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* r <br />, <br />Method 3. Empirical method ( Harbeck, 1962). Accoi'ding`to Harbeck, <br />reservoir evaporation in feet per day can be calculated using the <br />equation <br />E = Nu2 (eo - ea) <br />(5) <br />where N is 0.00028 for a 1 -acre surface area, u2 is wind speed <br />at 2 meters above water surface in miles per hour, e o is the <br />saturation vapor pressure in millibars, corresponding to the <br />temperature of the water surface, and e is the vapor pressure <br />of the air, in millibars. Mean monthly river temperatures were <br />estimated from data provided by the following: <br />Northside Waterworks, Pueblo; <br />Southern Colorado Power Company, Canon City; <br />Colorado Game Fish and Parks Department, Salida; <br />Otero Pumping Station, Homestake Project, Granite. <br />The evaporation rates determined by this method were much <br />lower during midsummer than rates determined by the other <br />methods. The reason is the importance of the wind factor <br />in this method and the fact that May, early June, and late <br />September are relatively windy above Canon City. <br />All lake evaporation rates determined by these methods were <br />multiplied by a factor of 1.7 which, according to Delay and Seaders <br />(1963), allows for higher evaporation rates from streams. Table 6 <br />shows the results of these determinations for the five common irri- <br />gation months. <br />Table 6.-- Summary of Arkansas River evaporation caZculations <br />Mean monthly river evaporation, <br />Method in feet per day <br />Standard pan evaporation -- <br />Modified pan evaporation -- <br />Empirical ----------------- <br />Average-------- - - - - -- <br />May <br />June <br />July <br />August <br />September <br />0.026 <br />0.030 <br />0.030 <br />0.026 <br />0.022 <br />.030 <br />.032 <br />.032 <br />.027 <br />.021 <br />.034 <br />.026 <br />.018 <br />.018 <br />.022 <br />.030 .029 .027 .024 .022 <br />24 0 <br />