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Study Period and Model Time Steps <br />The study period used for the Rio Grande application of RIBSIM was 1948 through 1985. This <br />period was chosen based primarily on statistical review of unadjusted gage records at the Rio Grande <br />Del Norte gage. Other factors considered in selecting a study period were the availability of <br />diversion and water use records, inclusion of drought years, consistency with previous basin studies, <br />public perception of the study period, and the desirability of including recent years to make the best <br />use of the most reliable data. <br />The Rio Grande model operates on a monthly time step over the period 1948 through 1985 <br />Model Operations <br />RIBSIM is a generalized computer program written in the Microsoft Basic programming language. <br />Various other programs are included with the RIBSIM program to facilitate creation of input data <br />sets and the summarization of the output data. <br />RIBSIM is intended for conducting water supply and demand planning studies. The model is <br />configured to a river basin by the definition of water demands, return flows, reservoirs, instream <br />flow requirements, etc. to be superimposed on a flow network. The model uses traditional <br />bookkeeping or accounting methodology to allocate water supplies to demands. The allocation is <br />performed for each month of the study period based on user assigned priorities of the water rights. <br />Modeling the Rio Grande Compact <br />Representation of the Compact in the Rio Grande model utilized amulti-step process to identify <br />delivery obligations, determine available storage amounts, and account for Compact credit and debit <br />situations. The process begins by using historical annual Compact delivery requirements for the Rio <br />Grande as provided in the Annual Compact Reports. From these annual delivery requirements were <br />subtracted: <br />• return flows between Alamosa and Del Norte; <br />• inflows between Alamosa and Lobatos; <br />• deliveries from the Closed Basin Project; <br />• flood flows in excess of diversion structure capacities; and <br />• a portion of the 10,000 acre-foot paper credit. <br />The annual delivery requirement was then determined by making further adjustments to reflect <br />previously accrued credits (subtraction) or debits (addition). <br />In the operation of the model, the adjusted delivery requirement was made available for storage in a <br />new reservoir. A separate effort to track storage in the Rio Grande Project (Elephant Butte <br />Reservoir) allowed examination of the effects of credit and debit situations on storable yields. Water <br />captured by storage in the model was then added to a debit account where it was kept until storage in <br />Elephant Butte fell below 600,000 acre-feet or there was a spill. In the case of storage falling below <br />600,000 acre-feet, the model released water from the new reservoir to meet the threshold Elephant <br />Butte storage requirement. If a spill occurred, any accrued debit was cancelled and storage in the <br />new reservoir was made available to users in Colorado. <br />C:Acdss\Task3Mem.doc Review Previous Modeling Efforts Apri16, 1999 -Page 10 of 11 <br />