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The second approach, method 2, is presented in Table 6. Method 2 uses GIS 1993 total acreage data, but <br />preserves the CAS 1985 crops type. As a result, some of the crop acreages have a zero value because <br />crop types from GIS do not match with CAS crop names. An explanation for each method is given in <br />Tables 5 and 6 after the table body in the form of a numerical example. <br />Both method 1 and method 2 increase the total irrigated acreage for the historical period 1985 through <br />1990 when compared to the original CAS data. The crop-type and acreage distributions were changed by <br />these methods of subdividing the county data into hydrologic units. Method 1, which preserved GIS <br />crop-type used a greater number of crop-types compared to method 2 as seen in Table 4. <br />Knowing that the GIS data will be used in the future for calculating the irrigated acreage in the Gunnison <br />River basin, and given the consistency in the crop types given in the GIS data, the CU Team preferred to <br />apply the first approach to recalculate the historical CAS data using GIS 1993. The IDS Group tested the <br />approach for Delta County with success. <br />The approach was expanded to estimate the irrigation acreage for the entire Gunnison River basin for the <br />years 1985 through 1990 using method 1. Table 7 shows the estimated irrigated acreage for the years <br />1985 through 1990 for all hydrologic units in the Gunnison River basin. Table 8 shows the summary of <br />the hydrological unit irrigated acreage form USBR 1985 and GIS 1993 and the estimated acreage 1985 <br />for the basin. <br />The tables developed in this memorandum show that the estimated irrigated acreage in each hydrological <br />unit in the Gunnison River basin does not accurately reflect what was grown in the years 1985 through <br />1990. By exploring the differences between the two methods, the commonly practiced use of CAS data <br />shows a consistent difference with the GIS approach. This memorandum has demonstrated, in a general <br />sense, that a gap exists between the GIS 1993 data and what the CAS 1993 and CAS 1985 through 1990 <br />data show. Neither method has confirming proof indicating validity; however, the GIS approach offers <br />consistency and detail not as conveniently available in the last decade. <br />Consumptive Use for Estimated Acreage <br />The consumptive use model that is developed for the CRDSS project uses the Gunnison River basin as a <br />test prototype. The 1985 through 1990 irrigated acreage is given in Table 7. The two methods that are <br />used to estimate the consumptive use are (1) using the Soil Conservation Service (SCS) Blaney-Criddle <br />(unmodified), and (2) adopting the CRDSS enhancements to the SCS Blaney-Criddle methodology (Task <br />Memorandum 1.14-14). <br />For each subarea, several combinations of crop and soil types are identified. A total of 33 combinations <br />are considered for the whole Gunnison River basin. Each crop-soil combination uses a single <br />planting/harvesting date regardless of location. The crops that are reported in these areas include alfalfa, <br />pasture, dry beans, corn grain, orchard, tree farms, small grains, and vegetables. <br />Figure 5 shows the annual consumptive use for 1985 through 1990 for the whole Gunnison River basin <br />as estimated by using the two forms of the Blaney-Criddle evapotranspiration estimation method (SCS <br />Blaney-Criddle with and without enhancements). The 6-year average annual use is 333,510 and 384,760 <br />acre-ft for the SCS Blaney-Criddle with and without enhancements, respectively. The peak use is in <br />1989 (115, 113 percent of the average annual use) while the lowest water use occurred in 1986 (80, 82 <br />percent). Figure 6 shows the 6-year average annual consumptive use by subarea. <br />3 <br />A 275 01.09.95 1.14-19 Al-azzawe <br />