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cfs. As such, only the very small (and less depletive) water rights are assumed to be left in <br />the gage. Demands for the included water rights are based on actual historical diversion data <br />and depletions will be estimated by assuming system efficiencies. The total consumptive use <br />will be verified during simulation operations by using estimates of historically irrigated <br />acreage, estimates of crop consumptive irrigation requirement and estimates of irrigation <br />efficiencies (this verification is similar to the procedures used in the YRBAS model). <br />Because of the inclusion of more structures in the Yampa Model, it is believed that the <br />demands in the basin are more clearly defined, in comparison to leaving the majority of the <br />rights in the gage as in the YRBAS. <br />The Yampa Model does not currently contain any provisions for future projected growth and <br />increased water demands. However, since it includes many more explicitly modeled nodes, it <br />would be relatively straightforward to add additional demands to any existing structure or <br />add new demand nodes in the network. <br />In the YRBAS, a number of the inflow nodes for the model (approximately 15) are included <br />to reflect return flows from existing and future basin demands. These are modeled as <br />occurring either in the same monthly time-step (municipal diversions and return flows) or are <br />lagged over a one to four month period (delayed timing pattern for subsurface irrigation <br />return flows). For the latter, a set of lag factors are specified which are then applied against <br />the amount of water delivered into a given node. Uses at the Hayden and Craig power plants <br />are assumed to be 100 percent consumptive. <br />In CDSS, a return flow pattern is assigned to every structure included in the model and <br />follows the same general timing pattern used in the YRBAS (typically a one to four month <br />lag for irrigation return flows). In addition, the location of those return flows has been <br />determined with respect to each downstream ditch structure that could potentially benefit <br />from returns from upstream structures. The latter was accomplished through detailed analysis <br />of irrigated acreage maps provided by the division engineer. Because many more structures <br />are modeled in CDSS, it is believed that the return flow patterns and locations are more <br />definitive than those in the YRBAS. <br />The YRBAS essentially models three existing reservoir: Stagecoach Reservoir, Steamboat <br />Lake, and Elkhead Reservoir, the reservoirs that were the main focus of evaluation for <br />potential enlargements and or changed operation to increase yield. Six other reservoirs in the <br />basin were assigned a node in the CRAM model, but are turned off in the model. Smaller <br />reservoirs are assumed to have a negligible effect on the flows in the river because of their <br />small size and/or method of operation. <br />For the larger reservoirs, the individual sub-accounts are connected only to aggregated <br />demands, particularly in the case of irrigation water. This is consistent with the overall <br />approach to aggregate the majority of the irrigation rights. Storage and releases from <br />reservoirs are driven by assignment of priorities relative to other direct flow demands in the <br />network. In general, releases are made if demands are unsatisfied by diversions of river flow <br />on a direct basis. In the CRAM allocation model, the storage, releases and accounting of <br />storage within sub-accounts in the reservoirs (ownerships, water right priorities, etc.) are <br />Yampa River Basin Information 5-3 <br />