WSP07506

• File Number: 822.600.10
• Description: Colorado River Annual Operating Plan - Annual Reports
• Basin: Colorado Mainstem
• Date: 1/1/1998
• Author: USDOI/BOR
• Title: Annual Operating Plan
• Water Supply Pro - Doc Type: Annual Report

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1998 WATER SUPPLY ASSUMPTIONS

For 1998 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
considerable uncertainty associated with streamflow forecasts made a year in advance, the data
are valuable in analyzing possible impacts on project uses and purposes. The most probable
inflow in water year 1998 is projected to be near normal. 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, 50 percent exceedance, and 90 percent exceedance, respectively) for
each reservoir for water year 1998. 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 1998 inflow and October I, 1997 reservoir
storage conditions were used as input to this model and monthly releases were adjusted until
release and storage levels accomplished project purposes.

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