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<br />2003 WATER SUPPLY ASSUMPTIONS <br /> <br />For 2003 operations, three reservoir unregulated inflow scenarios were developed and analyzed and <br />are labeled as probable maximum, most probable, and probable minimum. The attached graphs <br />show these inflow scenarios and associated release patterns and end of month contents for each <br />reservOIr. <br /> <br />Although there is considerable uncertainty associated with streamflow forecasts and reservoir <br />operating plans made a year in advance, these projections are valuable in analyzing probable impacts <br />on project uses and purposes. The magnitude of inflows in each of these three inflow scenarios for <br />2003 are below the historical upper decile, mean, and lower decile (10 percent exceedance, <br />50 percent exceedance, and 90 percent exceedance, respectively). The volume of inflow is reduced <br />in each of the three scenarios, due to dry antecedent conditions in the Colorado River basin resulting <br />from three consecutive years of below average precipitation. The National Weather Service's <br />Extended Streamflow Prediction (ESP) model was used to develop inflows for the three scenarios <br />for 2003. ESP modeling showed that even with average temperatures and precipitation in 2003, <br />runoff in the Colorado River Basin is likely to remain below average due to dry antecedent <br />conditions. Most probable inflow for Lake Powell for water year 2003 is 10.1 maf (12,500 mcm) or <br />84 percent of average. The three inflow scenarios for Lake Powell are shown in Tables 2(a) and <br />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 2003 inflow and October 1, 2002 reservoir storage <br />conditions were used as input to this model and monthly releases were adjusted until release and <br />storage levels accomplished project purposes. <br /> <br />December 13,2002 <br /> <br />7 <br /> <br />