EVAPORATION AND ITS MEASUREMENT (Faculty Guide) Prof. R.M.JADAV PREPARED BY:- SHAH PARITA 140283106021

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

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

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

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

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

Fitzgerald’s equation (1886): E=(0.4+0124V)(es-ew) Rohwer’s equation (1931): E=0.771(1.465-0.000732Pa) (0.44+0.0733V) (es-ea)

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.

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

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.

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

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