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. . . s~Jrc~lrea increases due to reservoir releases.--An evaporation loss can be determined for the increase in the river's surface area with a reservoir release. This increase in surface area for various release discharges is shown as a function of the river flow at the Wel1sville gage in figure 8. Figure 8 shows that the increase in river surface area resulting from an average reservoir release of 400 ft3/s ranges from 250 to 10 acres for antecedent river flows of from 280 to 3,200 ft3/s, respectively. These curves were developed from aerial photographs, U.S. Geological Survey topographic maps, and gaging sta- tion discharge measurements. Based on the average monthly evaporation rates shown in table 6, figure 9 shows the increase in evaporation with. an increase in river surface area. It indicates evaporation from the Arkansas River is increased by a maximum of 4.5 ft3/s due to increase in surface area resulting from a reservoir release. Used in conjunction with figure 8, these curves can be used to determine the incremental evaporation loss due to the increase in river surface area during reservoir releases. River temperature increases due to reservoir re~eases.--An in- crease in evaporation also occurs when reservoir releases increase the temperature of the river. Temperature profiles for lower Twin Lakes obtained by D. B. Hoffman (written commun., 1971) and Nolting (1968), and Twin Lakes river temperatures obtained daily by the Otero Pumping Station, indicate Twin Lakes release water to be from 5.F to 6.5.F warmer than average river temperature during June, July, August, and September. Data were not available for Turquoise Lake or Clear Creek Reservoirs. To calculate this component of the evaporation loss during re- leases from Twin Lakes, the difference between river evaporation before and during a reservoir release was computed for the length of river required for temperature equilibrium. Although the average monthly evaporation rates shown in table 6 were used in these calculations, the results showed insensitivity to monthly variations in evaporation and only averages were used. Evaporation due to the increase in river temperature during releases from Twin Lakes Reservoir made in June, July, August, or September is given in figure 10. Figure 10 shows this increase in evaporation to be as much as 1.3 ft3Js during low ante- cedent river flows. For releases from Turquoise Lake or Clear Creek Reservoirs., this component of evaporation loss is insignificant. Due to the physical nature of Twin Lakes Reservoir, the lower lake from which releases are made is fed from the surface waters of the upper lake and is, therefore, unusually warm. The temperature of Turquoise Lake and Clear Creek Res- ervoir release water will be considerably nearer the river's temperature. 25