WSP13009

• File Number: 8282.600.10.B
• Description: 2005 Annual Operating Plan
• Basin: Colorado Mainstem
• Author: USDOI/BOR
• Title: 2005 Annual Operating Plan for Colorado River System Reservoirs
• Water Supply Pro - Doc Type: Operating Principles/Plan

2005 ,,, ATER SUPPLY ASSUMPTIONS

For 2005 operations, three reservoir unregulated inflow scenarios were developed and analyz.ed, and
are labeled as probable maximum, most probable, and probable minimum. The attached graphs
show these inflow scenarios with associated release patterns and end-of-month contents for each
reservOIr.

Although there is considerable uncertainty 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 ofinflows in each of these three inflow scenarios for
2005 are below the historical upper decile, mean, and lower decile (10 percent exce:edance,
50 percentexceedance, and 90 percent exceedance, respectively). The volume of inflow is reduced
in each ofthe three scenarios due to dry antecedent conditions in the Colorado River Basin resulting
from five consecutive years of below average precipitation. The National Weather Service's
Ensemble Streamflow Prediction (ESP) model was used to develop inflows for the three scenarios
for 2005. The ESP modeling showed that even with average temperatures and precipitation in 2005,
runoff in the Colorado River Basin is likely to remain below average due to dry antecedent
conditions. Most probable inflow for Lake Powell in water year 2005 is 9.23 maf(11,380 mcm) or
77 percent of average. Most probable inflow is 2.83 maf (3,490 mcm) less than the 30 year a.verage
of 12.06 maf (14,870 mcm). Minimum probable inflow to Lake Powell in water year 2005 is 3.75
maf (4,630 mcm) or 31 percent of average (2.40 mafless than the statistical 90 percent exceedance
level). Maximum probable inflow is 15.3 maf(18,800 mcm) or 127 percent of average (2.89 maf
less than the statistical 1 0 percent exceedance level). The three inflow scenarios for Lake Powell are
shown in Tables 2(a) and 2(b).

The volume ofinflow 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. Proj ected water year 2005 inflow and October 1,2004, reservoir storage
conditions were used as input to this model; and monthly :releases were adjusted until release and
storage levels best accomplished project purposes.

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