WSP11586

• File Number: 8200.760
• Description: Yampa River General
• State: CO
• Basin: Yampa/White
• Water Division: 6
• Date: 1/1/3000
• Author: Unknown
• Title: Executive Summary - Task 9 Technical Memorandum - Draft
• Water Supply Pro - Doc Type: Report/Study

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Draft Task 9 Technical Memorandum

af; at the point of diversion for this plan, the average annual flow is approximately 30,000 af.
Diversions into Stagecoach Reservoir from Morrison Creek were not represented in the model. .

To test the sensitivity of the predicted demand shortages to the inflow hydrology at
Stagecoach Reservoir, a separate model run was made with the Stagecoach Reservoir inflows
reduced by 10%. Under these conditions, shortages to demands increased by an average of
300 af per year. The majority of this increase was due to additional shortages to existing
junior demands. TIiis is because these juniors rely heavily on direct flow water and have
relatively limited access to reservoir storage.

Juniper Contemplated Draft and Expected Draft of Existing Reservoirs

In Task 9 of the Feasibility Study, there were two activities which related to the size of
the Juniper Project contemplated draft. First, the size of the contemplated draft was re-
evaluated and model runs were made to test the sensitivity of demand deliveries to the size of
the draft. . Second, the size of the draft was reduced by an amount equal to the combined draft
of Stagecoach Reservoir, Steamboat Lake and the enlarged Elkhead Reservoir under 2015
demand conditions. The reduced contemplated draft was included in the model as an instream
flow right at Juniper Canyon and a 1954 priority.

Sensitivity to the Size of the Juniper Project Contemplated Draft

Two sensitivity runs were made with the Yampa model to evaluate the effects of the size
of the Juniper contemplated draft, as converted to an instream flow at Juniper Canyon, on
basin demands and reservoir contents. The sensitivity test involved increasing and decreasing
the original contemplated defined in the W.W. Wheeler study (1989) by 5% and examining the
differences in modeled shortages and reservoir contents. The original contemplated draft over
the period 1930 to 1982 averaged 862,011 afper year. The sensitivity model runs were made
using drafts of 818,911 af and 905,112 afper year.

Results of these model runs indicated some small changes to demand shortages and
reservoir contents with changes to the contemplated draft. The model run using the higher
contemplated draft results in Elkhead Reservoir operating an average of 1,670 af lower each
month. Future demands were shorted an average of 245 af per year more than under the
Wheeler contemplated draft model run, or approximately 6 percent. When the contemplated
draft was reduced by 5 %, modeled Elkhead Reservoir contents were an average of 1,590 af
higher. Modeled shortages to future demands were reduced by an average of 358 af per year,
or approximately 8 percent.

Estimated Draft of Existing Reservoirs

The average draft of the modeled reservoirs was evaluated to identify the potential need
for refill decrees and the significance of dates by which the reservoir fills are administered.
Under the 2015 demand level the average annual draft of the three reservoirs (with Elkhead
enlarged to 46,500 at) is approximately 33,000 af. This includes average drafts of 14,100 af
by Stagecoach, 4,040 afby Steamboat Lake, and 14,865 afby Elkhead. The maximum drafts
of each reservoir over a consecutive 12 month period are 29,030 af, 5,861 af, and 28,660 af
for Stagecoach, Steamboat and Elkhead, respectively. This suggests that refill decrees would
not be necessary under the 2015 demand level.

Under 2040 demand conditions, Stagecoach Reservoir is enlarged to 52,000 af. The
total average annual draft of Stagecoach, Steamboat and Elkhead is 46,600 af. The maximum

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