m3) liter, L (10-3 m3) liter, L (10-3 m3) liter, L (10-3 m3) liter, L (10-3 m3)
acre-inch cubic foot, ft3 cubic inch, in3 bushel, bu quart (liquid), qt cubic foot, ft3 gallon ounce (fluid), oz pint (fluid), pt
102.8 2.83 x 10-2 1.64 x IO-5 35.24 0.946 28.3 3.78 2.96 x 10-2 0.473
gram, g (10-3 kg) gram, g (10-3 kg) kilogram, kg kilogram, kg kilogram, kg megagram, Mg (tonne) tonne, t
pound, lb ounce, oz pound, lb quintal (metric), q ton (2000 lb), ton ton (U.S.), ton ton (U.S.), ton
454 28.4 0.454 100 907 0.907 0.907
Volume 9.73 X 10-3 35.3 6.10 x 104 2.84 x 10-2 1.057 3.53 X 10-2 0.265 33.78 2.11 Mass 2.20 x 10-3 3.52 x 10-2 2.205 0.01 1.10 x 10-3 1.102 1.102 Yield and Rate 0.893 7.77 X 10-2 1.49 X 10-2 1.59 X 10-2 1.86 X 10-2 0.107 893
kilogram per hectare, kg ha-1 pound per acre, lb acre-1 kilogram per cubic meter, kg m-3 pound per bushel, bu-1 kilogram per hectare, kg ha-1 bushel per acre, 60 lb kilogram per hectare, kg ha-1 bushel per acre, 56 lb kilogram per hectare, kg ha-1 bushel per acre, 48 lb gallon per acre liter per hectare, L ha-1 -1 tonnes per hectare, t ha pound per acre, lb acre-1
120
1.12 12.87 67.19 62.71 53.75 9.35 1.12 x 10-3
DRIP IRRIGATION
CONVERSION FACTORS (CONTINUED) Col.1 Col.2 multiply by
Column 1 SI Unit
Column 2 non-SI Unit
893 0.446 2.24
megagram per hectare, Mg ha-1 pound per acre, lb acre-1 megagram per hectare, Mg ha-1 ton (2000 lb) per acre, ton acre-1 meter per second, m s-1 mile per hour
Col.2 Col.1 multiply by 1.12 x 10-3 2.24 0.447
Specific Surface 10 square meter per kilogram, m2 kg-1 2 -1 per gram, cm g 0.1 1000 square meter per kilogram, m2 kg-1 per gram, mm2 g-1 0.001
square centimeter square millimeter
Pressure / Head 10 9.90 10 1.00 centimeter, g cm-3 2.09x 10-2 1.45X 10-4
Meter, m - water head atmosphere 6 atmosphere mega pascal, MPa (10 Pa) megapascal, MPa (106 Pa) bar megagram per cubic meter, Mg m-3 . 1.00 pascal, Pa pound per square foot, lb ft-2 pascai, Pa pound per square inch, lb in-2
0.1 0.101 0.1 gram per cubic 47.9 6.90 x 103
Temperature 1.00 (K - 273) 273) (9/5 0C) + 32
Kelvin, K
Celsius, 0C
1.00 (0C +
Celsius, 0C
Fahrenheit, 0F
5/9 (0F - 32)
British thermal unit, Btu calorie, cal erg foot-pound calorie per cm2 (langley) dyne calorie per cm2 minute, cal cm-2 min-1
1.05 x 103 4.19 10-7 1.36 4.19 x 104 10-5 698
Energy, Work, Quantity of Heat 9.52 x 10-4 0.239 107 0.735 2.387x 10-5 105 1.43 x 10-3
joule, J joule, J joule, J joule, J joule per square meter, J m-2 Newton, N watt per square meter, W m-2
Plane Angle 57.3 (angle), 0
radian, rad
degrees 1.75 x 10-2
Electrical Conductivity, Electricity, and Magnetism 1.0 104
decisiemen per meter, dS m-1 tesla, T
millimho per centimeter, mmho cm-1 gauss, G
10-4
acre-inches, acre-in cubic feet per second, ft3 s-1
102.8 101.9
1.0
Water Measurement 9.73 x 10-3 9.81 X 10-3
cubic meter, m3 cubic meter per hour, m3 h-1
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DRIP IRRIGATION
CONVERSION FACTORS (CONTINUED) Col.1 Col.2 multiply by
Column 1 SI Unit
Column 2 non-SI Unit
4.40
cubic meter per hour, m3 h-1 0.227 hectare-meters, ha-m hectare-meters, ha-m hectare-centimeters, ha-cm
U.S. gallons per minute, gal min-1
8.11 97.28 8.1 x 10-2
acre-feet, acre-ft acre-inches, acre-in acre-feet, acre-ft
Col.2 Col.1 multiply by
0.123 1.03 x 10-2 12.33
Concentration 1 centimole per kilogram, cmol kg-1 100 grams, meq 100 g-1 1 0.1 gram per kilogram, g kg-1 percent, % 1 milligram per kilogram, mg kg-1 parts per million, ppm
milliequivalents per 10 1
Plant Nutrient Conversion 2.29 1.20 1.39 1.66
Elemental P K Ca Mg
Oxide P2O5 K2 0 CaO MgO
122
0.437 0.830 0.715 0.602
DRIP IRRIGATION
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