The Rate of Natural Evaporation from Shallow Ponds

Abstract

The meteorological factors determining the rate of natural evaporation from " free " water surfaces are : the radiant energy supply from the Sun and atmosphere, the air temperature in contact with the surface, the absolute humidity of the air, and the wind velocity and turbulence over the surface. The relevant relationship of these factors has been expressed in a differential equation, whose solution, given proper numerical instantaneous values of the determining factors and their variation with time, yields the value of the depth of liquid evaporated in a given period from small areas of shallow ponds of different depths. Using different values of the parameters, a number of solutions of the equation were obtained by means of a. differential analyser. It is shown that, provided the depth of the water is greater than about 16 cm., a simpler equation using mean values of the radiant energy flux and the air temperature over short periods of time yields almost identical results. The use of this equation for the prediction of evaporation from shallow ponds exposed to known climatic conditions is discussed.