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Section 7 Availability of Existing Water Supplies estimating available supply in these basins. For those basins without developed DSS datasets - Arkansas, South Platte, and North Platte - information for physically and legally available flows were gathered from existing sources and studies. For example, USGS flow gages were used to help quantify physically available flows for all three of these basins. The Republican River basin was not studied in SWSI because of ongoing interstate litigation concerns. Additionally, legally available flows were quantified in the SECWCD Hydrologic Analysis Study (2000) for the Arkansas Basin, and in the Denver Water Chatfield Reservoir Reallocation PACSM modeling effort, the CWCB Lower South Platte Water Management and Storage Sites Reconnaissance Study, and the NCWCD NISP for the South Platte Basin. Table 7-1 Summary of SWSI Sources of Data: Supply Availability . . ~, Arkansas SECWCD Hydrologic Analysis (2000), USGS flow gages Colorado DSS/StateMod Dolores/San Juan/ San Mi uel DSS/StateMod Gunnison DSS/StateMod North Platte USGS flow a es Rio Grande DSS/StateMod South Platte PACSM model (Denver Water), NISP study (NCWCD), Lower S. Platte Water Management and Storage Sites Reconnaissance Study (CWCB) Yampa/VVhite/Green DSS/StateMod * Republican not studied For each basin where DSS StateMod runs were available, modeled flows were summarized at 4 to 10 key locations. Locations were chosen to provide a good spatial coverage across a given basin. These locations also generally correspond to downstream ends of delineated subbasins (counties and water districts). The simulated flows at these locations are therefore representative of total subbasin available flows and provide useful information for analyzing options at a subbasin level (Section 10). 7.1.3 Firm Yield Analysis The concept of "firm yield" is a common term used in water supply planning. The firm yield, as defined below, is analyzed for several locations to illustrate the storage to yield ratios under certain planning criteria. For these analyses, firm yield is defined as the maximum annual ~~ supply that can be reliably provided every year for the period of record with no monthly shortages. This firm yield definition is based on no monthly shortages for the period of record. The appropriate period of record to be used for firm yield analysis as well as the willingness to accept some shortages can significantly affect the firm yield analysis. Individual water planning agencies may have different criteria for the critical period of record that should be used. In addition, the planning agencies may determine that it is more cost- effective to manage infrequent shortages with demand modifications than to design the water system to deliver full supplies with no shortages over the critical period or period of record. As part of the supply availability summaries presented in this section, curves of firm yield versus total storage ("yield curves") are provided for one location in most basins. These curves were generated using Water Supply Investigation Tool (WatSIT), a screening-level reservoir and water supply model developed for this project. The model simulates the filling of a reservoir by a time series of monthly available river flows and the simultaneous emptying of the reservoir according to a user-defined monthly demand pattern. For a given reservoir size, firm yield is therefore calculated as the maximum total annual demand that can be met without shortages over the full period of record of river flows. A detailed description of WatSIT is provided in Appendix F. Firm yield calculations can be inaccurate for short periods of record, because assumed starting reservoir conditions affect calculated firm yields (Lester and Couch 2000). Starting reservoir conditions affect firm yields when the reservoir simulation fails to include at least one period of partial depletion followed by full recovery (to full capacity) before entering the critical hydrologic period (Zarriello 2002). Firm yields presented in this section were not affected by assumed starting conditions for any of the basins except the Rio Grande and the South Platte Basins (which had shorter periods of record than the others). As described below, for the Rio Grande Basin analysis, starting reservoir conditions were set equal to the maximum wet-weather predicted storage volume. For the South Platte Basin, the analysis was terminated at the point at which yields showed a dependence on starting conditions (at high total storage values). ~~ Sfvtewide Woter Supoly Initiofive 7-Z S:\REPORT\WORD PROCESSING\REPORT\S7 11-10.04.DOC