WSP13013

• File Number: 8282.600.10.B
• Description: 2004 Annual Operating Plan
• Basin: Colorado Mainstem
• Author: USDOI/BOR
• Title: 2004 Annual Operating Plan for Colorado River Systems
• Water Supply Pro - Doc Type: Operating Principles/Plan

January 7, 2004

8

2004 \V A TER SUPPLY ASSUMPTIONS

For 2004 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.

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 of inflows in each ofthese three inflow scenarios for
2004 are below the historical upper decile, mean, and lower decile (10 percent exceedance,
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 resulting
from four 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 2004. ESP modeling showed that even with average temperatures and precipitation in 2004,
runoff in the Colorado River Basin is likely to remain below average due to dry antecedent
conditions. Most probable inflow for Lake Powell for water year 2004 is 9.29 maf(11,500 mcm) or
77 percent of average. Most probable inflow was reduced by 2.78 maf (3,430 mcm) from the 30
year average of 12.06 maf (14,870 mcm). Minimum probable inflow was reduced by 2.41 maf
(2,970 mcm) from 51 percent of average (the statistical 90 percent exceedance level) to 31 percent of
average. Maximum probable inflow was reduced by 2.98 maf (3,680 mcm) from 151 percent of
average (the statistical 10 percent exceedance level) to 126 percent of average. 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 2004 inflow and October 1, 2003, reservoir
storage conditions were used as input to this model and monthly releases were adjusted until release
and storage levels accomplished project purposes.