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<br />000378 <br /> <br />Hydrosphere Yampa River Basin Model <br /> <br />In 1990 through 1993, Hydrosphere developed a hydrologic and water rights computer <br />model of the Yampa River in order to quantitatively evaluate various operational scenarios of <br />existing reservoirs and the utility of constructing new reservoirs at several potential sites in the <br />basin. The model represents the hydrology, water demands, reservoir operations, and water <br />rights administration of the basin on a monthly basis over a 53-year hydrologic study period <br />encompassing water years 1930 through 1982. This is the same study period used in previous <br />modeling efforts of the Yampa River Basin by the U.S. Fish and Wildlife Service. The period <br />includes a wide range of hydrologic conditions, from drought years to flood years, and permits <br />an assessment of the long-term effects of changes in water development and management. <br /> <br />Baseline Conditions <br /> <br />The "baseline" model scenario includes certain assumptions relating to basin water <br />demand levels, water rights priorities and operations of existing reservoirs. The baseline <br />scenario was defmed using a projected maximum demand level which could occur in the year <br />1989. This demand level included projected increases in agricultural and municipal water <br />depletions and water exports from the basin by roughly 8,000 af per year over estimated 1982 <br />conditions. It was assumed that these depletions could occur under existing water rights and <br />diversion facilities. Stagecoach Reservoir, Steamboat Lake and Elkhead Reservoir (at a <br />capacity of 13,700 af) were operated in the modeled baseline scenario according to historical <br />operating practices and to reflect a variety of contractual storage arrangements currently in <br />place. A more detailed description of model structure, input data and reservoir operating rules <br />is provided in the final report for the Yampa River Basin Alternatives Feasibility Study <br />(Colorado River Water Conservation District, 1993). <br /> <br />The baseline model run, as well as the runs based on projected future demand <br />conditions, do not reflect operations at Steamboat Lake to make annual water releases of up to <br />2,000 affor the benefit of the endangered fish in the Yampa River. Although these releases <br />have occurred in recent years, formal arrangements between the USFWS and the Colorado <br />Division of Parks have not been finalized. <br /> <br />Elkhead Reservoir <br /> <br />Under the modeled baseline conditions, Elkhead Reservoir experiences very little <br />fluctuation in storage contents (Figure 4): This is due largely to the ability of natural flows to <br />satisfy most basin demands, both on the Yampa River and on Elkhead Creek. Some drawdown <br />of Elkhead Reservoir would be expected during hydrologic conditions similar to those <br />experienced during the mid-1930's. Elkhead Reservoir typically spills during April or May of <br />each year. <br /> <br />Elkhead Creek <br /> <br />Table 3 shows modeled average monthly flows downstream of Elkhead Reservoir under <br />baseline conditions. Flows average less than 10 cfs during the fall and winter months and up <br />to 590 cfs during peak runoff periods. Modeled flows during August and September are often <br />less than 5 cfs. Average annual outflow from the reservoir is approximately 63,000 af, <br />reflecting annual evaporative losses of roughly 1,400 af. As a note, reported zero flow <br />conditions are, to some degree, an artifact of modeling assumptions. On average, a minimum <br />of 1 to 2 cfs are present in Elkhead Creek downstream of the reservoir resulting from reservoir <br />seepage and return flow. The seepage component of flow below the reservoir was not <br />represented in the model. <br /> <br />14 <br />