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CRDSS <br />TASK MEMORANDUM 1.15-29 <br />Water Rights Planning Model <br />Scenario Simulation - Instream Flow Rights <br />1.0 ISSUE <br />U.S. Fish and Wildlife Service bi ologists have asked the State of Colorado to consider granting <br />instream flow appropriations for several river reach es to promote the recovery of threatened and <br />endangered fish species under the Endangered Species Recovery Program. The additional flow would <br />help provide a more natural annual hydrograph for these endangered species. <br />On the average, more water origin ates in the Colorado River basin within Colorado than the State is <br />entitled to use under the apportionments made in the 1922 Colorado River Compact and the 1948 Upper <br />Colorado River Basin Compact. Moreover, Colorado ha s not developed the use of all of the water to <br />which it is entitled under the compacts. Given these circumstances, a number of questions arise: (a) <br />What magnitude of instream flow can be accommoda ted without impacting the rights of existing water <br />users? (b) When and how much water is available at a point in a basin for in stream flow appropriation? <br />(c) What are the impacts of instream flow rights on the development of new projects in the basin? <br />Similar considerations apply for establishing reserv ed water rights either by federal interests or by <br />Indian tribes. <br />The purpose of this task memorandum is to desc ribe and document the use of the Gunnison River <br />prototype water rights planning m odel (WRPM) to simulate a scenario involving a key water right <br />planning issue in the basin. For this task, an ex ample is developed to de monstrate how a hypothetical <br />instream flow requirement for endangered species can be added to an existing MODSIM network and <br />how the impacts of this appropriation on existing water rights can be evaluated. <br />2.0 DISCUSSION/ANALYSIS <br />The instream flow demand is added to an ex isting MODSIM network by creating a new demand node <br />and connecting it to the network at the desired loca tion. The attributes of the instream flow demand, <br />including the monthly hydrogr aph of flow demand (in cubic feet/s econd [cfs] or acre-feet/month), the <br />priority date, and the associated "flow through node" and "bypass credit link" are defined in the <br />MODSIM spreadsheets. The bypass credit link is th e link that is monitored by the instream flow <br />demand to determine how much water is alrea dy flowing through the str eam reach and how much <br />additional water must be called for by the demand to the satisfy the requirement. The procedures for <br />creating the new demand node and specifying the attr ibutes of the instream flow requirement are <br />described in the on-line documentation. <br />Two scenarios involving instream fl ow rights in the Gunnison River basin have been considered, as <br />described below. <br />Instream Flow at Redlands Power Canal <br />The first scenario is for imposing a new instream flow right for the Gunnison River for the river reach <br />just downstream of the diversion dam for the Redlands Power Canal. For purposes of this <br />demonstration, the demand is assumed to be at a year round flow rate of 300 cfs (approximately 18,000 <br />1 <br />A275 02.23.94 1.15-29 Fosha, Ritsch <br />