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<br />0'01270 <br />Draft Task 7 Technical Memorandum <br /> <br />. <br /> <br />. <br /> <br />SCENARIO V. ELKHEAD ENLARGEMENT WITH WILLIAMS FORK PROJECT <br /> <br />Key Assumptions of Scenario <br /> <br />In Scenario V the Elkhead Reservoir enlargement was modeled along with a new storage <br />project on the Williams Fork River. The Williams Fork Reservoir was modeled with a 70,000' <br />af capacity. This entire 70,000 af was modeled in a manner similar to the enlargement pools <br />of previous scenarios and was given a water right priority date of 1954. This means that water <br />was stored only after existing senior demands were met but prior to existing junior demands <br />and future demand increments. <br /> <br />Because the majority of basin demands are located above the confluence of the Williams <br />Fork with the Yampa River, Williams Fork Reservoir serves primarily as a source of exchange <br />water. The exchange was used to satisfy downstream senior water rights, i.e. the Maybell <br />Canal and the Juniper Canyon instream flow right, to allow additional upstream diversions by <br />junior rights (Figure 13). <br /> <br />Protocols to access reservoir storage were similar to the protocols described for <br />Scenarios I, II and III. In Scenario V, the model serves demands out of the Elkhead <br />enlargement pool prior to taking water from the Williams Fork Reservoir. <br /> <br />The outlet capacity of Williams Fork Reservoir was modeled as 450 cfs. <br /> <br />The amount of water that can be exchanged out of the Williams Fork Reservoir was <br />limited to the amount of physical water in the Yampa River mainstem between the mouth of the <br />Williams Fork River and the location of the junior diverting right. In order to prevent the <br />Yampa River from being dried up in occupied habitat reaches during the months of August and <br />September, an instream flow constraint was imposed on the mainstem just above the Williams <br />Fork River. The amount of this instream flow target was specified by the USFWS and <br />represented in the model as 150 cfs in August and 110 cfs in September. These were "soft" <br />flow targets in the model and act only to prevent the exchange from further depleting the <br />mainstem if flows were at or below the specified targets. There are no constraints on exchange <br />potential in other months of the year. <br /> <br />Results of Scenario <br /> <br />Demand Shortages <br /> <br />Shortages occurring in Scenario V for the existing senior demand level were the same as <br />previous scenarios and averaged 18 af/yr (Table 7). Existing junior demands were shorted an <br />average of 682 af/yr in this scenario. These shortages reflect the limitations of the use of <br />Williams Fork as an exchange supply and the constraints of the capacity of the outlet works of <br />Elkhead Reservoir. <br /> <br />Shortages to future demands averaged 42 af/yr and 804 af/yr for the 2015-level and <br />2040-level demands, respectively (Figure 14). The majority of these shortages occur to <br />demands in the Craig area. These shortages were generally a result a lack of physical supply, <br />constraints on storage levels in Elkhead Reservoir, or reflect the constraint placed on exchange <br />against water in Williams Fork Reservoir. <br /> <br />15 <br /> <br />J-~ <br />