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Evaporation Several methods have been used in the past to make this estimate, including pan evaporation, energy budget, and mass transfer. Each have advantages and all yield approximately the same value for gross evaporation, 5 to 5,5 feet per year, Annual net evaporation from Lake Powell is estimated by Reclamation to be about 600,000 acre-feet at full reservoir conditions, This annual rate includes a credit or reduction for native evapotranspiration from the reservoir basin, This credit accounts for the water that was used by plants and was evaporated from the river and adjoining lands in the area now inundated by the reservoir, After the dam was constructed and the reservoir inundated these areas, the water previously'lost was salvaged and is now applied as a credit against evaporative losses from the reservoir, Bank Storaie Since the reservoir is surrounded primarily by porous sandstone fonnations, a certain amount of water is absorbed from the reservoir. During the initial filling period, this volume of water could have been a substantial portion of the reservoir inflow as the water gradually seeped outward ,from the reservoir, Numerous wells have been drilled around the reservoir in an effort to detennine the effect of this seepage, but the problem is a difficult one to resolve with any real accuracy, During the annual fluctuation of the level of Lake Powell, the reservoir level rises during the spring runoff and falls during the remainder of the year. As the reservoir rises, it is assumed that a percentage of the increased storage is absorbed into the bank and that as the reservoir level falls, a percentage of this reservoir storage differential is returned to the lake, Reclamation currently uses 8 percent of the storage change as the volume estimated to move in and out of the bank, It is assumed that gradually, the rate of seepage into the bank has slowed, as the rock became saturated for a greater distance from the lake, Steady state conditions are expected as the water pressure from the lake is balanced by head loss through the sandstone, There may be some amount lost to evaporation from the rock but this has not been quantified, Sedimentation There have been published claims that Lake Powell will sustain enough loss through sedimentation that within 13 5 years, the sediment will reach the penstock intakes, That would take the powerplant out of operation, That estimate was based upon an assumption that reservoirs fill in with sediment from the bottom to the top going up the face of the dam, However, research conducted in 1986 for the 1986 Lake Powell Survey, published by Reclamation in December 1988, indicates that is not the pattern for Lake Powell, The reservoir is filling from the upper reaches of the reservoir, some 186 miles from the dam and trending downstream to the dam, The heavier, courser sediment is deposited at the head of the reservoir with fine clay and silt moving further downstream, At the rate measured, the best estimate is that it will be between 300 and 500 years before sediment reaches the penstock intake structures, 7