Final Evaporation Control in reservoirs

4.4
 
Mass Transfer or Humidity and Wind Velocity Gradient Method 
As the name suggests two important factors (i) humidity and (ii) wind velocity form the basis of 
calculating the evaporation. The basic assumptions involved in this method are: 
i)
If the moisture gradient exists in air, water vapours will move towards points of lower 
moisture content. 
ii)
The rate of movement of water vapour is accentuated by the intensity of turbulence in the 
air. 
The method is applicable to both land and water surfaces. However, relatively expensive and 
highly sensitive hygrometers and wind velocity meters are required to measure the corresponding 
factors simultaneously at two different elevations above the ground.
4.5
 
Energy Budget / Insolation Method 
This method was suggested by Angstorm in 1920 and is based on the conservation of heat within 
the body. For any given body of water, a balance must exist between heat gains and losses. Heat 
is normally gained by long and short-wave radiation, conduction and condensation. Heat loses 
result from direct land reflected radiation, conduction convection and evaporation. Radiation 
conduction, convection and changes in the energy storage in water may be measured and the 
evaporation or condensation may be computed. In this type of calculation some factors, such as 
heating due to chemical and biological processes, conduction of beat through the lake bottom and 
transformation of kinetic energy into thermal energy are considered insignificant. Different 
equations relating each of the parameters mentioned have been developed by various research 
workers in the field. This method suffers from the main draw back of measuring various 
parameters accurately with the help of sensitive and costly equipment. 

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