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ALTERNATIVE PLANS Most of the surface irrigated fields in Virgin Valley need improved onfarm and distribution systems and management, One-half of the 1iater withdrawn for irrigation goes to deep percolation. This percolating water dissolves salts from the underlying strata, resulting in increased salt loading of !lubsurface flows. Major irrigation system improvements and better irrigation water management would increase efficiency and reduce deep percolation. Formulation of irrigation alternatives considered: (1) water control and measuring devices; (2) using the irrigation system and method best adapted to the soils, crops and level of management desired; and (3) applying the proper amount of water at the proper time to meet consumptive use plus management practice (leaching, sluicing, prewetting, etc.) requirements. Improved water management would alleviate some drainage problems caused by over irrigation, Land leveling, and land planing on a regular basis are important conservation practices included in the plans for better distribution of irrigation water on the field. Land planing will continue as a part of the ongoing program regardless of which alternative is implemented. The SCS computer program for Irrigation Methods Analysis (IRMA) was used to estimate irrigation efficiencies, gross and net application, net runoff and deep percolation for the alternative water management plans (see Table 8). Water and salt budgets (Table 9) were prepared for alter- native plans using IRMA output information for crop net application and deep percolation to ground water. Future land use was assumed, The pre- dominate surface irrigation method is graded borders, The alternatives minimize changes in irrigation methods. Minor changes in crops and field sizes are suggested, Changes in irrigation methods, crops, field sizes and land use are dictated by physical and institutional constraints, such as road location, land ownership and water rights, The sample of fields processed in the computer analysis is representative of crops and their location. Computer analyses evaluated 600 acres (17 percent) of the total surface irrigated land, Gully and channel. erosion of saline soils contribute both salt and sediment to Virgin River, Water erosion results in salts being removed through solution and the sediment detached and moved by surface waters, Vegetative measures are difficult to establish because of low rainfall, Structural measures which trap sediment are not recommended because of the high cost compared to salinity benefits. Erosion control measures are limited to those sssociated with improved irrigation water management, Changes in land use were considered for the irrigated area. Only a small portion of the subevaluation unit is in private ownership. Purchase of irrigated lands for conversion to nonirrigated land was considered, but was not recommended by Virgin Valley irrigators, Future state and local 38 l": t 1(:" ,') ..., ."""v"\..r't