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Where Table 18 shows a reduction in the volume of water to be delivered under improvement alternatives the reduction in farm delivery will be achieved by adjusting the frequency of irrigation and the application time to the field. The flow rate delivered to the farm headgate will not be changed. Reduced ditch seepage, deep percolation and field runoff under improved management account for the indicated return flow reductions. Analysis reveals that present irrigation practice provides 68 percent of crop consumptive use requirements, however, subsurface water must be supplying additional water to support plant growth and present crop yields. The estimated consumptive use requirement for the 60,000 acres is 138,000 ac.ft. per year. Meeting the crop consumptive use require- ment is an objective of the four alternative improvement levels; increasing the efficiency of irrigation is another objective obtained. Under Alternative One, more water will be needed at the farm to overcome the present consumptive irrigation shortages. less water is needed for Alternative Two as irrigation practice becomes more efficient. Reducing runoff by reducing the flow rate and time of set is the key factor in increasing irrigation efficiency. Deep percolation losses increase as flow rate decreases and the duration of each irrigation or number of irrigations increases. The effect of these conditions is reflected in higher deep percolation for Alternatives 2A and 3A. SALT BUDGET ANALYSIS The salt budget accounts for salt pickup through erosion, deep perco- lation from field irrigation, and seepage from field ditches. Irrigation water delivered to the farms has an average salt concentration of 500 mg. per Ijter. Base flow returning to the river in the drains and washes has concentrations ranging from 9,000 to l,600 mg. per liter with an average of 4,200 mg. per liter based on measurements taken at 75 stations. Salt carried by the irrigation water percolates through the soil profile and i~ concentrated as plants extract water needed for growth. In addition to the effect of concentration water lost through deep percolation, seepage from earth ditches that carry irrigation water to each field, and seepage from ditches that collect tailwater runoff has the capacity to increase its salt load. Although the deeper man- made drains and natural washes ultimately convey excess surface water back to the river, flow occurs near the groundwater level and essentially nO seepage losses occur. For this budget volumes of seepage and deep percolation were summarized for each drainage area. The salt concentration from each area was used to calculate the salt pickup attributed to that area. The total salt pickup attributed to irrigation is the sum of the individual salt loads calculated for each drainage area. Table 19 shows the salt budget for various levels of improved irrigation systems and management practices. 1656 49 ~