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CRDSS <br />TASK MEMORANDUM 1.15-10 <br />Water Rights Planning Model <br />Aggregation of Water Rights <br />1.0 ISSUE <br />Each of the river basins that will be modeled in the CRDSS contains literally thousands of individual <br />water rights, ranging from large decrees exceeding 1,000 cubic feet per second (cfs) to a large <br />majority of the rights being less than 5 cfs. Given the time and budget constraints of the CRDSS <br />project, it is not feasible or practical to model the operation of each individual water right. <br />Therefore, some means of aggregating water rights for modeling purposes is required. This <br />memorandum presents preliminary concepts for the aggregation methods suggested for incorporation <br />into the water rights planning model. These concepts will be refined as development of the <br />Gunnison River prototype model proceeds in Phase II of Task 1.15. <br />2.0 DISCUSSION/ANALYSIS <br />It is possible that the methods of aggregating water rights in each of the basins to be modeled in the <br />CRDSS project may be slightly different, depending on the number of small water rights in the basin <br />and the estimated magnitude of their depletions compared to the more major water rights in the <br />basin. There are, however, a number of procedures that can be used as the basis for the aggregation <br />and simplification of the modeling effort. <br />Exclusion of Certain Water Rights <br />The first concept relates to establishing a threshold flow rate (in cfs and/or acre-feet) for small water <br />rights that would not be included in the modeling effort but rather would be "left in the gage", i.e., it <br />depletions <br />is assumed that the attributable to the historic use of these rights is reflected in the <br />historic gaged stream flow records. Fundamental to this concept is the presumption that these small, <br />non-critical water rights would continue to operate in the future as they had in the past, and there <br />would be no need to model the actual diversions and return flows. This concept was adopted in the <br />CORSIM modeling efforts in the Colorado and White River basins and is also being used by Denver <br />Water as they develop the BESTSM model for the Colorado and upper South Platte basins. Denver <br />Water is apparently taking this concept one step further and has predetermined a given number of <br />"key" water rights in the basins that will be included in the model (between 200 and 250 rights); the <br />depletive effects of all other rights are being left in the gage. <br />There are clear advantages to this concept. From a modeling standpoint, the fewer rights that are <br />included simplifies the setup of the network and provides a higher level of confidence in the limited <br />data (diversions, acreage, consumptive use, etc.) that may be available for those rights. Similarly, a <br />high level of confidence is obtained with respect to the depletive effects of all of the rights that can <br />be left in the gage, without the introduction of error due to inaccurate or incomplete assumptions <br />about the historic or future use of a right. Another advantage is that the modeling effort, including <br />the development of data, can be focused on the key water rights in the basin that are most likely to <br />1 <br />A275 05.10.94 1.15-10 Fosha, Hyre <br />