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2.2 Crop Distribution for each Diversion Structure <br />The crop distribution of the irrigated areas served by each diversion structure is obtained using <br />the DMI demandts as shown in Table 3. In cases in which the crop distribution for a particular <br />structure cannot be identified, the crop distribution of the entire county-huc combination was <br />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 CR DSS-CU Blaney-Criddle model has the <br />capability of using a soi l moisture budget. For this phase of CRDSS, the State decided to use the <br />SCS Blaney-Criddle method without the soil moisture budget. <br />In addition to the soil moisture budget, the CRDSS-CU model has the capability of computing <br />CU using the Penman-Monteith equation. Some preliminary studies showed the potential for <br />significant differences in the CU values computed using the SCS Blaney-Criddle and the <br />Penman-Monteith. The State has a task force reviewing the results of the two equations. Based <br />on extensive prior work by many researchers , there is a well established body of knowledge that <br />shows that as meteorological data becomes available the Penman type equations can provide <br />more accurate estimates of crop CU when us ed properly. However, since this area did not have <br />sufficient weather data or the quality of these data was inconsistent, the SCS Blaney-Criddle was <br />selected for this modeling effort. <br />The weather stations that were recommended to be used in White River Basin as part of the <br />CRDSS project (Task Memorandum 2.09-15) were adopted. The weather stations and associated <br />weights for the county-huc combinations in the White River Basin are shown in Table 4. The <br />weights of each weather station for each key structure is obtained by adopting the weights <br />associated with the county-huc combination where the key structure is located. For the Upper <br />Colorado River Basin the county-huc assigned to each structure is based on the location of the <br />structure and not the location of the area it serves. <br />2.4 Water Supply results from STATEMOD <br />The average water supply for all structures being modeled is 22,772 acre-ft, the usable water <br />supply is equal to the estimated CU which is 22,706 acre-ft (Table 8). while the average <br />Irrigation Water Requirement (IWR) is 23,124 acre-ft (Table 6) which represents an average <br />shortage of 418 acre-ft. Although the annual water supply is close to the IWR, the years 1988 and <br />1990 have by far the largest shortages as can be seen in Table 7. <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 <br />being modeled is pro-rated (using GIS data) to estimate the total CU for each county-HUC. This <br />is done by generating a GIS report for all the structures in each county-huc and determining the <br />2 <br />12/16/96 2.09-06 CSU/IDS <br />