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<br />2361 <br /> <br />. <br /> <br />that enter the river. The Boyle estimate includes subsurface seepage <br />to and from the Reservation, a minor amount of irrigation water <br />pumped directly from the river, and surface runoff entering the <br />Reservation. Referring again to Table 1, these items produce a net <br />inflow of 82,810 acre-feet, containing a net 93,471 tons of salt. When <br />this tonnage is added to the Bureau's 1974 estimate of 45,273 tons <br />based on measured flow, the sum is approximately 139,000 tons <br />retained on the Reservation. This is close to the Boyle estimate of <br />131,428 tons in Table l, and indicates a general harmony between the <br />two semi-independent estimates. <br />These two estimates are based on the inflows to and outflows <br />from the Reservation, and do not indicate the processes within the <br />Reservation that cause salt retention. Consequently the salt retention <br />was analyzed by focusing on several factors that affect salt retention. <br />This analysis produced an estimate of approximately 80,000 tons of <br />salt retained in 1974, which was used for the purpose of this report. <br />This approach also made it possible to synthesize estimates of salt <br />retention for each year of the past decade, as explained below. <br />Factors Affecting Salt Discharge <br />The major factors affecting the Reservation's salt discharge or <br />retention appear to be consumptive use of water by natural vegeta- <br />tion, initial flushing of salts from newly irrigated areas, and flushing <br />or displacement of preexisting ground water. <br />Natural Vegetation. Subsurface water flows to areas of natural <br />growth where it is consumptively used, but the dissolved salts there- <br />in tend to remain in the root zone. Boyle Engineering has estimated <br />that water use by native vegetation was 110,739 acre-feet in 1974. <br />The estimate is based on a consumptive use of 2.0 acre-feet per acre <br />by vegetation upstream from Palo Verde Diversion Dam, and 3.8 <br />acre-feet per acre downstream. [4] <br />The major sources of the water are deep percolation from irriga- <br />tion, canal seepage, and subsurface inflow of river water to the <br />Reservation. The salt content of the water varies from approximately <br />1 ton per acre-foot in river water to 3 tons or more per acre-foot in <br /> <br />14 <br />