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also be considered, or at least some studies should be conducted to quantify the errors that might be <br />introduced by the current approach. <br />In the San Juan, Dolores and San Miguel River Basins it appears that some shortages occu r later in the <br />growing season (Jun. Jul, Aug, and Sep), this can be seen in ditches such as 301013 (Florida Canal). This <br />ditch in 1989 only shows water supply in April, May, and June showing a total shortage for the year of <br />over 15,000 acre-ft. The historical data in the sanjuan.ddh shows that this ditch diverted in all months of <br />the 1989 irrigation season, therefore the reasons for the shortages should be further studied. Similarly, <br />ditch 322006 (Dove Creek Canal) shows a shortage of over 28,000 acres-ft in 1990. It shows water <br />supply for April and May in 1990 but it historically (sanjuan.ddh) diverted in all months of the 1990 <br />irrigation season. These are just two significant examples where shortages are present that might need to <br />be reviewed. <br />There are other ditches such as 990707 (Gurley Irrig) that are considerably wa ter short (in excess of <br />14,000 acre-ft shortages in 1989 and 1990) but historically diverted small volumes of water compared to <br />Head Gate Requirements (which are computed based on the IWR values and the efficiency). In these <br />cases the diversion records and the area assigned to these ditches should be considered for future review <br />to determine the cause of such big discrepancies. <br />Since some ditches have been determined to be water short, it is likely that the opposite might also be the <br />case. There might be ditches for which the Head Gate Requirements are small compared to the historical <br />diversions for that ditch. <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 crop distribution than the total irrigated areas (this is <br />more likely when the percent of area modeled is small). The second issue relates to the fact that we used <br />the location of the diversion structures to determine what county/HUC to use for the weather stations. <br />The GIS uses the location of the lands to determine what county/HUC the lands served by a diversion <br />structure are located. When determining what percent of a county/HUC is being modeled we sum all the <br />areas associated with the structures being modeled in a given county/HUC. If the lands and the diversion <br />structure that serves them are not located in the same county/HUC an error would be introduced in <br />determining the percentage of land being modeled. Also, the weather stations to use for a structure <br />would change. It would be better in the future to assign county/HUC locations to the diversion structures <br />based on the location of the majority land-area served by each structure and not the location of the <br />structure. <br />3.0 CONCLUSION <br />The crop consumptive use and amount of water short in a county-huc combination are estimated using <br />the results of CU calculations at the structure level. The pro-rated 6-year annual average estimated CU <br />for the irrigated areas in the 19 county-huc combinations being modeled is 264,046 acre-ft. The pro-rated <br />6-year annual amount of water short for the total irrigated areas in the 19 county-huc combination being <br />modeled is 58,199 acre-ft. <br />Based on the issues presented in the issues for review section of this task memo, it appears that some of <br />the shortages presented are created by the fact that no soil moisture accounting is being used. Also in <br />ditches with large discrepancies between historical diversions and CU demands the diversion records and <br />the areas assigned to the structure should be verified. <br />4 <br />12/16/96 2.09-08 CSU/IDS <br />