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UPCO/Summit County Water Supply Study <br />December 15, 2004 <br />Storage Sites <br />Approach <br />To determine the physically and legally available water supply at the proposed storage sites, <br />DW's PACSM model was used.l Generally, Boyle reviewed characteristics of the simulations <br />with DW's modeling staff and the UPCO study managers, after which DW modified input files, <br />executed the model, and delivered results to Boyle. The "starting point" for the modeling was a <br />scenario from Phase II of the UPCO study, entitled "PACSM 3- New 15KAF North System <br />Supply and Full Demand" in the Phase II report (see Section 3.2.2). In this scenario, DW's <br />service area demand averages 390,000 acre-feet per year. This number represents maximum use <br />of DW's current system, plus an additional 15,000 acre-feet demand met through expansion of <br />the northern (Moffat) system. This scenario was selected because it represents a scenario being <br />considered in DW's Moffat Collection System EIS, currently in progress. The west slope <br />municipal and industrial demand is that demand contemplated at full buildout, expected to occur <br />at approximately 2030. This scenario is refened to within this study as the "baseline run", <br />because it represents conditions without an "UPCO alternative". The PACSM modeling period is <br />1947 through 1991. <br />As with the Phase II modeling, tributary hydrology had to be developed in this phase for areas <br />not explicitly modeled in PACSM until now. Specifically, Indiana Creek and the upper Swan <br />River were added to DW's model. DW used methods originally developed in the Suminit County <br />Small Reservoir Feasibility Study (Hydrosphere, 1989) and adopted during Phase II for this <br />effort, as described in Section 2.2.2 of the Phase II report. <br />For each storage alternative, DW modified PACSM to include an unconstrained diversion to <br />storage under either a new water right or a conditional water right if it existed. That is, no <br />reservoir size was assumed, and diversion to storage was not limited to physical capacity of the <br />hypothetical reservoir. Senior diverters and instream flow rights at and below the reservoir were <br />included in the determination of divertible flow, so the diversion was limited in that sense. This <br />step identified the legally available supply to the water right without the benefit of an exchange. <br />The model run also delivered information about physical supply at the reservoir site, and at each <br />node located between the reservoir site and Dillon Reservoir. From the physical supply data, <br />combined with an understanding of instream flow commitments, it was possible to estimate the <br />potential for storing water by exchange assuming that a replacement supply could be made <br />1 The PACSM model was reviewed during Phase II prior to its application to the UPCO study. See Section 3.2.1 <br />PACSM Verification Process, of the Phase II report. The results of any particular PACSM simulation are based <br />upon thousands of operating assumptions and conditions. Several assumptions involve Green Mountain Reservoir <br />operations. For example, a model assumption in PACSM is that payment for power interference is allowed and the <br />1,726 cfs Green Mountain Reservoir Power Plant direct flow water right is effectively not operated.