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2374 a crop consumptive use of 4.02 acre-feet per acre, derived from the 1974 cropping pattern for the Reservation. Water Supply Optimization Conveyance System Efficiency. In 1974, the distribution system was operated with an apparent efficiency of 72.5 percent, where operational waste was 13.3 percent of the gross diversion, and seep- age plus evaporation was 14.1 percent. In order to determine the maximum distribution system efficiency attainable, the operation of several projects in the Yuma area was examined. The lined distribution system of the Wellton-Mohawk Irrigation and Drainage District seems to provide the best example. Records indicate that it is operated at an efficiency of between 89 percent and 92 percent while serving about 65,000 irrigated acres. Two smaller systems that have a combination of lined laterals and buried pipe laterals have higher efficiencies--the Yuma Irrigation District serving about 10,000 acres has loss and waste of only 3 to 4 percent, and the Yuma Mesa Project serving about 20,000 acres has system loss and waste of only 6 to 8 percent. Based on these examples, it is estimated that the Reservation's distribution system could attain an 88 percent operating efficiency if all canals were lined and operational waste were reduced to the un- avoidable minimum. Using information from the 1974 operation, it is estimated that operational waste could be reduced from 13.3 percent to 7 percent. Thus the progression of conveyance system efficiency improvement would be: System efficiency in 1974 72.5 percent System efficiency with operational waste reduced to 7 percent 79 percent System efficiency with operational waste at 7 percent and with all laterals lined 88 percent In order to reduce operational waste to a minimum, it will be necessary to develop a more precise water scheduling procedure on the project. Greater coordination will be needed between orders for 25