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<br />.. I }. t <br /> <br />Colorado. Only minor amounts of depletions are made in the San Juan River Basin in <br />Utah and Arizona. <br /> <br />B. Approach <br /> <br />This hydrologic investigation considers and uses many of the same basic assumptions as <br />the 1988 Hydrologic Determination. Both investigations assume use of the CRSS natural . <br />flows at Lee Ferry, minimum releases from Lake Powell of between 7.48 maf and 8.23 <br />maf annually, an allowable overall shortage of no more than 6 percent for a critical <br />period, either maintenance or use of the minimum power pools at CRSP units, reduced <br />storage capacity in Lake Powell due to sedimentation, and inclusion of bank storage. The <br />CRSS natural flows at Lee Ferry for the period 1971-1980 were increased to reflect <br />recalculation of historic irrigation depletions in the Upper Basin using the Soil <br />Conservation Service (SCS) modified Blaney-Criddle method with SCS effective <br />precipitation. The revised CRSS natural flows fbr 1971-1980 are consistent with the <br />CRSS natural flows at Lee Ferry determined for the remainder of the 1906-2000 period <br />of record. Also, sedimentation in Lake Powell was adjusted to reflect a 2060 planning <br />horizon, and a 4-percent bank storage factor w~ uSled in this investigation consistent with <br />Reclamation's current CRSS model. <br /> <br />The Upper Colorado River Commission does not agree with the modeling assumption of <br />an objective minimum release of 8.23 maf and the assumed delivery of 0.75 ma,f each <br />year toward the Mexican Treaty obligation included therein. At the request of the <br />Commission, this hydrologic investigation considers for planning purposes both the <br />objective minimum release of 8.23 maf and a minimum release from Lake Powell of 7.48 <br />maf annually. However, this hydrologic determination does not quantify the Colorado <br />River Compact Article ID( c) requirement or make or rely on a critical compact <br />interpretation regarding Article ID(c). The 1988 Hydrologic Determination also showed <br />the Upper Basin yields under both minimum releasc~ scenarios. <br /> <br />Mass balance analyses were used to analyze potential water use by the Upper Basin under <br />2060 conditions. The mass balance considers Upper Basin reservoir storage, natural <br />flows at Lee Ferry, deliveries to the Lower Basin, c:onsumptive use demands in the Upper <br />Basin, and CRSP evaporation as a function of storage volume. All existing Upper Basin <br />storage capacity was included in the analysis because all storage supports water use in the <br />Upper Basin and impacts stream flows. The CRSP and non-CRSP reservoirs as groups <br />were assumed to be the same percent full each year, and CRSP storage was assumed to <br />be distributed between units in accordance with the average historic storage distribution. <br />The CRSP reservoir evaporation that is used in the mass balance analyses includes <br />evaporation from Lake Powell, Flaming Gorge Reservoir and the Aspinall Unit that is <br />shared among the Upper Division States, but excludes evaporation from Navajo <br />Reservoir which is chargeable to the states based on use. Shared CRSP reservoir <br />evaporation is modeled using a regression equation relating historic shared CRSP <br />reservoir evaporation from Lake Powell, Flaming Gorge Reservoir and the Aspinall Unit <br />to the aggregate historic storage volume in these reservoir plus Navajo Reservoir. <br />Evaporation equations were developed for both active and live storage, and were applied <br /> <br />3 <br />