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Model Input Data Requirements <br />The RIBSIM model required two primary types of data to operate. A "Water Use Network" <br />provides information to the model physical features (ditches, reservoirs, instream flows, etc.) to be <br />operated in a simulation. Included are the location, priority, maximum water use level and type of <br />right of these features. The second type of data required by the model is a "Flow Base." <br />Flow Base <br />The Flow Base is a set of monthly flows at various locations in the river basin on which to <br />superimpose the modeled features of the basin. These flows usually consist of gaged flows adjusted <br />for the historical operation of those features being modeled. The Flow Base for the Rio Grande <br />application of RIBSIM was generated by estimating monthly incremental flow originating in each <br />of several flow sectors during the study period. Flow sectors were defined as distinct flow <br />reaches where there is a significant change in flow regime affecting modeled basin features. During <br />operation of the model, incremental flow sectors above a given point are summed to arrive at the <br />total flow available at that point. This Flow Base represents the flow available to the features being <br />modeled, but does not represent undepleted or natural flows. <br />Water Rights <br />RIBSIM allocates water by user assigned priorities of the water rights in a manner consistent with <br />the prior appropriation doctrine. Because of the large number of water rights decreed along the Rio <br />Grande, a simplification scheme was used to group certain rights whose diversion patterns were no <br />expected to be effected by model assumptions. Several groups of smaller rights were also developed <br />to further ease computational and interpretation burden. <br />The model included the operation of the major water rights located between Del Norte and Alamosa <br />in Water District 20 which could significantly affect water availability at potential reservoir sites. <br />Other smaller rights were not represented under the assumption they would not significantly affect <br />water availability. Operation of rights in other river reaches was implicitly considered by the <br />incorporation of gains and losses from the study period calculated Flow Base. <br />The water rights of the Centennial, Excelsior and Rio Grande Piedra canals, because of their very <br />senior priorities, were aggregated into a single senior priority right. This right was assigned typical <br />diversion levels experienced over the study period. Also included in this grouping were <br />approximately 150 rights located between Del Norte and Lobatos that are senior to the primary <br />eleven ditch rights. <br />Over 70 percent of diversions from the Rio Grande River occur under water rights (approximately <br />110) owned by the so called Big Eight: 1) Rio Grande, 2) Empire, 3) Farmers Union, 4) Monte <br />Vista, 5) San Luis Valley, 6) Prairie, 7) Costilla and 8) Rio Grande & Lariate canals. Rather than <br />representing each right individually in the model, they were combined into 40 groups of rights. <br />The combination process preserved the nature of the major rights (greater than 20 cfs) by combining <br />the smaller decrees with the larger decrees for each ditch. Smaller rights were generally assigned to <br />the nearest priority of the larger decrees modeled. The 40 groups of combined rights represented <br />approximately 4,500 cfs in appropriations. <br />Storage releases from major reservoirs upstream of Del Norte (Rio Grande, Santa Maria, and <br />Continental) to downstream users have made significant contributions to the yield of several ditches. <br />Releases made during the study period were removed from the gaged records and maintained in a <br />special account for delivery to the appropriate ditches and in the pattern historically experienced. <br />C:\cdss\Task3Mem.doc Review Previous Modeling Efforts Apri16, 1999 -Page 9 of 11 <br />