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1 <br />?J <br /> <br />?B <br />gg8 <br />CS <br />Lh <br />Figure 1: Comparison of hourly evaporation rates determined from measurements in a shallow <br />pan with those computed via the combined approach. <br />1.0- opm vramr <br />25 ApW 1961 <br />0.6 <br />0.6 <br />0.4 <br />0.2- <br />00'1- 06 12 12 <br />Time (4 <br />24 MST <br />Additionally, we have run the calculation at one of our Northwest Colorado mountain sites <br />• (elevation = 8,200 feet) that uses an evaporation pan to determine the local correlation. <br />CALCULATE PAN EVAPORATION 11ACKGROUN0 <br />For our on site calculations we use the following equation to calculate the vapor pressure <br />difference: <br />eg - ea = 33.86[(0.00738Ta + 0.8072)8 - (0.00738Td +0.8072)8] <br />The temperature terms T. and Td represent the air temperature and the dew point in degrees <br />Celsius respectively. This equation represents the actual vapor pressure difference to within <br />1 % for all temperatures above 27 degrees Celsius. The dewpoint temperature is calculated <br />from the relative humidity measured on site. <br />The ambient parameters measured on site to calculate the pan evaporation are wind speed, <br />temperature, and relative humidity. <br />Comparability. <br />The Penman combined equation has been shown to give reliable estimates of the daily pan <br />evaporation. A study by Van Bavel in 1966investigated the correlation between the Penman <br />equation and a shallow pan in Tempe, AZ (Figure 1). <br />Aks&EnpCa1C.TECNMEM0062120 -ft <br />2