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5. Yampa Basin Modeling Efforts <br />5.1 YRBAS Model <br />The CDSS data collection effort called for review of the assumptions, databases, and results <br />of prior modeling efforts in the Yampa River basin. This section summarizes the objectives, <br />key modeling assumptions and results of recent water rights modeling efforts conducted by <br />Hydrosphere Resource Consultants (Hydrosphere) for the Colorado River Water <br />Conservation District (CRWCD) and the CWCB for the "Yampa River Basin -Alternatives <br />Feasibility Study" (YRBAS). This study examined current and future water supply and <br />water-based recreation needs in the Yampa River basin. It also evaluated the potential for <br />establishing an instream flow right on the Yampa main stem using a portion of the <br />CRWCD's conditional water rights decreed to the Juniper Project. The study focused on the <br />potential for development of new storage facilities to meet existing and future water needs in <br />the basin while enhancing a river regime conducive to the recovery of endangered fish <br />species in the lower Yampa/Green Rivers. The Phase I Study was completed in March 1993 <br />and recommended anear-term enlargement of the existing Elkhead Reservoir and the long- <br />term enlargement of Stagecoach Reservoir. The new storage would serve as an augmentation <br />source for future uses by junior water rights in the basin without impacting the instream <br />flows available under the Juniper water rights. In March 1995, a Phase II Study was <br />completed and provided additional detailed analyses of the simulated operations of the <br />recommended alternative to enlarge Elkhead Reservoir. This memorandum provides a review <br />of the procedures and assumptions adopted for the modeling efforts for both phases of the <br />study, and discusses the relevance of the modeling work to the current efforts of the CDSS. <br />For the YRBAS, a network flow modeling system developed by Hydrosphere, known as the <br />Central Resource Allocation Model (CRAM), was utilized. The network for this model <br />consists of a system of nodes and arcs. The nodes typically represent points of inflow, <br />outflow and junctions of flow in the stream system network, while the arcs connect the nodes <br />and typically represent paths through which water must flow, subj ect to certain prescribed <br />constraints. The flow through any arc is assigned a positive or negative cost, which in water <br />rights modeling generally relates to the priority of a water right. The solution of the network <br />is the set of arc flows that produces the minimum total cost (or maximum value) for the <br />entire network. As such the network solution employed by the CRAM model can be <br />considered a form of optimization model. For CDSS, the model StateMod has been adopted <br />for the Yampa Model. This model can be considered as an allocation model that distributes <br />available water to the demands in the network in strict accordance with a described set of <br />priorities (water rights). As a result of fundamental differences in the two modeling <br />approaches, there is little opportunity to directly import data files and assumptions related to <br />complex administrative issues from the CRAM platform to the StateMod platform. <br />In the YRBAS, 25 USGS stream gages were identified to assist in estimating inflows to <br />different locations in the model. The majority of these gages were also used in the CDSS, <br />Yampa River Basin Information 5-1 <br />