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PROCEDURE In the late 1950's several evaporation pans were installed near Lake Powell, Flaming Gorge, and Navajo Reservoirs. Data from pans at the following locations were collected for the May-October period for 1958 through 1962 and converted into an average annual evaporation rate. Average Annual Gross Evaporation Rate in Inches Location of Pan Hite Mexican Hat Hoab Page Total Average 59.87 73.17 58.52 69.13 260.73 65.18 A pan was established at Green River, Utah, but the data was not used. A pan was installed at Wahweap in 1962, but the data was not available when the average annual evaporation rate was determined. A method was established to determine the evapo-transpiration from the pre- reservoir Lake Powell area and data was collected before the Lake started to fill. The amount of evaporation caused by Lake Powell was estimated by multiplying the evaporation rate (65.18 inches) by the lake area and subtracting the evapo-transpiration from the equivalent pre-reservoir area. This evaporation has been used to the present time (Table 8). The method and the computations for determining the evaporation from the equivalent pre-reservoir is in the files of the Water Operations Branch of the Upper Colorado Region. Studies of several methods of determining evaporation had been made at Lake Hefner in Oklahoma and Lake Mead. It was decided that collecting data for the energy-budget method would be very expensive for periods of time less than 1 year. Since it was desirable to determine evaporation for shorter periods of time, it was decided not to use the energy-budget method. It was also desirable to make a determination independent of the pan method. Therefore, the mass-transfer method was selected. Three rafts were constructed, instrumented, and placed at Wahweap Bay, Padre Bay, (also called Crossing-of-the-Fathers), and Bullfrog Bay as shown in Figure I. The data was processed in the early 1970's but there was some concern as to the coefficient to use in the equation. At that time scien- tists from several universities were investigating several aspects of Lake Powell. The studies, sponsored by the National Science Foundation, were named the Lake Powell Research Project. A cooperative agreement was made to make an evaporation study as part of their water budget analysis. The Bureau furnished instruments and another raft which was installed at Hite. The Bureau participated in this to obtain data to calibrate a mass-transfer coefficient for the data we had collected. New cassette recorders and sensors were installed on the rafts along with the old equipment which was still being used. Data was collected for about 2 years and analyzed by the energy-budget and mass-transfer methods and compared with pan measurements. 2