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<br />Description of Existing Model <br /> <br />7 <br /> <br /> <br />.OOlZM <br /> <br />,. <br /> <br />Umitalions in Geographical Scope <br /> <br />The North and Smith Forks contain substantial water development facilities but are represented <br />in the existing model only as net inflows to the mainstem. Some additional depletion of the North Fork <br />gaged outflow was modeled to reflect development of conditional water rights. but existing reservoirs <br />and diversion systems were not represented in the model. Similarly, the Tongue, Surface, and Kannah <br />Creek basins (with their many reservoirs and irrigation ditches) were not represented in the modeL The <br />net contributions from these tributaries. and from most of the tributaries entering the Uncompahgre <br />River from the west, are embedded in modeled reach gains. <br /> <br />The representation of these areas simply as net inflows means that the existing model cannot be <br />used to investigate in detail the hydrology and water use operations of those areas. If detailed study of <br />changes in hydrology and operations in these areas is necessary, the new planning model should <br />represent these areas in more detail. <br /> <br />Aggregation of Water Rights <br /> <br />The representation of smaller water rights as aggregated depletions has two important implica. <br />tions. First, the aggregation means that the existing model cannot be used to evaluate the effects of ad. <br />ministration on individual water rights. Second, the representation of water uses as depletions rather <br />than diversions and return flows means that local water rights calls and storage release requirements will <br />generally be underestimated. <br /> <br />For river basin planning purposes the aggregation of water rights is probably an acceptable con- <br />struct; the fact that the Gunnison River basin contains literally tens of thousands of water rights mellns <br />that some aggregation is probably unavoidable. The aggregation points used in the existing model could <br />be reviewed, however. to determine if there are significant water rights that should be pulled out and <br />represented individually. <br /> <br />The use of depletions rather than diversions and return flows is a drawback primarily in areas <br />where local \Vater rights calls against upstream juniors are common or where reservoir storage for water <br />supply is being evaluated. In these areas it is probably desirable to represent water uses as diversions <br />with return .f1ows. The modeling of diversions and return flows implies an iterative solution procedure <br />that is not required when water uses are treated as depletions; this increased computational burden <br />should be kept in mind when considering how many depletion points in the existing-model should be <br />reconfigured as diversions. <br /> <br />Tributary Hydrology <br /> <br />To achieve the then-desired level of detail in the existing model, it was necessary to define <br />numerous inflow points in tributary sub. basins that were often themselves ungaged or contained only a <br />single gage. Theseinllows were developed by deregulation of gaged flows and disaggregation of these <br />deregulated flows based on area and elevation. Though this procedure is technically defensible, it relies <br />solely on records of flows and diversions and data obtainable from maps. <br /> <br />In some areas, local hydrologic and geologic conditions may also affect the distribution of <br />deregulated flows into various inflow points. These conditions can best be ascertained by field recon. <br />naissance and interviews with knowledgeable local people. In areas where the existing model is <br />suspected of misrepresenting inflow hydrology, such field reconnaissance and interviewing should be <br />considered and inflows adjusted Or redefined accordingly. <br /> <br />