MEASUREMENTS OF HYDROLOGICAL VARIABLES

Shariq Pervez Civil Engineering UET Peshawar

MEASUREMENTS OF HYDROLOGICAL VARIABLES

Hydrological Variables  Precipitation

 Evaporation  Transpiration

 Wind Velocity  Humidity  Radiation  Evapotranspiration

Precipitation  Precipitation is any form of water that falls to the Earth's surface. Different

forms of precipitation include rain, snow, sleet, hail and drizzle.

How Precipitation Is Measured  Standard rain guage

 Weighing precipitation gauge  Tipping bucket rain gauge

 Optical rain gauge  Weather station  Radar

Standard Rain Guage  Often have a funnel opening into a cylinder

guage  Come in a variety of shapes and sizes  Calculate the rainfall (in mm) by dividing the volume of water collected by the area of the opening of the cup.

Standard Rain guage

Placement of Rain Gauges  Gauges are affected by wind pattern, trees

and the gauge itself, therefore it is important to have the gauge located and positioned properly.  1m above ground level is standard.  2 to 4 times the distance away from an isolated object (such as a tree or building).

Rainfall Depth

The Rain Gauge

The cross-sectional of measuring tube is 1/10 the crosssectional area of the outer tube.

So, 10 mm of rain falling into the outer tube looks like it fills the measuring tube 10 cm high. Measurement is 10 mm!

Monthly Climatic Report

Evaporation  Evaporation is the process by which water is

converted from its liquid form to its vapor form and thus transferred from land and water masses to the atmosphere. OR  Process by which water is changed from the liquid or solid state into the gaseous state through the transfer of heat energy (ASCE, 1949).

Methods For Estimating Evaporation  Water budget methods

 Energy budget methods  Mass transfer techniques (e.g., Meyer,

Thornthwaile-Holzman)  Combination of energy budget and mass transfer methods (e.g.,Penman)

Energy Budget Method Total solar radiation - Rt Net energy advected (net energy content of incoming and outcoming water -

Ee

Reflected solar radiation - Rr Energy used for evaporation (latent heat)- Ee

Sensible heat loss from the water body to the atmosphere - Hn

Energy stored - Es

Net long-wave radiation exchange between the atmospere and the water body- R1

Energy Budget Method

g cal Es cm2 - day

Ea

Rt

Rr

Ee

Hn

R1

Energy Budget Method Characteristics:  Most accurate method (evaporation is a

function of the energy state of the water system).  Difficult to evaluate all terms.  Energy balance equation has to be simplified.  Empirical formulas are used.

Water Budget Method

Precipitation - P Inflow- Q

Evaporation- E Surface runoff - Qr

Subsurface runoff - Qs Outflow- Q0

S t

P Q Qr

Qs

Subsurface seepage losses- Qd

Q0 Qd

E

E

Water Budget Method Characteristics:  Simple

 Difficult to estimate Qd and Qs  Unreliable, accuracy will increase as Δt

increases

Transpiration  Transpiration is the evaporation occurring

through plant leaves (stomatal openings).  Transpiration is affected by plant physiology and environmental factors, such as:  Type of vegetation

 Stage and growth of plants  Soil conditions (type and moisture)  Climate and weather

How Transpiration is Measured

Capillary tube

Movement of meniscus is measured over time

Wind Velocity  Wind velocity can be measured by a device

called anemometer.  Two types of anemometers  Velocity anemometers  Pressure anemometers

Anemometer

Anemometer

Anemometer  The air flow past the cups in any horizontal

direction turning the cups.  The no. of turns is proportional to the average wind speed.  The needle show the direction of the wind.

Humidity  Humidity is the amount of water vapor in the

air.  Types of humidity are:  Absolute humidity  Relative humidity  Specific humidity

 Humidity during rain

 Sling psychrometer is used to measure

humidity.

Sling Psychrometer

Hygrometer  Hygrometer is used for measuring relative humidity.

 Psychrometer is the simplest form of hygrometer.

Radiation  Solar radiation

entering Earth’s atmosphere consists of direct, diffuse, and albedo radiation.

Radiation

How Radiation is Measured Radiation can be measured by:  Pyrheliometer  Pyranometer

Pyrheliometer

Pyranometer

Evapotranspiration  Combined “loss” of water

vapor from within the leaves of plants (“transpiration”) and evaporation of liquid water from water surfaces, bare soil and vegetative surfaces.

Evapotranspiration Measurement  Evapotranspiration can be measured by:  Lysimeter

Rain Transpiration Evapo-transpiration

Irrigation

Evaporation

Runoff Root Zone

Below Root Zone

Water Storage

Drainage

Lysimeter

Lysimeter

Lysimeter

Evapotranspiration Report

Weather Station  A weather station is a facility with

instruments which can measure  Temperature  Humidity  Wind velocity  Barometric pressure

 Precipitation

Weather Station

References  http://www.wikipedia.org/

 Research papers

Thanks