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Of the 254 diversion structures, there are 10 structures for which GIS data is not available with a total <br />area of 43,811 acres, these structures are shown in Table 4. <br />2.2 Crop Distribution for each Diversion Structure <br />The crop distribution of the irrigated areas served by each diversion structure is obtained using the DMI <br />demandts. The header and a small sample of the crop distribution file generated by demandts is shown <br />in Table 3. In cases in which the crop distribution for a particular structure cannot be identified, the crop <br />distribution of the entire county-huc combination was adopted. <br />2.3 Irrigation Water Requirement Calculation <br />The CU for each structure being modeled is estimated using the SCS Blaney-Criddle method as <br />implemented in the CRDSS-CU model. The CRDSS-CU Blaney-Criddle model has the capability of <br />using a soil moisture budget. For this phase of CRDSS, the State decided to use the SCS Blaney-Criddle <br />method without the soil moisture budget. <br />In addition to the soil moisture budget, the CRDSS-CU model has the capability of computing CU using <br />the Penman-Monteith equation. Some preliminary studies showed the potential for significant <br />differences in the CU values computed using the SCS Blaney-Criddle and the Penman-Monteith. The <br />State has a task force reviewing the results of the two equations. Based on extensive prior work by many <br />researchers, there is a well established body of knowledge that shows that as meteorological data <br />becomes available the Penman type equations can provide more accurate estimates of crop CU when <br />used properly. However, since this area did not have sufficient weather data or the quality of these data <br />was inconsistent, the SCS Blaney-Criddle was selected for this modeling effort. <br />The weather stations that were recommended to be used in the Upper Colorado River Basin as part of the <br />CRDSS project (Task Memorandum 2.09-14) were adopted. The weather stations and associated weights <br />for the county-huc combinations are shown in Table 5. For the Upper Colorado River Basin the county- <br />huc assigned to each structure is based on the location of the structure and not the location of the area it <br />serves. <br />2.4 Water Supply results from STATEMOD <br />The average water supply for all diversion structures being modeled is 235,758 acre-ft.The usable water <br />supply is equal to the estimated CU which is 235,285 acre-ft (Table 9). The average irrigation water <br />requirement (IWR) is 275,253 acre-ft (Table 7) which represents an average shortage of 39,968 acre-ft <br />Although the annual water supply is close to the IWR, the years 1988, 1989, and 1990 have the largest <br />shortages as can be seen in Table 8. <br />2.5 Pro-ration of CU of modeled Structures to CU of County-HUC <br />At this time only a portion of the irrigated area is being modeled. Therefore, the CU for the area being <br />modeled is pro-rated (using GIS data) to estimate the total CU for each county-HUC. This is done by <br />generating a GIS report for all the structures in each county-huc and determining the percentage of <br />irrigated acreage modeled for each county-huc in the basin. The demand, supply and shortages can then <br />be pro-rated based on this percentage. <br />2 <br />Page <br />12/16/96 2.09-05 CSU/IDS <br />