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INTRODUCTION In the early 1950's it was expected that a highly credible determination of evaporation from reservoirs in the Colorado Basin was required because of the need to divide the water between the Upper and Lower Basins according to the Colorado River Compact of 1922. The average evaporation from Colorado River Storage Project Reservoirs (Lake Powell, Flaming Gorge, Navajo, Blue Mesa, Morrow Point and Crystal) is estimated at about 600,000 acre-feet per year. About 80 percent of this is evaporated from Lake Powell. Therefore, a plan was developed to measure and collect data. Since that time it has been determined that the Upper Basin will probably not be able to develop the 7,500,000 af per year allotted by the Colorado River Compact*. Therefore, at the present time, the accuracy of the evap- oration determination is not so critical. But the determination is still needed for several purposes including water use and availability studies, streamflow forecasting, and water budget studies. This report describes and comparies several methods of determining evapora- tion from Lake Powell. METHODS OF DETERHINATION Although evaporation from a reservoir cannot be measured directly, there are several ways of determining it indirectly. One way is termed the water budget method. In this method, the inflow must equal the outflow plus or minus the change in storage. But this method cannot be applied to Lake Powell because not all of the inflow can be measured and because the bank storage cannot be measured. Evaporation pan measurements can be used. This method is being used as a check but it has disadvantages in that pans do not account for the change in energy storage nor for advected energy. A third way is the energy-budget method. If all of the energy entering and leaving the Lake can be measured then the energy required to balance the energy-budget is that needed for evaporation. This method is used as a check at Lake Mead for annual values. This method requires data which is expensive to obtain for periods of time shorter than I year. Since surface area is changing during the year it was desirable to use a method for which it was practical to obtain data to complete evaporation for periods of time shorter than a year. Another method is based on Dalton's law which says that the rate of evapor- ation is a function of the differences in the vapor pressure at the water surface. This is termed the mass transfer method and the one used in this study. *Nothing in this report is intended to interpret the provisions of the Colorado River Compact (45 Stat. 1057), the Upper Colorado River Basin Compact (63 Stat. 31), the Water Treaty of 1944 with the United Mexican States (Treaty Series 994, 59 Stat. 1219), the Decree entered by the Supreme Court of the United States in Arizona v. California et al. (379 U.S. 340), the Boulder Canyon Project Act (45 Stat. 1057), the Boulder Canyon Project Adjustment Act (54 Stat. 774; 43 U.S.C. 618a), the Colorado River Storage Project Act (70 Stat. 105.; 43 U.S.C. 620), or the Colorado River Basin Project Act (B2 Stat. B85; 43 U.S.C. 1501). Pursuant to the Colorado River Basin Project Act (Public Law 90-537) of 1968.