CRDSS_Task1_14-26_IrrigationCULosses_SanJuanBasin

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the effects of water-short areas. The effect of water shortage is taken into account by identifying alfalfa-short and pasture-short areas. The weather stations recommended by the USBR (shown in Table 2) are used. Five weather stations are identified; multiple stations are not assigned to any one subarea. The data sets are prepared to support the input requirements for estimating evapotranspiration by SCS Blaney-Criddle. This gives two consumptive use estimates: (1) by using the SCS Blaney-Criddle (XCONS2); and (2) by adopting the CRDSS enhancements to the SCS Blaney-Criddle methodology. The data set for Penman-Monteith calculations cannot be prepared for the whole San Juan River basin, mainly because of a lack of representative daily weather data for the whole basin. Data Set 1 - Water-short Areas Not Considered For each subarea, several unique combinations of crop and soil types are identified using the GIS data. A total of 33 combinations are considered for the whole San Juan River basin. It should be noted that the same crop and soil combination in a subarea (i.e. alfalfa in clay-loam) could have different crop (i.e. planting/harvesting dates), soil (i.e. water holding capacity, AWC) and management (i.e. management allowed depletion [MAD]) characteristics. Permutations of these other characteristics would dramatically increase the number of combinations. For this data set, a constant planting/harvesting date is specified for each annual crop regardless of its spatial position in the basin. The dates for perennial crops are automatically specified by the model based on earliest moisture use in the spring and latest moisture use in the fall. The MAD varies according to the type of crop. Figure 1 shows the annual irrigation water requirement (IWR) for 1985-1990 for the whole San Juan River basin as estimated by using the two forms of the SCS Blaney-Criddle evapotranspiration estimation method (SCS Blaney-Criddle with and without enhancements). The 6-year average annual use is 203,050 acre-ft and 242,340 acre-ft for the SCS Blaney-Criddle with and without enhancements, respectively. The peak use was in 1989 (131, 124 percent of the average annual use) while the lowest water use occurred in 1986 (72, 78 percent). Figure 2 shows the 6-year average annual IWR by subarea. It should be noted that the highest volumetric water use by subarea, by county, and by HU is in Montezuma2 (54,760 and 63,660 acre-ft), La Plata (109,650 and 131,400 acre-ft), and HU14080101 (66,850 and 79,320 acre-ft), respectively. Figure 3 shows the 6-year average annual IWR in terms of unit depth for each subarea. The average value for the whole San Juan River basin is 14.4 and 17.20 inches using the SCS Blaney-Criddle with and without enhancements, respectively. The highest per unit depth water use by subarea, by county, and by HU is in Montezuma7 (17.4 and 20.0 inches), Montezuma (17.2 and 19.9 inches), and HUC14080107 (17.4 and 20.0 inches), respectively. The per unit depth IWR for Hinsdale2 is only 9.2 and 12.2 inches for the SCS Blaney-Criddle with and without enhancements, respectively. The IWR estimate by the two methods differs by approximately 39,000 acre-ft, where the enhanced version of the SCS Blaney-Criddle method gives the lower estimate. The bulk of this difference can be attributed to an assumption that in the first year (1985), the soil moisture capacity is full. In the enhanced version, where soil moisture budget is performed, the crop is assumed to be able to deplete this soil moisture storage. Other factors that contribute to the difference in the estimates are (1) replenishment of the soil moisture storage (from winter precipitation) prior to the growing season, and (2) revised values for effective rainfall. The latter could be a positive or a negative change Page 2 A275 03.07.95 1.14-26 Garcia