Figures 4.5 and 4.6 show the effect of wind speed on evaporation rates. Generally, higher 

 wind speeds (up to 6 mile/hr) result in greater evaporation . This is because the wind is 

 preventing the build-up of a diffusion barrier (Moore and Runkles, 1968). Figures 4.7 and 4.8 

 show the evaporation rate as a function of concentration as affected by wind speed, air 

 temperature and humidity. At high humidity a smaller vapor pressure difference is present 

 which in turn reduces the evaporation rate. Salinity decreases the vapor pressure of the film 

 next to the water surface. Therefore, evaporation decreases as salinity increases. Evaporation 

 decrease with increasing specific gravity is plotted in Figure 4.9. 



Lakshman (1975) studied the influence of wind and water temperature profiles overthe 

 water surface on evaporation rate using the following Dalton's based formula: 



E = N LP-* (es - ea) (1) 



where: 



and 



E is the evaporation in inches/hr 



N is the mass transfer coefficient 



U is the wind speed in mile/hr 



es is saturation vapor pressure in mbar 



ea is the vapor pressure of air at 2 m in mbar. 



N = [((3.9 X 10^) m«=n/{(m+l)'«(2m+l)o^] (C/2)'^°'(P/A)''2 (2) 



where: 



m is the wind profile exponent 



fi is the thickness of the turbulent boundary layer in meter 



P is the perimeter of the water body in feet 



A is the water surface in square feet 



For smaller bodies of water (e. g. sloughs and small reservoirs), the transfer coefficient 

 (N) can be simplified to give; 



N = (2.62 X 10^) (P/Ay 2 (3) 



A comparison between some of the computed (eq. 2) and experimental values of N in the 

 experiment is in the following table: 



Study Area (sq. fl.) N computed N experimental Error 



1 Blucher Dugout 6.3 x lO' 



2 Lake Hefrier 1.0 x 10» 



3 Wascana Lake (1969-1970) 2.3 x 10' 



The empirical equations 2 and 3 need to be verified with San Joaquin valley conditions 

 in which the evaporation rate from the evaporation ponds can be estimated accurately. 



page 4.3 



