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n Executive Summary that water is allocated to network arcs in accordance with user supplied priorities, that these allocations are always within user-supplied constraints reflecting decree limits, physical diversion capacities, and available water supply, and that mass balance is preserved throughout the system. ' Modeled inflows represent virgin inflow 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 estimates from previous studies of water yields at various points in the basin (Morrison-Knudson, 1987; Tipton and Kalmbach, 1980). This approach utilized existing d stream flow data to derive physical water yield models (regression equations) which accepte tributary area, average elevation, and aspect as independent variables. Existing basin water uses are generally represented implicitly in the model through the use of gaged flow hydrology rather than virgin flow hydrology. Implicit in this method is the assumption that historical and current water use patterns continue in the future as they have in the past, unaffected by any additional water development represented explicitly in the basin model. Future demands, as well as certain demand adjustments, are represented in the model as demand increments, i.e., only the differences between current demands and future demands are defined in the model. To the degree possible, demands were placed in the model network where they occur, or will most likely occur in the future. The water rights of the various demands are represented in the model only in terms of their priorities relative to each other and relative to the Juniper Project right. Generally speaking, existing uses are considered senior to the Juniper rights while future uses, both 2015 and 2040 level, are considered junior to the Juniper rights. There are two exceptions to this general rule: 1) certain existing agricultural uses are modeled as having rights junior to the ' Juniper project, and 2) the Hayden generating station has sufficient rights senior to the Juniper Project to serve its projected long-term demand. In model scenarios representing an instream flow right for the endangered fish, this right was assigned a priority equal to the Juniper Project right. Reservoir Operations Three existing reservoirs were operated in the Yampa River Basin Model: Stagecoach Reservoir, Steamboat Lake, and Elkhead Reservoir. The operations of all other existing reservoirs (most of which are located in headwaters areas) were assumed to have negligible ' effect on flows below Stagecoach Reservoir either because of their small size or because of their mode of operation. The model network was configured to represent nine reservoirs in all. Six of these reservoirs are currently undeveloped and were effectively "turned off" in the ' model runs except in scenarios in which they were specifically being evaluated. The inactive reservoirs were all included in the model to allow them to easily be operated should their development be considered at a future time. i Reservoir evaporation and storage carryover are automated in the model. Storage and release operations are driven by the relative priorities between reservoirs and demands. Reservoirs in the model were allowed to store an amount of water equal to their physical capacity one time each water year; carryover storage in reservoirs was not counted against the reservoirs' annual fill limit. In terms of strict water rights administration this implies that each modeled reservoir had a refill decree of sufficient size to support storage limited only by ' available capacity and inflow. S-17