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001141 4 that reservoirs are allowed (with the exception of possible minimum release requirements) to store all inflows in excess of 1989-level demand, All future demands are considered junior to the Juniper rights and junior to storage tn the three existing reservoirs. These demands are satisfied- first through direct flow diversions; remaining deficiencies are then made up by reservoir releases or by subordination of the Juniper water rights. REVIEW OF USFWS MODELING Background In 1986, the U.S. Fish and Wildlife Service (USFWS) applied the HYDROSS methodology to the Yampa and Green River basins to assist in assessing the cumulative impacts of water developments for irrigation, power, municipal, and recreation. HYDROSS is a system of computer programs developed by the USFWS for conducting monthly water supply studies along traditional accounting lines, Application to the Yar;npa and Green Rivers was aimed primarily at identifying the impacts <).ssodated with the Stagecoach Reservoir Project near Steamboat Springs. Four separate scenarios were investigated and included simulating historical conditions, present level of development, a future. level of development with all proposed projects likely to be developed, and a baseline plus the proposed Stagecoach Project, These scenarios were used to define Interim Flow Recommendations at Deerlodge Park. The corresponding flows at Maybell are referred to as the "Environmental Baseline", Data and assumptions used in the Yampa River Basin HYDROSS application have been reviewed in depth in an effort to fully understand the meaning of the environmental baseline flows, The review included the modeled hydrology, water demands, an assessment of reliability of demand forecasts, and identification of water development projects included in the environmental baseline definition. The assumptions of the HYDROSS modeling were represented in a network model of the basin to further assist in developing this understanding, ' Hydrology and Demands Represented in the HYDROSS Model Inflow data for the HYDROSS model runs were generated using the V.S, I\ureau of Reclamation Colorado River Simulation System (CRSS) data base. CRSS "natural flows" are established by taking the observed flows at certain USGS gaging stations and adding estimates of all historic depletions from the stream. CRSS flows are hardwired into the HYDROSS model. In order to simulate historical conditions along the Yampa River, the USGS gage data at Maybell were substituted in place of the HYDROSS Natural Flow Database. This was achieved in the Yampa Basin model using a "correction" depletion as the difference between the CRS$ natural flow and the recorded flow at the Maybell gage. This effeCtively set the flow tei the historical Maybell record, To simulate existing demand conditions. depletions were modeled up to the year when they commenced. Because Ihe model is based on gage hydrology, there is no need to model the demand after the depletion came on line; it is already Implied ,in the gaged record, The change from historical to existing conditions involved adding depletions over portions of the 1930- I 982 period. These adjustments were applied for Hayden Units I and 2 (7,100 at) prior to 1966 and Craig Units 1 and 2 (12,800 af) prior to 1981. The proposed Cheyenne Stage I project (8,000 at) is located on the little . Snake River and was not considered in defining conditions under the existing demand. " "