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<br />COMPUTATIONS <br /> <br />Equation (3) was used to compute a daily evaporation rate. It was devel- <br />oped from equation (2) which was developed specifically for Lake Mead. <br /> <br />E = 3.27xlO-3u(eo-0.005(Hmax+Hmin)eas) <br /> <br />(3) <br /> <br />Where <br /> <br />E = evaporation in inches per day <br />u = average wind speed in miles per hour <br /> <br />eo = saturation vapor pressure at water surface temperature in <br />millibars <br /> <br />Hmax = maximum relative humidity as percentage <br /> <br />Hmin = minimum relative humidity as a percentage <br /> <br />eas = saturation vapor pressure in millibars at average daily air <br />temperature. <br /> <br />The vapor pressure of the air is approximated by multiplying the average <br />daily humidity by the saturation vapor pressure at the average daily air <br />temperature. Thus the .005 in the equation is from dividing by 100 to con- <br />vert from percentage and dividing by 2 to get an average. The computations <br />were done by a computer program termed EVAP. The daily evaporation rates <br />were totaled for each month. They are shown in Table 2. <br /> <br />COMPARISON OF EVAPORATION RATES <br /> <br />Table 3 is pan evaporation as measured at Wahweap. Table 4 is the Lake <br />Powell evaporation rate in inches from the adjusted class A pan at Wahweap <br />Arizona. This adjustment is described in Lake Powell Research Bulletin <br />Number 48. The average annual evaporation rate for the evaporation pan <br />method was 69.44 inches for the 1962-1975 periOd while the rate for the <br />mass-transfer method was 68.32 inches for the same period. <br /> <br />GROSS EVAPORATION <br /> <br />The gross evaporation is computed by multiplying the lake surface area by <br />the evaporation rate in Table 2. This is shown in Table 5 and column (1) <br />of Table 8. <br /> <br />NET EVAPORATION <br /> <br />Net evaporation which is the amount of increased evaporation caused by the <br />reservoir is needed for some purposes. It is needed to establish water <br />supply data that is homogeneous for the period before and after Lake Powell <br />started to fill. It is also needed to determine depletion. <br /> <br />Net evaporation is determined by subtracting from the gross evaporation the <br />evapotranspiration that would have occured from the reservoir basin if the <br /> <br />8 <br />