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UPCO/Summit County Water Supply Study <br />December 15, 2004 <br />available to Dillon Reservoir. PACSM's daily time step made it particularly appropriate to this <br />analysis, because it is possible, even typical, for exchanges to be operable intermittently over the <br />spring and sumuler. These exchange "windows" may last for only a few days at a time. Finally, <br />the model also demonstrated when Green Mountain Reservoir would be unable to fill. In these <br />"substitution years", exchange potential for the reservoir alternative was assumed to be zero. <br />PACSM showed that for each reservoir, some exchange potential was available occasionally <br />from November through April, but these amounts were minor. Given the impracticality of <br />intermittent winter operations, it was assumed for the purpose of yield analysis that these winter <br />exchanges would not be realized. <br />Total supply available to each storage alternative was computed as the sum of water available to <br />a new or conditional water right, plus water available by exchange. To help understand the <br />relationship between storage capacity and yield for each of the three new reservoir sites, a simple <br />operational spreadsheet was created for each one. The spreadsheet covered the same period as <br />PACSM (1947-1991) but used a monthly time step. It featured the calculated total supply as <br />inflow, an annual demand with the monthly distribution shown in Table 1 as the schedule of <br />releases, and evaporative losses based on average historical monthly evaporation data at Dillon <br />Reservoir. The monthly demand pattern was developed from the UPCO Phase II buildout <br />demands for Suminit County providers, and reflects snowmaking, irrigation, and <br />domesticlmunicipallcommercial uses in correct proportion for the providers in aggregate. For <br />each of several assumed reservoir sizes, the annual demand was varied until firm yield was <br />identified. Firm yield is the maximum amount that can be delivered in each and every year of the <br />operational model, under the assumed release pattern. At this point, no allowance was made for a <br />dead pool. That is, demand was increased until the entire reservoir capacity was drained in one <br />year. <br />Table 1 <br />Percent of Annual Demand Occurring in Each Month <br />Combined Summit County Water Providers <br />Oct Nov Dec Jan Feb Mar A r May Jun Jul Au Se <br />8 11 14 7 5 6 5 6 9 12 9 8 <br />Reservoir yield is a function of the time distribution of demands on the reservoir, that is, the <br />manner of operating the reservoir. If the reservoir was used as a supplemental supply only half of <br />the time, it could support a higher annual draft for the years when it was used. The overall <br />average yield of the reservoir, however, would be lower than when the reservoir is used year in <br />and year out. To understand reservoir yield at each site when operating as a supplemental supply, <br />the operations spreadsheets were next populated with a demand that was zero in half the years, <br />and the same positive value in all other years. This value was varied iteratively in order to find <br />the maximum annual draft that the reservoir could supply in one out of two years, for this <br />particular hydrologic sequence. Years in which the supplemental supply would be needed were <br />determined by sorting annual shortages to Summit County providers in the Phase II model. The <br />SOt" percentile of years with the greater shortages were classified as "supplemental supply <br />years:"