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depending on whether the excess rainfall is stored in the soil root zone for later use or lost through <br />deep percolation. <br />A more detailed tabular result is included in Section 4.0. <br />Data Set 2- Alfalfa-Short and Pasture-Short Crops Considered <br />Parts of alfalfa and pasture areas are assigned short. These areas are calculated by using a ratio (area <br />short/total area) that the USBR adopted in the Colorado River System Consumptive Uses and Losses <br />Report, 1981-85 . The ratio is constant for each subarea for each crop (alfalfa, pasture) as shown in <br />Table 3 and is applied to all years. <br />The length of the growing season for alfalfa-short and pasture-short crops is assumed to be equal to <br />the average of the values that are reported in the USBR Colorado River System Consumptive Uses <br />and Losses Report for 1981-85 . Similarly, the specified planting and cutoff dates in the report are <br />averaged in each subarea to obtain equivalent numbers for the 1985-1990 report. Thus, each crop <br />combination is assigned a single planting and cutoff date for all years, though these numbers can be <br />varied in each year. This increases the number of crop-soil combinations to 47. <br />Figure 4 shows the total annual IWR for the whole San Juan River basin computed using the two <br />forms of the SCS Blaney-Criddle evapotranspiration estimation method (SCS Blaney-Criddle with <br />and without enhancements). As expected, the 6-year average annual IWR is lower than the previous <br />data set run, i.e. 170,590 acre-ft (16.0 percent lower) and 211,230 acre-ft (12.8 percent lower) for <br />SCS Blaney-Criddle with and without enhancements, respectively. <br />Figure 5 shows the 6-year average annual consumptive use by subarea. The highest volumetric <br />water use by subarea, by county, and by HU is in Montezuma2 (51,960 and 61,190 acre-ft), La Plata <br />(84,380 and 107,010 acre-ft), and HU14080101 (56,280 and 69,270 acre-ft), respectively. <br />In the USBR Colorado River System Consumptive Uses and Losses Report, 1981-85 , the <br />determination of alfalfa-short and pasture-short areas has become a prerequisite for consumptive use <br />calculation. This is because of the limitation of the XCONS2 program, which does not <br />automatically account for water supply information in the calculation. <br />A more detailed tabular result is included in Section 4.0 <br />3.0 CONCLUSIONS/RECOMMENDATIONS <br />Depending on the level of detail desired, several different data sets can be prepared for estimating <br />IWR in the San Juan River basin. Smaller subareas may be chosen to increase the representativeness <br />of the weather stations. <br />The IWR, as estimated by the SCS Blaney-Criddle method with enhancements, is consistently lower <br />than the estimate of the SCS Blaney-Criddle without enhancements. This is mainly because of the <br />assumption that in the very first year of the estimation, the soil moisture storage is at full capacity. <br />The other factors causing the difference are the winter precipitation contribution to soil moisture <br />storage at the beginning of the growing season, and the difference in the estimate of the effective <br />rainfall. <br />Page 3 <br />A275 03.07.95 1.14-26 Garcia <br />