1. EVAPORATION AND ITS MEASUREMENT
(Faculty Guide)
Prof. R.M.JADAV
PREPARED BY:-
SHAH PARITA

2. CONTENT INTRODUCTION MEASUREMENT OR ESTIMATION OF EVAPORATION
EMPIRICAL FORMULA
METHOD OF ACTUAL OBSERVATION
PAN OBSERVATION

3. INTRODUCTION
Evaporation is the process by which water is converted into gaseous state and is returned to the atmosphere as vapour.

4. The rate of evaporation is dependent on:
The vapor pressure at the water surface and air above
Air and water temperature
Wind speed
Atmospheric pressure
Quality of water
Size of water body

5. MEASUREMENT OR ESTIMATION OF EVAPORATION
The following methods are adopted to evaluate the rate of evaporation from a reservoir:
Empirical formulae
Water budget method
Energy budget method
Mass transfer method
Actual observations
Pan observations

6. 1. EMPIRICAL FORMULAE
The various variables used in the formulae in estimating evaporation from a reservoir are as follows:
E= Evaporation in mm/day
es= Saturated vapour pressure in mm Hg
ea= Actual vapor pressure in mm Hg
Pa= Mean barometric pressure in mm Hg
V= Wind velocity at the water surface in km/h
Ta= Average air temperature in C
Tw= Average water temperature in C

7. Fitzgerald’s equation (1886):
E=( V)(es-ew)
Rohwer’s equation (1931):
E=0.771( Pa) ( V)
(es-ea)

8. 2. WATER BUDGET METHOD
Evaporation from a reservoir or water body can be determined by the following water budget equation:
E = P + I – (Ui-Oo) – O ± ∆S
where, E= Evaporation
P= Precipitation
I= Surface inflow
Ui= Underground inflow into the reservoir
Uo= Underground outflow from the reservoir
O= Ouflow from reservoir
∆S= Change in storage
This method is not accurate may give rough idea for evaporation.

9. 3. METHOD OF ACTUAL OBSERVATIONS
Evaporation can be measured by an instrument known as atmometer.
The atmometers are provided with some special surface which are kept wet and from these surfaces the evaporation takes place.
The water loss observed at all the source is and indication of evaporation.
PICHE ATMOMETER

10. 4. PAN OBSERVATION
The five methods of estimation of evaporation are not directly applicable in design problem and also in some cases required data are not easily available.
Therefore in most of the design problems, evaporation is measured by evaporation pans which are called evaporimeters.
Pans are most commonly used to measure evaporation.
A pan is a metal container, square or circular, and of uniform cross section. Normally it is circular.
The commonly used pans are Colorado Sunken pan, US geological survey floating pan, ISI standard pan and the US Weather Bureau Class A pan.

11. US WEATHER BUREAU CLASS A PAN
122 cm
Wooden support
25.4 cm
15 cm
Galvanized steel

12. THANKYOU