WSP05551

• File Number: 822.600.10
• Description: Colorado River Annual Operating Plan - Annual Reports
• Basin: Colorado Mainstem
• Date: 1/1/2002
• Author: USDOI/BOR
• Title: Annual Operating Plan
• Water Supply Pro - Doc Type: Annual Report

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2002 WATER SUPPLY ASSUMPTIONS

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For 2002 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 and end of month
contents for each reservoir.

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Although there is considerable uncertainty associated with streamflow forecasts and reservoir
operating plans made a year in advance, these projections are valuable in analyzing possible
impacts on project uses and purposes. The magnitude of inflows in each of these three inflow
scenarios for 2002 are near, but moderately below, the historical upper decile, mean, and
lower decile (10 percent exceedance, 50 percent exceedance, and 90 percent exceedance,
respectively), With the exception of Navajo Reservoir, the volume of inflow is reduced in
each of the three scenarios, due to dry antecedent conditions in the Colorado River basin
resulting from two consecutive years of below average precipitation. Most probable inflow
for Lake Powell for water year 2002 is 11.0 million acre-feet (13,600 million cubic meters) or
94 percent of average. The three inflow scenarios for Lake Powell are shown in Tables 2(a)
and 2(b).

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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 2002 inflow and October 1, 200) 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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October 15, 200 I

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