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1,084,000 acres (Table 2). Hence, approximately 25 percent of the l.ano remains fallow in a given year. Nonirrigated acreages based upon harvested acreage and total availahle dry croplands also are compared (Table 5). Overall, approximately 55 percent of the dry cropland was harvested in 1978. The variation in harvested acreage to total dry cropland acreage verifies that nonirrigated crops, such as wheat and barley, are typically grown only in alternate years. Consumptive Use The net consumptive use values for crops grown in the basin were estimated for each of the 3 generalized zones (Table 6). Net consumptive use is defined as the total consumptive use minus effective precipitation, where total consumptive use is the water lost to the atmosphere from plant transpiration and evaporation from soil and water surfaces. Effective precipitation is defined as precipitation that falls during the growing season of the crop and is available to meet the consumptive water requirements of the crop. The total consumptive use values were prepared for 14 sub-basins for crops grown in the basin (U.S. Army Corps of Engineers, 1977). These data were aggregated into data for the 3 zones of the basin (Mountain, Transition, and Plains zones) in order to estimate the net consumptive use values. The consumptive use value for each crop, based upon the 1978 crop mix is given in Table 7. Using a weighted average net consumptive use figure of 1.15 acre-feet per acre and the total irrigated acreage of 793,000 acrp.s, the total net consumptive use is about 914,000 acre-feet for the 1978 crop mix (Table 7). Water requirements for irrigation to meet the net consumptive use of the crops grown are influenced by the efficiency at the farm. Typically, farm irrigation efficiencies for sprinkler irrigation are higher than for -24- l I I I I I I I I I I I I I I I I I I~