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CRPM is currently configured to run for a period of 21 years between 1970 and 1990. This period <br />encompasses a range of hydrologic conditions and relatively consistent Compact administration in <br />the basin. <br />The Conejos model is based on a time step of approximately 10 days, although the time step actually <br />varies throughout the year, from between 8 and 11 days, so that there are 3 time steps per month, and <br />36 time steps per year. This approach was selected to closely correspond with the 10-day period <br />over which the Division 3 Engineer makes decisions related to administration of the Rio Grande <br />Compact. <br />Model Operations <br />At a fundamental level the Conejos model is "driven" by the individual water rights owned by the <br />irrigation ditches, a compact "call" representing water which can not be diverted, inflows from the <br />Conejos, Los Pinos and Rio San Antonio, and by operations at Platoro Reservoir. Specifically, the <br />model takes a set of natural inflows, Closed Basin pumping targets, Norton Drain return flows, Rio <br />Grande Compact requirements and Platoro Reservoir storage levels for a particular time step and <br />computes a corresponding allocation of the water to specific demands within the system. Included in <br />the calculations are allocations to diversions, consumptive use, return flows, evaporation from the <br />reservoir, changes in Platoro storage levels, and outflow from the Conejos River to the Rio Grande. <br />Natural flows are computed from gage records and adjusted to reflect all upstream diversions or <br />reservoir operations. The model maintains a running account of deliveries and obligations to the Rio <br />Grande Compact. <br />The solution to the CRPM network occurs in several steps. A maximum of six solution steps occurs <br />during every time period. Because a solution must often be developed through an iterative process, <br />the actual out-of-kilter algorithm may be used to solve the network multiple times during any one of <br />the six solution steps. Each of the steps is described more fully below. <br />Step 1 -Allocation of Direct Flow to Individual Water Rights <br />The first solution step in the CRPM is called the water rights step. In this step, natural inflows from <br />the Conejos, Los Pinos and Rio San Antonio rivers are allocated among numerous water users which <br />compete for the water based on their individual water rights priorities. The total available water for <br />allocation to direct flow rights includes inflows to the stream system, immediate return flows from <br />ditches diverting that season, and lagged return flows from past diversions. <br />Existing water rights for individual ditches are represented explicitly in the model and the allocation <br />of available water adheres to the fundamental principle of strict administration according to priority. <br />In this step water may be stored by Platoro Reservoir subject to the storage water right priority, i.e. <br />all water rights senior to 1973 are first delivered their full allotments before any water is stored. <br />Step 2 -Storage in Individual Storage Accounts at Platoro <br />In the second step of the model, water may be stored in Individual Storage Accounts under the same <br />priorities as the direct flow rights of the individual ditches. Water may be stored up to a specific <br />percentage of the amount identified as excess supply. Excess supply is water that the ditch is <br />entitled to divert but has no need for in the current time step. Individual ditches may store water in <br />Platoro Reservoir only if they are participating in the District's storage program. The storage space <br />in Platoro for individual storage accounts is allotted among the individual ditches by the model user. <br />The model user may select which ditches can store water at Platoro in individual storage accounts. <br />C:Acdss\Task3Mem.doc Review Previous Modeling Efforts Apri16, 1999 -Page 3 of 11 <br />