WSP11761

• File Number: 8441.400
• Description: Colorado Big Thompson Project - Reports
• State: CO
• Basin: South Platte
• Water Division: 1
• Date: 4/1/1987
• Title: Joint-Use Reservoir and Green Mountain Exchange Projects - Summary Report
• Water Supply Pro - Doc Type: Report/Study

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The model Incorporates over 800 major diversion structures each having an aggregate water

right greater than 5 cubic feet per second (cfs). This ac.counts for more than 90 percent of the total

diversions in the basin. The total number of water rights associated with these structures is

approximately 1600. All existing major reservoirs are simulated in the model. These include Lake

Granby, Willow Creek, Green Mountain, Williams Fork, Dillon, Homestake, and Ruedi Reservoirs.

Other reservoirs such as Shadow'Mountain Reservoir and Grand Lake were excluded from the model

simulations because of their relatively small impact on streamflows.

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Utilizing BESTSM, a reservoir operation analysis was performed to estimate the firm annual
yields of the proposed reservoirs. Firm annual yield is defined for the purpose of this study as the
quantity of water that can be supplied every year without any shortage during the study period of
1951 through 1983. This Study period includes the two significant drought periods of 1954-1956
and 1977. Through drought frequency analysis, it was estimated that the 1954-1956 drought would
occur in the study area once in 50 to 100 years while the 1977 drought would occur in the study area
.once in 30 to 50 years, depending on location of the stream. Accordingiy, the firm annual yield
presented in this report indicates the amount of water that can be supplied during these two drought
periods.

WATER YIELD FOR RESERVOIRS AND RESERVOIR COMBINATIONS: Firm annual yields

for the six reservoirs, selected and for the four representative combinations of reservoirs are

presented in Table S.1. The firm annual yield estimates were based on a projected high future-level

of water development in the basin with two different Green Mountain operating conditions.

The high future-level of water development anticipates increased diversions by existing

transmountain diversion systems which are not presently operating at their full capacity, and an

approximately 60 percent increase in in-basin use over the present use. The first Green Mountain

Reservoir operating condition assumes that the Reservoir would continue to operate as originally

prescribed in Senate Document No. 80. The second condition assumes that the pumpback system

from Green Mountain Reservoir to Dillon Reservoir and the associated water exchange arrangement

would be in operation.

Some of the proposed reservoir sites are in effect alternative locations for regulating the

same water. When more than one reservoir is added in the upper portion of the basin, the yield of

the combination of reservoirs is less than the sum of the yields of individual reservoirs,

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