CRDSS_Task2-09-5_CropCUEstimates_UpperColoBasin

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The total irrigated lands, lands being modeled, and percent of lands being modeled for each county-huc are shown in Table 6. This information can be used to evaluate the validity of the pro-rated estimates of CU and amount of water short in a county-huc (i.e. whether the county-huc combination is adequately modeled by the structures). Sinc e a number of structures are not present on the GIS coverage (Table 4) their areas were assumed to be part of the county-huc the structures was determined to be located at. The avrage pro-rated estimate of CU is 341,170 acre-ft (Table12). The average pro-rated estimate of shortage is 60,429 acre-ft (Table 11). The average pro-rated estimate of IWR is 401,600 acre-ft (Table 10). Table 13 shows the average water supply, IWR and water short for each structure. Table 14 shows the same information by year. In Table 14 diversion structures with water shortages greater than 50%, between 25% and 50%, and between 10% and 25% are marked with an ***, **, or * respectively. Table 8 shows that 1988, 1989, and 1990 are by far the years with the greatest water shortages. The water supply for ditches showing shortages in the range of 25%-50% or more than 50%, should be checked to make sure these shortages are correct. These ditches can be identified using Table 14. 2.6 Issues for Review The results presented here for the CU Model indicate a pattern of middle a nd late growing season shortages. Some of the shortages might be caused by the fact that no soil moisture is being modeled. These shortages could reflect the practice of having spring diversions in excess of what is needed to meet the IWR in the Spring, in order to replenishing the soil profile from losses through the winter. This soil moisture is used in the middle and late growing season when the diversions are less than the IWR. This pattern of consumptive use shortages indicates that using a soil moisture budget might yield b etter results in the future. The weights assigned to each weather station serving a county-huc are based on the area of the county- huc served by each weather station. This is probably a good assumption when modeling the whole irrigated area, but could introduce some errors when modeling individual diversion structures. The reason for this, is that a structure might be located in an area of the county-huc that is represented by only one weather station. Therefore, when modeling individual diversion structures consideration should be given to further breaking the county-huc areas by the zones where each weather station could be applied. If any ditches are considerably water short but historically divert small volumes of wa ter compared to Head Gate Requirements (which are computed based on the IWR values and the efficiency), the diversion records and the area assigned to these ditches should be considered for future review to determine the cause of these discrepancies. If any ditches serving irrigated areas have a large discrepancy between the historical diversion and the calculated Head Gate Requirement, the diversion records and the area assigned to these ditches should be considered for future review to determine the cause of the discrepancy. In the process of doing the pro-ration between the areas being modeled by structures and the total irrigated areas calculated based on the GIS maps, two issues were discovered. First, the areas represented by the structures could have a different crop distribution than the total irrigated areas (this is more likely when the percent of area modeled is small). The second issue relates to the fact that we used the location of the diversion structures to determine what county/HUC to use for the weather stations. The GIS uses the location of the lands to determine what county/HUC the lands served by a diversion 3 Page 12/16/96 2.09-05 CSU/IDS