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Reservoir Operating Studies suite of operating studies sought to establish an "Environmental Baseline" characterization of flows at Maybell for use in preparing a preliminary Biological Assessment for the enlargement project based on previous Section 7 consultation. Later studies moved away from the Juniper- based instream flow decree concept and investigated the efficacy of making storage releases to maintain specific seasonal minimum flow targets at Maybell. The discussion below provides a background description of the basin model used in these studies and then presents an overview of the earlier operating studies. This chapter concludes with a thorough description of the final operating studies reflecting storage releases to meet proposed Maybell flow targets. Details of certain of these studies are contained in Appendices to this report. YAMPA RIVER BASIN MODEL Background As part of the Phase I Study, Hydrosphere developed a hydrologic and water rights computer model of the Yampa River basin in order to quantitatively evaluate the operation of existing reservoirs and the utility of constructing new reservoirs at several potential sites in the basin. The computer model represents the hydrology, water demands, water rights administration, and reservoir operations in the basin on a monthly basis using a 53-year hydrologic study period encompassing water years 1930 through 1982. This is the same study period used in previous modeling efforts of the basin carried out by the U.S. Fish and Wildlife Service (USFWS). This study period includes a wide range of hydrologic conditions, from drought years to flood years, and permits an assessment of the long-term effects of changes in water development and management. The Yampa River basin model represents inflows, river reaches, diversions, and reservoirs of the basin as a network of arcs and nodes. The model accepts inflows, simulates the allocation of these flows among water rights and uses in the basin, and predicts the resulting streamflow conditions in reaches of interest. The mathematical solution of the network insures that water is allocated in accordance with water right priorities, that these allocations are always within the physical and institutional constraints (e.g., decree limits, storage capacities) specified by the model user, and that mass balance is preserved throughout the system. Hydrology Model inflows represent either virgin inflows or net monthly reach gains (or, in some cases, depletions) in various reaches of the Yampa River system. In the headwaters areas, inflows generally represent virgin flows. Lower down in the basin they represent net reach gains or losses computed by mass balance techniques from gage flow records. A regional hydrologic analysis was conducted based on records from the U.S. Geological Survey (USGS) streamflow gages and on previous estimates of water yields at various points in the basin (Morrison-Knudson, 1987; Tipton and Kalmbach, 1980). This approach utilized existing stream flow data to derive physical water yield models (regression equations) which accepted tributary area, average elevation, and aspect as independent variables. 2-2