WSP03527

• File Number: 822.600.10
• Description: Colorado River Annual Operating Plan - Annual Reports
• Basin: Colorado Mainstem
• Date: 1/1/2003
• Author: USDOI/BOR
• Title: Annual Operating Plan
• Water Supply Pro - Doc Type: Annual Report

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2003 \VATER SUPPL \' ASSmlpTlO:\S

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For 2003 operations. three reservoir unregulated inflow scenarios were dc\"clopcd and analyzed and
are labeled as probable maximum. most probable. and probable minimum. The attached h'Taphs
show these inflow scenarios and associated release patterns and end of month contents for each
reservoir.

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Although there is considerable uncertainly associated with streamflow forecasts and reservoir
operating plans made a year in advance. these projections are valuable in analyzing probable impacts
on project uses and purposes. The magnitude of inflows in each of these three inflow scenarios for
2003 are below the historical upper decile. mean. and lower decile (10 percenl excecrlance.
50 percent exceedance. and 90 percent exceedance. respectively). The volume of inflow is reduced
in each of the three scenarios. due to dry antecedent conditions in the Colorado River basin resulling
from three conseculive years of below average precipitation. The National Weather Service's
Extended Streamflow Prediction (ESP) model was used to develop inflows for the three scenarios for
2003. ESP modeling showed that even with average temperatures and precipitation in 2003. runoff
in the Colorado River Basin is likely 10 remain below average due to dry antecedent conditions.
Most probable inllow for Lake Powell for water year 2003 is to. 1 maf (12.500 Il1cm) or 84 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 l4-month period. Projected water year 2003 inflow and October 1. 2002 reseryoir storage
conditions were used as input to this model and monthly releases \Vere adjusted unlil release and
storage levels accomplished project purposes.

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December 13. 2002

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