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<br />1998 WATER SUPPLY ASSUMPTIONS <br /> <br />For 1998 operations, three reservoir unregulated inflow scenarios were developed and analyzed <br />and are labeled as probable maximum, most probable, and probable minimum. The attached <br />graphs show these inflow scenarios and associated release patterns, end of month contents, and <br />end of month elevations for each reservoir. <br /> <br />The National Weather Service Extended Streamflow Prediction (ESP) computer model was <br />employed to develop each of these inflow scenarios. This model uses current basin conditions <br />as well as historical data to predict the range of possible future streamflows. Although there is <br />considerable uncertainty associated with streamflow forecasts made a year in advance, the data <br />are valuable in analyzing possible impacts on project uses and purposes. The most probable <br />inflow in water year 1998 is projected to be near normal. Therefore, the magnitude of inflows <br />in each of the three inflow scenarios are near the historical upper decile, mean, and lower decile <br />(10 percent exceedance, 50 percent exceedance, and 90 percent exceedance, respectively) for <br />each reservoir for water year 1998. The three inflow scenarios for Lake Powell are shown in <br />Tables 2(a) and 2(b). <br /> <br />The volume of inflow resulting from these assumptions was used as input into Reclamation's <br />monthly reservoir simulation model. This model is used to plan reservoir operations for the <br />upcoming 24-month period. Projected water year 1998 inflow and October I, 1997 reservoir <br />storage conditions were used as input to this model and monthly releases were adjusted until <br />release and storage levels accomplished project purposes. <br /> <br />September 3, 1997 <br /> <br />5 <br />