5/27/2018 WATER LOSSES

1/28

5/27/2018 WATER LOSSES

2/28

Evaporation:(a) from water surface, i.e., reservoirs, lakes,

ponds, river channels, etc.

(b) from soil surface, appreciably when the ground

water table is very near the soil surface.

Transpirationfrom plant leaves.

Evapotranspiration for consumptive usefromirrigated or cropped land.

Infiltrationinto the soil at the ground surface.

Watershed leakageground water movementfrom one basin to another or into the sea.

http://localhost/var/www/apps/conversion/tmp/scratch_5/%E2%96%B6%20Evaporation%20and%20Transpiration%20[720p].mp4http://localhost/var/www/apps/conversion/tmp/scratch_5/%E2%96%B6%20Evaporation%20and%20Transpiration%20[720p].mp4

5/27/2018 WATER LOSSES

3/28

Evaporation from free water surfaces and soilare of great importance in hydro-meterological studies. (?)

Evaporation from water surfacesThe factors affecting evaporation are air and

water temperature, relative humidity, windvelocity, surface area (exposed), barometricpressure and salinity of the water.

Not : The a minor effect of evaporation are barometric pressureand salinity of the water

5/27/2018 WATER LOSSES

4/28

This is done by the following methods Using evaporimeters

Using empirical equations

Using analytical methods

5/27/2018 WATER LOSSES

5/28

Evaporimeters These are pans containingwater which are exposed to the atmosphere.Loss of water by evaporation from these pansare measured at regular intervals (daily).

Meteorological data such as humidity, windvelocity, air and water temperatures, andprecipitation are also measured and notedalong with evaporation.

5/27/2018 WATER LOSSES

6/28

255 mm

150 mm

5/27/2018 WATER LOSSES

7/28

WMOrecommends the following values of minimumdensity of evaporimeters

Arid Zones 1 station for every 30,000 sq.km Humid Temperate Zones 1 station for every 50,000

sq.km

Cold regions 1 station for every 1,00,000 sq.kmA typical hydro-meteorological station has thefollowing: Recording raingauge and non-recording raingauge Stevenson box with maximum, minimum, wet, and

dry bulb thermometers Wind anemometer and wind vane Pan evaporimeter Sunshine Recorder etc

http://localhost/var/www/apps/conversion/tmp/scratch_5/WMO%20in%20Brief%20-%20World%20Meteorological%20Organization%20(WMO).htmhttp://localhost/var/www/apps/conversion/tmp/scratch_5/WMO%20in%20Brief%20-%20World%20Meteorological%20Organization%20(WMO).htm

5/27/2018 WATER LOSSES

8/28

Daltonslaw of evaporationThe rate of evaporation is a function of the differences in

vapour pressure at the water surface and in the atmosphere,and the Daltonslaw of evaporation is given by .

5/27/2018 WATER LOSSES

9/28

5/27/2018 WATER LOSSES

10/28

Accounts for the effect of pressure inaddition to the wind speed effect

5/27/2018 WATER LOSSES

11/28

1) Water Budget Method

can only be measured

5/27/2018 WATER LOSSES

12/28

The following are some of the methods ofestimating evapotranspiration:

(i) Tanks and lysimeter experiments

(ii) Field experimental plots

(iii) Installation of sunken (colarado) tanks (iv) Evapotranspiration equations as developed by

Lowry-Johnson, Penman, Thornthwaite, Blaney-Criddle, etc.

(v) Evaporation index method, i.e., from panevaporation data as developed by Hargreavesand Christiansen.

5/27/2018 WATER LOSSES

13/28

However, two well known methods arediscussed here.

(i) Blaney-Criddle method.

(ii) Evaporation Index method..

5/27/2018 WATER LOSSES

14/28

This method is used throughout the world forthe consumptive use determinations and isgiven by :

5/27/2018 WATER LOSSES

15/28

5/27/2018 WATER LOSSES

16/28

5/27/2018 WATER LOSSES

17/28

Analysis of data on consumptive use indicate ahigh degree of correlation between panvaporation values and consumptive use. Therelationship between the evapotranspiration

(Et) and pan evaporation (Ep) is usuallyexpressed as

5/27/2018 WATER LOSSES

18/28

5/27/2018 WATER LOSSES

19/28

5/27/2018 WATER LOSSES

20/28

Water entering the soil at the ground surfaceis called infiltration.

Infiltration is important for: Soil water recharge (e.g., for plants)

Groundwater recharge

River base flow

Contaminant movement

builds up the ground water table.

5/27/2018 WATER LOSSES

21/28

f0ft=fc+ f0-fc)e-ktfcfinfiltration

time twhen if fa=fi< f fa=iwhere, fa=actual infiltration ratef =infiltration capacity

i =intensity of rainfallinfiltr tion c p city t time t fter the beginning of the stormft=fc+ f0-fc)e ktwhere, fo=initial infiltration capacityfc=limiting constant value of the infiltrationThe values of fc, foand k depends upon the type and condition ofsoil

5/27/2018 WATER LOSSES

22/28

soil moisture type of soil medium permeability

vegetal cover compaction of soil available storage in soil stratum depth of surface detention

temperature of water other factors

5/27/2018 WATER LOSSES

23/28

2. Rainfall simulator-type infiltrometer

5/27/2018 WATER LOSSES

24/28

Single Ring Double Ring

http://upload.wikimedia.org/wikipedia/en/e/e9/Double_ring.JPGhttp://upload.wikimedia.org/wikipedia/en/a/aa/Single_ring.JPG

5/27/2018 WATER LOSSES

25/28

An average constant value of infiltration iscalled infiltration index.

Two types of infiltration indices arecommonly used.-index

1. -index2. w index

There are extremely used for the analysis ofmajor floods when the soil is wet and the

infiltration rate becomes constant

5/27/2018 WATER LOSSES

26/28

The value of f-index can be derived from the rainfallhyetograph and the resulting surface runoff volume by trial anderror.

The unshaded area below the horizontal line is assumed that alllosses are due to infiltration only.

For determination of - index, a horizontal line is drawn on thehyetograph such that the shaded area above that line is equalto the volume of surface runoff.

If the shaded area is not equal to the volume of measuredsurface runoff, the horizontal line is shifted upwards ordownwards till this condition is satisfied.

5/27/2018 WATER LOSSES

27/28

w-index=(P-R-S)/tfwhere P=total storm precipitation (cm)

R=total surface runoff (cm)S=depression and interception losses (cm)

tf=time period (in hours)The w-index is more accurate than the -index

because it excludes the interception and depression

5/27/2018 WATER LOSSES

28/28

For a storm of 3-hr duration, the rainfall rates are as follows

1.2.0.2.8.4.4ainfall rate cm/hr)3000000Time period (min)

If the surface runoff is 3.4 cm, determine the -index and w-index.Solution

Assume that f-index is more than 1.4 cm/hr. Therefore, surface runoffR=[(3.4-)+(4.8-)+(3.2-)+(2.0-)]30/60

3.40=6.7-2=1.65 cm/hrAs the computed value of f-index is greater than 1.4 cm/hr, the assumption wastotal precipitation P=(1.4+3.4+4.8+3.2+2.0+1.2)30/60

=8 cmW-index=(P-R-S)/tf, assume S = 0

=(8 0-3 4)/3 0 =1 53 cm/hr