SURFACE IRRIGATION: Surface irrigation may be classified as flooding method, furrow method and contour farming method, with
further
divisions
1. Flooding Method: In flooding irrigation, water is allowed to cover the surface of land in a continuous sheet, the water standing just long enough in the field for the soil to absorb the water applied to refill the root zone. A properly designed size of irrigation stream aims at proper balance against the intake rate of soil, the total depth of water to be stored in the root zone and the area to be covered so as to give reasonably uniform coverage of water over the entire field. The flooding may be (i) wild, and (ii) controlled. (a) Wild Flooding: It is also called uncontrolled flooding. It is the primitive and most inefficient method of irrigation. In this method water is spread over the smooth or flat field without much control over the flow or prior preparation. The water is spread into the field from the ditch excavated either on the contour or up and down the slope. This method is applicable to inundation irrigation system or for pastures or forage crops where water is available in abundance at the highest elevation and is inexpensive or the crop values do not justify adoption of better method. The water distribution is quite uneven. The method is suitable for all medium to fine texture soils. It has low cost and does not interfere with tillage. The disadvantages of the method are (i) wasteful use of water, (ii) non-uniform distribution of water, (iii) excessive soil erosion on steeper slopes, (iv) require drainage arrangement to reduce
ponding.
(b) Controlled Flooding: (i) Free flooding. Also called ordinary flooding. It is the commonly adopted method where irrigation water is in abundance and cheap. The land is divided into plots or kiaries of suitable size depending on porosity of soil. Water is spread over the field from watercourse. The irrigation operation begins at the higher area and proceeds towards the lower levels. The flow is stopped when the lower end of the field has received the desired depth of water. The field watercourse is properly spaced, the spacing depends on the topography, oil texture, depth of soil and size of stream. The spreading may vary from less than 15 m to more than 60 m. Porous soil requires close spacing than tight soil. The method is most suitable for soils of medium t exture and with moderate slopes.
SURFACE IRRIGATION It is by far the most common type of irrigation. In surface irrigation method, since water is applied to the field in varied quantities and at different times, the flow remains unsteady. The method involves diverting a stream of water from the head of a field into furrows or borders, and allowing it to flow downward. Water infiltrates into the soil while traversing the furrow. By subsequent ponding and lateral movement, the soil is restored to its full water holding capacity to a depth that depends on the quantity of water introduced, the duration and rate of stream flow, the gradient and the soil structure and texture. Generally under open-ditch conveyance and surface irrigation methods, less than one-half of the water released reaches plants. Highly efficient irrigation can be achieved by an appropriate combination of size of the irrigation stream, uniform application of water, minimum soil erosion, minimum labour requirement, maximum land use, size and shape of field and use of machinery. The
surface
irrigation
is
essentially
supplemented
with
efficient
water
disposal
system.
Advantages: (i)
Allows use of machinery for land preparation, cultivation and harvesting, and
(ii)
(ii) Helps to store the required amount of water in the capillary zone of the soil for supply to the root zone of plants.
Disadvantages: (i)
Greater loss of water by surface runoff and deep percolation, larger requirement of water per unit area,
(ii)
Water is lost in infiltration and deep per colation,
(iii)
Low efficiency due to imperfect control over the water flow,
(iv)
Inferior quality crops with a low yield,
(v)
Wasteful use of water compared to better irrigation methods, and
(vi)
Costly and time consuming preparation of land.
Sub-surface irrigation: In sub-surface or sub-irrigation water is applied beneath the ground by creating and maintaining an artificial water table at some depth, usually 30 to 75 cm, below the ground surface. Moisture moves upwards towards the land surface through capillary action to meet requirements of the crops in plant roots. Water is applied through underground distribution system consisting of a properly designed main field ditches, laterals, laid 15 to 30 m apart. Water may be obtained from wells, streams, lakes etc. Water is introduced into soil profile through open ditches, mole drains or tile drains. Open ditches are preferred because they are relatively inexpensive and suitable for all types of soils. Tiles and mole drains are suitable only for organic soils. Sub-surface irrigation requires little field preparation and labour. It entails minimum evaporation loss and surface waste. The irrigation water is essentially required to be of good quality to prevent excessive soil salinity. The flow rate in supply ditches is required to be low to prevent waterlogging of the field. The use of sub-irrigation is limited because it requires certain soil condition that is the soil is permeable in root zone, underlain by an impervious horizon or high water table. Essential
Requirements
The essential requirements for a successful sub-surface irrigation are (i) Availability of adequate supply of good quality water throughout growth period of the crop, (ii) Fields must be nearly level and smooth. Ground slope is moderate. Land is approximately parallel to water table, (iii) Availability of a layer of permeable soil such as sandy loam or loam immediately below the surface soil to permit free and rapid movement of water laterally and vertically, (iv) Availability of a relatively impervious layer at 2 to 3 m in the substratum to prevent deep percolation of water or a permanently high natural water table on which an artificial water table can be built, (v) A well planned distribupon system of main ditches, field laterals, etc., which raises the water table to a uniform depth below the ground surface over the entire area, (vi) Availability of adequate outlet for drainage of the area so irrigated particularly in humid areas, (vii) Subsoil water table is within 2 to 3 m below the ground surface, (viii) Topographical conditions are
uniform, and (ix) Soil is capable of lifting moisture from the water table to the root zone. Also the soil permits lateral and downward movement of water. The efficiency of water use depends on soil characteristics, topography and operation and maintenance management. In good system, t he efficiency is 70-75 per cent. The essential requirements for adoption of any irrigation method are (1) Application is within desirable limits. Stream flow is adequate so that the quality of irrigation is such that the depth of wetting and hence the stand of crop are approximately uniform. The irrigator achieves high productivity so that during a day’s work he can irrigate a large hectarage, (2) Afford a uniform water distribution in root zone of
a
crop
with
as
small
as
6
cm
applications
for
light
irrigation,
(3)
Allow
heavy uniform application of 15 to 20 cm of water depth and under some conditions as much as 25cm per irrigation for salt leaching where such a problem exists, (4) Allow use of large concentrated water flows for reduction of conveyance losses, field channel network and labour cost, (5) Suitable for use with economical conveyance structure, (6) Facilitate mechanized farming, (7) Occupy minimum land under bunds, etc, (8) Inexpensive and economically justifiable, (9) High efficiency of water application i.e., the ratio of water stored in the root zone to that delivered to the field should be maximum, (10) Minimum wastage of water either through surface runoff or through deep percolation below
the
root
zone
of
a
crop.
A suitable irrigation method shall ensure maximum yield at optimum water utilization, i.e., conservation of water resource. The choice of an irrigation method suitable to a specific situation is an important aspect of water management of crops.
The design, equipment and technique of replenishing the soil water deficit by applying irrigation water is referred to as irrigation method. Five basic methods of the application of irrigation water to the fields to suit different crops, soil type, topography, water availability and its quality, climatic conditions, individual holdings and costs are (i) flooding irrigation, (ii) furrow irrigation, (iii) sprinkler irrigation, (iv) drip irrigation, and (v) sub irrigation, besides various subclasses. The primary objective of any irrigation method is to supply water to soil so that moisture will be readily available at all times for crop growth but without indiscriminately adding to the water table, as well as avoiding influence of soil salinity. An efficient irrigation method is that which best suits local conditions such as (i) soil characteristics, (ii) kind of crop and its age, (iii) crop rotation, (iv) topographic condition, (v) available water flow, (vi) underground water table condition (high or low), and (vii) state of soil salinity.
METHODS OF IRRIGATION Irrigation water can be applied to crop lands using one of the following irrigation methods : (i)Surface irrigation (a)Uncontrolled (or wild or free) flooding method, (b)Border strip method, (c)Check method, (d)Basin method, and (e)Furrow method (ii)Subsurface irrigation (iii)Sprinkler irrigation (iv)Trickle(Drip) irrigation. Each of the above methods has some advantages and disadvantages, andthe choice of the method depends on the following factors: (i)Size, shape, and slope of the field, (ii)Soil characteristics, (iii)Nature and availability of the water supply subsystem, (iv)Types of crops being grown, (v)Initial development costs and availability of funds, and (vi)Preferences and past experience of the farmer. Surface Irrigation In all the surface methods of irrigation, water is either ponded on the soil or allowed to flow continuously over the soil surface for the duration of irrigation. Although surface irrigation is the oldest and most common method of irrigation, it does not result in high levels of performance. This is mainly because of uncertain infiltration rates which are affected by year-to-year changes in the cropping pattern, cultivation practices, climatic factors, and many other factors. As a result, correct estimation of irrigation efficiency of surface irrigation is difficult. Application efficiencies for surface methods may range from about 40 to 80 per cent. (a) Uncontrolled Flooding When water is applied to the cropland without any preparation of land and without any levees to guide or restrict the flow of water on the field, the method is called µuncontrolled, wild or µfree flooding. In this method of flooding, water is brought to field ditches and then admitted at one end of the field thus letting it flood the entire field without any control. Uncontrolled flooding generally results in excess irrigation at the inlet region of the field and insufficient irrigation at the outlet end. Application efficiency is reduced because of either deep percolation (in case of longer duration of flooding) or flowing away of water (in case of shorter flooding duration) from the field. The application efficiency would also depend on the depth of flooding, the rate of intake of water into the soil, the size of the stream, and topography of the field. Obviously, this method is suitable when water is available in large quantities, the land surface is irregular, and the crop being grown is unaffected because of excess water. The advantage of this method is the low initial cost of land preparation (b) Border Strip Method Border strip irrigation (or simply µborder irrigation¶) is a controlled surface flooding method of applying irrigation water. In this method, the farm is divided into a number of strips. These strips are separated by low levees (or borders). Water from the supply ditch is diverted to these strips along which it flows slowly towards the downstream end and in the process it wets and irrigates the soil. When the water supply is stopped, it recedes from the upstream end to the downstream end. The border strip method is suited to soils of moderately low to moderately high intake rates and low erodibility. This method, however, requires preparation of land involving high initial cost.
(c) Check Method The check method of irrigation is based on rapid application of irrigation water to a level or nearly level area completely enclosed by dikes. In this method, the entire field is divided into a number of almost levelled plots (compartments or Kiaries) surrounded by levees. Water is admitted from the farmers watercourse to these plots turn by turn. This method is suitable for a wide range of soils ranging from very permeable to heavy soils. The farmer has very good control over the distribution of water in different areas of his farm. Loss of water through deep percolation (near the supply ditch) and surface runoff can be minimised an dadequate irrigation of the entire farm can be achieved. Thus, application efficiency is higher for this method. However, this method requires constant attendance and work (allowing and closing the supplies to the levelled plots). Besides, there is some loss of cultivable area which is occupied by the levees. Sometimes, levees are made sufficiently wide so that some µrow¶ crops can be grown over the levee surface. (d) Basin Method This method is frequently used to irrigate orchards. Generally, one basin is made for one tree. However, where conditions are favourable, two or more trees can be included in one basin. (e) Furrow Method In the surface irrigation methods, the entire land surface is flooded during each irrigation. An alternative to flooding the entire land surface is to constructs mall channels along the primary direction of the movement of water and letting the water flow through these channels which are termed as furrows, creases or corrugation. Furrows are small channels having a continuous and almost uniform slope in the direction of irrigation. Water infiltrates through the wetted perimeter of the furrows and moves vertically and then laterally to saturate the soil. Furrows are used to irrigate crops planted in rows. Furrows necessitate the wetting of only about half to one-fifth of the field surface. This reduces the evaporation loss considerably. Furrows provide better onfarm water management capabilities for most of the surface irrigation conditions, and variable and severe topographical conditions.For example, with the change in supply conditions, number of simultaneously supplied furrows can be easily changed.In this manner, very high irrigation efficiency can be achieved. The following are the disadvantages of furrow irrigation: i. Loss of water at the downstream end unless end dikes are used, ii. The necessity of furrow construction, iii. Possibility of increased erosion, and iv. Furrow irrigation requires more labour than any other surface irrigation method Subsurface Irrigation Subsurface irrigation (or simply sub irrigation) is the practice of applying water to soils directly under the surface. Moisture reaches the plant roots through capillary action. The conditions which favor sub irrigation are as follows: (i)Impervious subsoil at a depth of 2 meters or more, (ii)A very permeable subsoil, (iii)A permeable loam or sandy loam surface soil, (iv)Uniform topographic conditions, and (v)Moderate ground slopes. In natural sub irrigation, water is distributed in a series of ditches about 0.6 to 0.9 meter deep and 0.3meter wide having vertical sides. These ditches are spaced 45 to 90 meters apart.
Sometimes, when soil conditions are favorable for the production of cash crops (i.e., high-priced crops) on small areas, a pipe distribution system is placed in the soil well below the surface. This method of applying water is known as artificial sub-irrigation. Soils which permit free lateral movement of water, rapid capillary movement in the root-zone soil, and very slow downward movement of water in the subsoil are very suitable for artificial subirrigation. The cost of such methods is very high. However, the water consumption is as low as one-third of the surface irrigation methods. Sprinkler Irrigation Sprinkling is the method of applying water to the soil surface in the form of a spray which is somewhat similar to rain. In this method, water is sprayed into the air and allowed to fall on the soil surface in a uniform pattern at a rate less than the infiltration rate of the soil. This method started in the beginning of this century and was initially limited to nurseries and orchards. In the beginning, it was used in humid regions as a supplemental method of irrigation. This method is popular in the developed countries and is gaining popularity in the developing countries too. Rotating sprinkler-head systems are commonly used for sprinkler irrigation. Each rotating sprinkler head applies water to a given area, size of which is governed by the nozzle size and the water pressure. Alternatively, perforated pipe can be used to deliver water through very small holes which are drilled at close intervals along a segment of the circumference of a pipe. The trajectories of these jets provide fairly uniform application of water over a strip of cropland along both sides of the pipe. With the availability of flexible PVC pipes, the sprinkler system can be made portable too. Sprinklers have been used on all types of soils on lands of different topography and slopes, and for many crops. . The following conditions are favorable for sprinkler irrigation: i. Very previous soils which do not permit good distribution of water by surface methods, ii. Lands which have steep slopes and easily erodible soils, iii. Irrigation channels which are too small to distribute water efficiently by surface irrigation, and iv. Lands with shallow soils and undulating lands which prevent proper leveling required for surface methods of irrigation
.
Advantages: i. Low water loss (efficiency up to 80%) ii. Saving in fertilizer iii. Suitable for any topography iv. No soil erosion v. Better seed germination, free aeration of root zone vi. Uniform application of water Disadvantages i. High initial cost, cannot adopt by ordinary farmers ii. Poor application efficiency in windy weather and high temperature iii. High evaporation losses iv. Water should be free of debris v. Equipment need careful handling vi. Physical damage to crops by application of high intensity spray vii. Power requires for running pumping Trickle (Drip) Irrigation Trickle irrigation (also known as drip irrigation) system comprises main line, sub mains, laterals, valves (to control the flow), drippers or emitters (to supply water to the plants), pressure gauges, water meters, filters (to remove all debris, sand and clay to reduce clogging of the emitters), pumps, fertilizer tanks, vacuum breakers, and pressure regulators. The drippers are designed to supply water at the desired rate (1 to 10 liters per hour) directly to the soil. Low pressure heads at the emitters are considered adequate as the soil capillary forces causes the emitted water to spread laterally and vertically. Flow is controlled manually or set to automatically either (i) deliver desired amount of water for a predetermined time, or (ii) supply water whenever soil moisture decreases to a predetermined amount. A line sketch of a typical drip irrigation system is shown in Fig.
Advantages: i. Low water loss and hence saves water ii. Enhances plant growth and plant yield iii. Saves labor and energy iv. Control weed growth v. No soil erosion vi. Improves fertilizer application efficiency Disadvantages: i. High skill in design, installation, and subsequent operation ii. Clogging of small conduits and openings in emitters due to sand, clay particles, debris, chemical precipitates and organic growth iii. Not suitable for closely planted crops such as wheat and other cereal grains.
Quality of irrigation water Irrigation water must not have direct or indirect undesirable effects on the health of human beings, animals, and plants. The irrigation water must not damage the soil and not endanger the quality of surface and ground waters with which it comes into contact. The presence of toxic substances in irrigation water may threaten the vegetation besides degrading the suitability of soil for future cultivation. Surface water, ground water, and suitably treated waste waters are generally used for irrigation purposes. In examining the quality of irrigation water, attention is focused on the physical, chemical, and biological properties of such water. The various types of impurities, which make the water unfit for irrigation, are classified as : i. Sediment concentration in water ii. Total concentration of soluble salts in water iii. Proportion of sodium ions to other ions iv. Concentration of potentially toxic elements present in water v. Bacterial contamination
