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<br />Evaporation <br /> <br />Several methods have been used in the past to make this estimate, including pan evaporation. <br />energy budget. and mass transfer, Each have advantages and all yield approximately the same <br />value for gross evaporation. 5 to 5,5 feet per year, Annual net evaporation from Lake Powell is <br />estimated by Reclamation to be about 600,000 acre-feet at full reservoir conditions, This annual <br />rate includes a credit or reduction for native evapotranspiration from the reservoir basin, This <br />credit accounts for the water that was used by plants and was evaporated from the river and <br />adjoining lands in the area now inundated by the reservoir, After the dam was constructed and <br />the reservoir inundated these areas, the water previously lost was salvaged and is now applied as a <br />credit against evaporative losses from the reservoir. <br /> <br />Bank Storai'e <br /> <br />Since the reservoir is surrounded primarily by porous sandstone formations, a certain amount of <br />water is absorbed from the reservoir, During the initial tilling period, this volume of water could <br />have been a substantial portion of the reservoir intlow as the water gradually seeped outward <br />, from the reservoir, Numerous wells have been drilled around the reservoir in an effort to <br />determine the effect of this seepage. but the problem is a dimcult one to resolve with any real <br />accuracy, During the annual tluctuation of the level of Lake Powell, the reservoir level rises <br />during the spring runoff and tails during the remainder of the year, As the reservoir rises, it is <br />assumed that a percentage of the increased storage is absorbed into the bank and that as the <br />reservoir level falls, a percentage of this reservoir storage differential is returned to the lake, <br /> <br />Reclamation currently uses 8 percent of the storage change as the volume estimated to move in <br />and out of the bank, It is assumed that gradually, the rate of seepage into the bank has slowed, as <br />the rock became saturated for a greater distance from the lake, Steady state conditions are <br />expected as the water pressure from the lake is balanced by head loss through the sandstone, <br />There may be some amount lost to evaporation from the rock but this has not been quantified, <br /> <br />Sedimentation <br /> <br />There have been published claims that Lake Powell will sustain enough loss through <br />sedimentation that within 135 years, the sediment will reach the penstock intakes, That would <br />take the powerplant out of operation, That estimate was based upon an assumption that <br />reservoirs fill in with sediment from the bottom to the top going up the face of the dam, <br /> <br />However, research conducted in 1986 for the 1986 Lake Powell Survey. published by <br />Reclamation in December 1988. indicates that is not the pattern for Lake Powell, The reservoir is <br />filling from the upper reaches of the reservoir. some 186 miles from the dam and trending <br />downstream to the dam, The heavier. courser sediment is deposited at the head of the reservoir <br />with fine clay and silt moving turther downstream, At the rate measured, the best estimate is that <br />it will be between 300 and 500 years before sediment reaches the penstock intake structures, <br /> <br />II <br />