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, I . OG:1392 Description of Existing Model 1 limitations in Geographical Scope The North and Smith Forks coma in substantial vr.ller development facilities but are represented in the existing model ollly as net inflows to the mainstem. Some additional depletion of the North Fork gaged outflow was madded to reflect development of conditional water rights. but existing reservoirs and diversion systems were not represented in the model. Similarly, the Tongue. Surface. and Kannah Creek basins (with their many reservoirs and irrigation ditches) were nm represented in the model. The net contributions from these tributaries, and from most of the tributaries enlering the Uncompahgre River from the west, are embedded in modeled reach gains The represemation of these areas simply as net inflows means that the existing model cannot be used to investigate in detail the hydrology and water use operations of those areas. If detailed study of changes in hydrology and operations in these areas is necessary, the new planning model should represent these areas in more detail. Aggregation ofWatcr RiJ:hlS The represenuuion of smaller water rights as aggregated depletions has two imponant implica- tions. First, the aggregation means that the existing model cannot be used to evaluate the effects of ad. ministration on individual water rights. Second, the represenmlion of water uses as depletions rather than diverSlOns and return flows means that \ocal water rights calls and storage release requirements will generaUy be underestimated. For river basin planning purposes the aggregation of water rights is probably an acceptable con. struct; the fact that the Gunnison River basin contains literally tens of thousands of water rights means that some aggregal10n is probably unaVOidable. The aggregation poims used in the existing model could be reviewed, however. to determme If there ::Ire sigmricanl water rights [hat should be pulled out and represented individually. The use of depletions rather than diversions and return nows is a draw:,ack primarily in areas where local water rights calls agamst upstream juniors are common or where reservoir storage for water supply is being evaluated. In these areas it is probably desirable to represent water uses as diversions with return flows. The modeling of diversions and return flows implies an itcrati\'c solution procedure that is not required when water uses are treated as depictions; this increased computational burden should be kept in mind when considering how many depiction points in the existing model should be recon6gured as diversions. Tribuwy Hydrology To achieve the then-desired level of detail in the existing model, it was necessary to define numerous inflow points in tributary sub-basins that were often themselves ungaged or contained only a single gage. These inflows were developed by deregulation of gaged flows and disaggregation of these deregulated flows based on area and elevation. Though this procedure is technically defensible, it relies solely on records of flows and diversions and data obtainable from maps. In some areas, local hydrologic and geologic conditions may also affect lhe distribution of dere61Ulated flows into various inflow points. These conditions can best be ascerlamed by field recon. naissance and interviews with knowledgeable local people. In areas where lhe existing model is suspected of misrepresenling inflow hydrology, such Ocld rl'COnniJ.lSSanCe and mterviewmg should be considered and inflows adjusted or redefined accordingly.