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'.; 001H() ." 3 BASIN WATER RIGHTS AND MODELING APPROACH Modeling Approach The Yampa River Basin Model was developed using ,a network modeling system. Inflows, diver. 'sions, demands. reservoirs, and river reaches were represented in the model as a network of arcs and nodes. The mathematical solution of the network insures that water is allocated in strict accordance with user supplied priorities, that allocations are always within user-supplied constraints reflecting decree limits and physical diversion capacities and that mass balance is vigorously preserved throughout the system. ~i 11 1 The Yampa model runs on a monthly time step over a hydrologic study period encompassing water years 1930 through 1982, inclusive. TI1is study period is the same as that used in generating the U.S. Fish and Wildlife Service Environmental Baseline and is the basis for the interim flow recommenda. tions. The use of the same study period permits more convenient comparison of modeled hydrologic regimes with the regime presented in the Environmental Baseline. Existing basin water uses are represented in the model through the use of gaged flow hydrology rather than virgin flow hydrology. Implicit in this method are the assumptions that historical and cur. rent water use patterns continue in the future as they have in the past, unaffected by any additional water development, and that existing uses are senior, in the water rights sense, to any uses represented explicitly in the basin model. Future demands are represented in the model as demand increments, i.e., only the differences between current demands and future demands are defined in the model. Water Rights , " Water rights are represented in the basin model only to the extent necessary to properly allocate water supplies among demandirtcremems. Water rights supporting historical uses were implicitly con. sidered senior to all new development by the use of gaged 'flow hydrology. Water right priorities are modeled through the use of ranks placed on modeled demand increments; these ranks are set according to the relative priorities among water rights. The water right priorities assigned to demand increments are described below. _:' The use of gaged flow hydrology in the model implies that historical water uses (which are em- bedded in the gage record) are effectively senior to any water uses represented explicitly in the model. Since the 1989.level demancls lire really a set of "adjustments" to bring historical water uses up to 1989 conditions, they must also logically be considered senior to other water uses represented explicitly in the model. In 'any case, the 1989.level demands can 'all safely be considered senior to the Juniper rights either by virtue of their own priorities or by virtue of their being assumed to benefit from the blanket 65,000 af Juniper subordination. Thus the 1989.level demands are represented in the model as being senior to the Juniper rights; the relative priorities among the 1989-level demands increments simply reflect whether the demand in question is senior or junior to the Maybell Canal. The 1989.level demands were also considered senior to the ""'sting reservoirs which were oper- ated in the basin model (i.e., Stagecoach, Steamboat Lake, and Elkhead). TI1is assumption reflects the fact that these reservoirs have yet to be systematically called upon to meet basin water demands and have heretofore been left largely full year.around. Storage in the 'three existing reservoirs was assumed to be senior to the Juniper rights by virtue of specific subordinations of the Juniper rights. TI1is means