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Sufkco Irea increases due to reservoir releases. - - An evaporation <br />loss can be determined for the increase in the river's surface area <br />with a reservoir release. This increase in surface area for various <br />release discharges is shown as a function of the river flow at the <br />Wellsville gage in figure 8. Figure 8 shows that the increase in river <br />surface area resulting from an average reservoir release of 400 ft /s <br />ranges from 250 to 10 acres for antecedent river flows of from 280 to <br />3,200 ft /s, respectively. These curves were developed from aerial <br />photographs, U.S. Geological Survey topographic maps, and gaging sta- <br />tion discharge measurements. <br />Based on the average monthly evaporation rates shown in table 6, <br />figure 9 shows the increase in evaporation with an increase in river <br />surface area. It indicates evaporation from the Arkansas River is <br />increased by a maximum of 4.5 ft /s due to increase in surface area <br />resulting from a reservoir release. Used in conjunction with figure <br />8, these curves can be used to determine the incremental evaporation <br />loss due to the increase in river surface area during reservoir <br />releases. <br />River temperature increases due to reservoir releases. - - An in- <br />crease in evaporation also occurs when reservoir releases increase <br />the temperature of the river. Temperature profiles for lower Twin <br />Lakes obtained by D. B. Hoffman (written commun., 1971) and Nolting <br />(1968),and Twin Lakes river temperatures obtained daily by the Otero <br />Pumping Station, indicate Twin Lakes release water to be from 5 ° F to <br />® 6.5 ° V warmer than average river temperature during June, July, August, <br />and September. Data were not available for Turquoise Lake or Clear <br />Creek Reservoirs. <br />To calculate this component of the evaporation loss during re- <br />leases from Twin Lakes, the difference between river evaporation before <br />and during a reservoir release was computed for the length of river <br />required for temperature equilibrium. Although the average monthly <br />evaporation rates shown in table 6 were used in these calculations, <br />the results showed insensitivity to monthly variations in evaporation <br />and only averages were used. Evaporation due to the increase in river <br />temperature during releases from Twin Lakes Reservoir made in June, <br />July, August, or September is given in figure 10. Figure 10 shows this <br />increase in evaporation to be as much as 1.3 ft /s during low ante- <br />cedent river flows. <br />For releases from Turquoise Lake or Clear Creek Reservoirs,, this <br />component of evaporation loss is insignificant. Due to the physical <br />nature of Twin Lakes Reservoir, the lower lake from which releases are <br />made is fed from the surface waters of the upper lake and is, therefore, <br />unusually warm. The temperature of Turquoise Lake and Clear Creek Res- <br />ervoir release water will be considerably nearer the river's temperature. <br />25 <br />