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influence future water resource planning policies without spending unnecessary time and expense <br />developing data for rights that likely will never change to a significant degree. <br />If there is a disadvantage to the concept of leaving certain water rights in the gage, it might be the <br />argument from the water user community that the CRDSS is neglecting the needs of the small water <br />rights in the planning process. The procedure would also make it more difficult to quantify true <br />"virgin flows" since, by definition, this would require a determination of the depletive effects of all <br />water rights in the basin. Accordingly, if development of a true virgin flow database is to be a <br />fundamental objective of the CRDSS, it would be necessary to estimate the depletions attributable to <br />all of the water rights that are designated to be left in the gage. A final concern relates to the <br />procedures and policies necessary to define the threshold flow rates and storage levels, below which <br />the water rights would be considered in the gage. This threshold would likely need to be determined <br />for each river basin, with assistance and input from the division engineers, the water commissioners, <br />and the major water users. <br />Aggregation of Rights <br />The second procedure for realistic simplification of the water rights modeling effort is through the <br />process of aggregating water rights, i.e., collecting numerous small decrees into larger groups for <br />consideration in the model. This is the method being used in the Gunnison River basin model. For <br />the Gunnison model, it appears that non-critical water rights were aggregated by specific location in <br />the river basin network, based on their physical proximity and a common access to available inflows. <br />The rights were also aggregated based on a defined "priority class", reflective of the average priority <br />dates of the aggregated rights in relation to the priorities of a number of key water rights, i.e., rights <br />that have the potential to significantly affect basin-wide water resource planning. Ten priority <br />classes were adopted for that analysis, ranging from rights that are senior to the first key right (the <br />senior right of the Uncompahgre Valley Water Users Association) to a class that is senior only to <br />recent junior filings, including those of the CWCB, the Taylor Park Reservoir refill, and others. <br />The main advantage of this method of aggregation is that all water rights are theoretically considered <br />in the model's allocation of water supplies, even if the level of detail for each right is lacking. This <br />also enables the user to develop an estimate of the true "virgin flows" (if desired) and to some degree <br />allows for some "what if" scenarios for smaller water rights in the basin, although only as a group. A <br />second advantage is that the CRDSS will apparently have access to and use of the basic data derived <br />for the Gunnison model, and the work effort required to develop the prototype would presumably be <br />reduced. Unfortunately, this same level of prior data collection and modeling is not available for all <br />of the basins to be included in the CRDSS, so the value of these prior data sets should not be <br />overemphasized. <br />There are a number of disadvantages associated with this method of aggregation. The first concerns <br />the uncertain availability of reliable data for all of the rights, even if they are aggregated. For <br />example, if the diversion demand for an aggregated group of rights is determined using irrigated <br />acreage, evapotranspiration estimates, and assumed efficiencies, what is the degree of confidence <br />that these assumptions apply universally across the entire range of rights within a class. Also, what <br />is the comfort level with the model if the water supply, diverted in priority, is insufficient for the <br />aggregated demand, given the potential wide range of priorities within a class. It is our opinion that <br />greater confidence can be realized by leaving the effects of these non-critical rights in the gage, <br />where it is known that the depletive effects are accurately represented. A second concern relates to <br />the potential situation where the "river call" falls immediately senior or junior to the "weighted" <br />2 <br />A275 05.10.94 1.15-10 Fosha, Hyre <br />