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percentage of irrigated acreage modeled for each county-huc in the basin. The demand, supply <br />and shortages can then be pro-rated based on this percentage. <br />The total irrigated lands, lands being modeled and percent of lands being modeled for each <br />county-huc are shown in Table 5. This information can be used to evaluate the validity of the <br />pro-rated estimates of CU and amount of water shor t in a county-huc (i.e. whether the county-huc <br />combination is adequately modeled by the key structures). The average pro-rated estimate of CU <br />is 31,757 acre-ft (Tables 11). The average pro-rated estimage of shortage is 794 acre-ft (Table <br />10). The average pro-rated estimate of IWR is 32,550 acre-ft (Table 9). <br />Table 12 shows the average IWR, water supply and water short for each structure. Table 13 <br />shows the same information by year. In Table 13 structures with water shortages greater than <br />50%, between 25% and 50%, and between 10% and 25% are marked. Table 7 shows that 1988 <br />and 1990 are by far the years with the greatest water shortages. The water supply for ditches <br />showing 25%-50% or more than 50% should be further studied. These ditches can be identified <br />using Table 13. <br />2.6 Issues for Review . <br />The weights assigned to each weather station serving a county-huc are based on the area of the <br />county-huc served by each weather station. This is probably a good assumption when modeling <br />the whole irrigated area, but could introduce some errors when modeling individual structures. <br />The reason for this, is that a structure might be located in an area of the county-huc that is <br />represented by only one weather station. Therefore, when modeling individual structures <br />consideration should be given to further breaking the county-huc areas by the zones where each <br />weather station could be applied. <br />If any ditches serving irrigated areas have a large discrepancy between the historical diversion <br />and the calculated Head Gate Requirement, the diversion records and the area assigned to these <br />ditches should be considered for future review to determine the cause of the discrepancy. <br />In the process of doing the pro-ration between the areas being modeled by structures and the total <br />irrigated areas calculated based on the GIS maps, two issues were discovered. First, the areas <br />represented by the structures could have a different c rop distribution than the total irrigated areas <br />and therefore the pro-rated values would not be a good representation of the total area (this is <br />more likely when the percent of area modeled is small). The second issue relates to the fact that <br />we used the location of the diversion structures to determine what county/HUC to use for the <br />weather stations. The GIS uses the loc ation of the lands to determine what county/HUC the <br />lands served by a diversion structure are located. When determining what percent of a <br />county/HUC is being modeled we sum all the areas associated with the structures being modeled. <br />If the lands and the diversion structure that serves them are not located in the same county/HUC <br />then the weather stations that should be used would be different. This means that if you use the <br />location of the diversion structure you would get one set of weather stations to use and if you use <br />the location of the lands you would get a different set of weather stations to use. It would be <br />3 <br />12/16/96 2.09-06 CSU/IDS <br />