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I I I I II I I I I I I I I I I I I I I . 1997 WATER SUPPLY ASSUMPTIONS For 1997 operations, three reservoir unregulated inflow scenarios were developed and analyzed and are labeled as probable maximum, most probable, and probable minimum. The attached graphs show these inflow scenarios and associated release patterns, end of month contents, and end of month elevations for each reservoir. The National Weather Service Extended Streamflow Prediction (ESP) computer model was employed to develop each of these inflow scenarios. This model uses current basin conditions as well as historical data to predict the range of possible future streamflows. Although there is a wide confidence band associated with streamflow forecasts made a year in advance, the data are valuable in analyzing possible impacts on project uses and purposes. Soil moisture conditions throughout much of the basin were maintained by the snowpack that was experienced in early 1996, but became much drier after June from an extreme lack of precipitation. Despite these conditions, the most probable inflow in water year 1997 is expected to be near nonnal. Therefore, the magnitude of inflows in each of the three inflow scenarios are near the historical upper decile, mean, and lower decile (10 percent exceedance, SO percent exceedance, and 90 percent exceedance, respectively) for each reservoir for water year 1997. The three inflow scenarios for Lake Powell are shown in Tables 2(a) and 2(b). The volume of inflow resulting from these assumptions was used as input into Reclamation's monthly reservoir simulation model. This model is used to plan reservoir operations for the upcoming 24-month period. Projected water year 1997 inflow and October 1, 1996 reservoir storage conditions were used as input to this model and monthly releases were adjuSted until release and storage levels accomplished project purposes and priorities. 5