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' 5. EVALUATION OF FUTURE FLOWS <br />As discussed in Section -2.3, the RIP initiated a study to consider the effect of the Juniper water <br />right on instream flows and other water uses. This initial study lead Hydrosphere to develop a <br />Yampa River Basin model to simulate flows in the basin under different operational scenarios. <br />' In Phase I studies, Hydrosphere determined the Elkhead Reservoir enlargement was the most <br />feasible alternative to meet short -term demands for augmenting late season flows on critical <br />habitat. This section summarizes results of selected model runs during studies Hydrosphere <br />.,. conducted between 1991 and 1996. <br />In 1998, the Yampa Management Team asked the CWCB to model Yampa River flows using the <br />CRDSS to evaluate the impacts of existing and projected future water depletions to satisfy <br />human needs. These data were further analyzed to determine the frequency, magnitude and <br />duration of transgressions of a 93 cfs daily flow target during July- October and estimate the <br />volume of water necessary to augment stream flows to meet this target in its historical context <br />(Appendix B). <br />5.1. Definition of Scenarios <br />Numerous model scenarios were run using Hydrosphere's Yampa River Basin Model in <br />connection with Phase I and Phase II planning studies completed in 1993 and 1995, respectively. <br />Subsequent to the completion of the Phase II study, additional model scenarios were developed <br />to further examine a variety of operation and management options ( "O &M Studies "). Of the more <br />than 50 model scenarios run over the course of these efforts, 6 models that simulated storage <br />releases from an enlarged Elkhead Reservoir are described below. Table 3 summarizes the <br />instream flow rights and targets represented in these six scenarios. <br />5. The Flow Rec. - Junior - No Runoff Aug. scenario, which simulates the same <br />operation as the Flow Rec. - Junior scenario except that no storage releases are <br />made for flow augmentation during the months of April, May and June. <br />6. The CWCB - Junior - No Runoff Aug. scenario, which simulates the same operation <br />as the CWCB - Junior scenario except that no storage releases are made for flow <br />' augmentation during the months of April, May and June. <br />15 <br />1. The Phase I Study scenario, which simulates an instream flow water right based on <br />the contemplated draft of the Juniper Project and having a priority date of 1954. <br />Reservoir storage releases are made to satisfy existing and future in -basin demands, <br />some of which are called out by the 1954 instream flow right. <br />' <br />2. The Phase II Study scenario, which simulates flow targets, based on Preliminary Flow <br />Recommendations developed in 1994. In -basin demands are assumed to divert free <br />' <br />from any call for instream flows. Reservoir releases are then made year -round to <br />bring Maybell flows to the level the targets, if they are not met otherwise. <br />up of <br />3. The Flow Rec. - Junior scenario, which simulates flow targets based on Revised Flow <br />' <br />Recommendations made by the USFWS. Reservoir releases are made year around to <br />bring Maybell flows up to the target level. <br />4. The CWCB - Junior scenario, which simulates flow targets based on the now - <br />' <br />withdrawn CWCB instream flow filing. Reservoir releases are made year around to <br />bring Maybell flows up to the target level. <br />5. The Flow Rec. - Junior - No Runoff Aug. scenario, which simulates the same <br />operation as the Flow Rec. - Junior scenario except that no storage releases are <br />made for flow augmentation during the months of April, May and June. <br />6. The CWCB - Junior - No Runoff Aug. scenario, which simulates the same operation <br />as the CWCB - Junior scenario except that no storage releases are made for flow <br />' augmentation during the months of April, May and June. <br />15 <br />