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CRDSS <br />TASK MEMORANDUM 1.14-21 <br />Consumptive Use Model <br />Differences Between XCONS2 and CRDSS CU (XCONS2) <br />1.0 ISSUE <br />The Soil Conservation Service (SCS) Blaney-Criddle evapotranspiration estimation method implemented <br />in the CRDSS consumptive use (CU) model and the original XCONS2 program have slightly different <br />output results; this task memorandum identifies the source(s) of difference between the programs. <br />2.0 DISCUSSION/ANALYSIS <br />A first attempt was made to generate consumptive use values using both the XCONS2 and the CRDSS <br />CU programs. Unfortunately, because of some differences in the input data for both programs, the <br />results of this work were not conclusive. One of the first observations concerning the difference in <br />results was that most of the time, the CU model was giving higher values for the net consumptive use <br />than the original XCONS2 program. Also, some specific county and hydrologic unit combinations were <br />identified where the differences were more consistent, and the case with the highest difference (namely <br />Delta5) was selected for further investigation (Table 1). <br />In a second attempt at comparing the results of both programs used the same input data taken from the <br />technical Appendix (volume 2) of the Colorado River System Consumptive Uses and Losses Report, <br />1981-1985 was used. An overview of the output data and the different steps leading to the estimation of <br />the net consumptive use showed that the differences in the two programs originated with the estimation <br />of the crop evapotranspiration using the SCS Blaney-Criddle method. <br />In order to compare and appreciate the output of both programs, manual computations of the crop <br />evapotranspiration were performed using the SCS Blaney-Criddle method as described by Technical <br />Release No. 21 titled ?Irrigation Water Requirements? (USDA/SCS, revised September 1970). The <br />worst case in terms of differences in the output data for the net consumptive use (Delta5) and the worst <br />case in terms of crop evapotranspiration data (vegetables) were selected for analysis. Crop <br />evapotranspiration for alfalfa was also evaluated, which provides closer results, in order to have a <br />broader view of the problem. <br />Summary for Net Consumptive Use (CRDSS versus XCONS2 <br />) <br />Table 2 gives a summary of the computations of net consumptive use in acre-feet using both programs <br />(CRDSS CU and XCONS2) as compared to values presented in the Colorado River System Consumptive <br />Uses and Losses Report, 1981-1985 . <br />From Tables 1, 2, and 3 it can be seen that (1) sometimes there are differences between the XCONS2 <br />output data and the book values, and (2) most of the time there are differences between the XCONS2 <br />values and CRDSS CU values. These can be positive and negative differences, but most of the time the <br />differences between CRDSS CU and XCONS2 are positive, indicating that CRDSS CU is giving higher <br />results. <br />1 <br />A275 01.09.95 1.14-21 CSU-IDS <br />