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• <br />Consumptive Uses and Losses 7 <br />Reservoir Evaporation <br />A comprehensive listing was developed of all reservoirs in the Upper Colorado River Basin <br />that included the latitude, longitude, elevation, and surface area at total capacity for each <br />reservoir. <br />Monthly evaporation estimates or content records were obtained for those reservoirs for <br />which records are available. The average annual water - surface area was determined for <br />each year of the reporting period. For those reservoirs lacking records, a "fullness factor" <br />was estimated on the basis of reservoir use and historical hydrologic conditions. These <br />"fullness factors" were then used to obtain estimates of average annual water surface area <br />for the unreported reservoirs. For all reservoirs without monthly evaporation estimates, <br />annual free water surface (FWS) evaporation rates were used to determine reservoir <br />evaporation. <br />The FWS evaporation value was taken from NOAH Technical Report NWS 33, <br />"Evaporation Atlas for the Contiguous 48 United States ", June 1982, Map 3 of 4 Annual <br />FWS Evaporation based on the reservoir location information. An account was taken of <br />precipitation and runoff salvage to determine net evaporation rates. The net evaporation <br />rates were applied to the estimates of average annual water- surface area to yield the <br />values of annual reservoir evaporation. <br />An exception to this procedure was the determination of evaporation from what are called <br />the main stem reservoirs shown in table UC =1. Predetermined' average evaporation rates <br />were applied to historical surface areas to yield values of evaporation on a monthly basis. <br />Ground Water <br />Currently, all ground -water pumping is counted as consumptive use charged against <br />the Colorado River Basin. Obviously,' this is not necessarily true. Depending on the <br />location and depth of the well and what types of soils are present in the area, it is possible <br />that little or none of the water pumped would have contributed to the Colorado River <br />System for hundreds or even thousands of years. It has recently been proposed that an <br />interagency study team be put together consisting of personnel from various State <br />Engineers Offices, Bureau of Reclamation, and any other pertinent agencies. This study <br />team would establish guidelines for computing what amounts of ground water pumped <br />should be charged against the Colorado River Basin. These guidelines will need to be <br />established on an area by area basis rather than one set percentage for the entire basin. <br />Results of this study will be incorporated in future Consumptive Uses and Losses Reports. <br />However, until these guidelines are established, the Consumptive Uses and Losses <br />Reports will continue to report all ground -water pumping as depletion from the basin. <br />Although significant ground -water usage occurs in Arizona, and New Mexico, for purposes <br />of this report ground -water overdraft has not been taken into account in the computation of <br />tributary consumptive use. <br />0 <br />