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2. 1999 Water Supply Assumptions <br />For 1999 operations, three reservoir unregulated inflow scenarios were <br />developed and analyzed and are labeled as probable maximum, most probable and <br />probable minimum. Although there is considerable uncertainty associated with <br />streamflow forecasts and reservoir operating plans made a year in advance, these <br />projections are valuable in analyzing possible impacts on project uses and purposes. <br />The most probable inflow in water year 1999 is projected to be near normal. <br />Therefore, the magnitude of inflows in each of the three inflow scenarios are near <br />the historical upper decile, mean and lower decile (10 percent exceedance, <br />50 percent exceedance and 90 percent exceedance, respectively) for each reservoir <br />for water year 1999. The three inflow scenarios for Lake Powell are shown in <br />Tables 2(a) and 2(b1. <br />The volume of inflow resulting from these assumptions was used as input <br />into Reclamation's monthly reservoir simulation model. This model is used to plan <br />reservoir operations for the upcoming 24-month period. Projected water year 1999 <br />inflow and October 1, 1998 reservoir storage conditions were used as input to this <br />model and monthly releases were adjusted until release and storage levels <br />accomplished project purposes. <br />Table 21a1. Projected Unregulated Inflow <br />Into Lake Powell for Water Year 1999 <br />(Metric Units: MCM) <br />Time Probable Maximum Most Probable <br />Period Probable Minimum <br />10/98 - 12/98 2,552 1,850 1,462 <br />1 /99 - 3/99 2,595 1,729 1,485 <br />4199 - 7/99 15,803 9,541 4,199 <br />8199 - 9/99 1,393 1,342 797 <br />10!99 - 12/99 1,850 1,850 1,850 <br />WY 1999 22,343 14,463 7,943 <br />CY 1999 21,641 14,463 8,331 <br />57 <br />