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<br />Summary and Conclusions <br />Approximately 92,000 acres of land are irrigated in the Palo <br />Verde Irrigation District, most of which are on the flood plain of the <br />Colorado River. The District's entitlement to Colorado river water is <br />stipulated as the amount necessary to irrigate a gross area of 104,500 <br />acres, and there is no diversion limitation. <br />An analysis based on 1974 operational data indicated that the <br />914,000 acre-feet diverted from the river contained 945,000 tons of <br />salt, and that 467,000 acre-feet of return flow to the river contained <br />1,097,000 tons of salt. The di fference of 152,000 tons of salt was <br />the net discharge to the river. For analysis, the District was <br />divided into seven subareas, which were found to vary greatly in their <br />salt discharge. Based largely on 1974 data, five subareas were found <br />to discharge various amounts of salt and two were found to retain <br />salt. The variation among them apparently results mainly from dif- <br />ferences in the quality of the underlying ground water. <br />The subarea wi th the greatest di scharge by a substantial margi n <br />is the Palo Verde Subarea in the southwestern part of the District, <br />which discharged 144,000 tons. This Subarea was found to be underlain <br />by a sizable body of saline ground water that is gradually being <br />flushed out by percolating irrigation leaching water and canal seep- <br />age. In the future the salt di scharge is expected to continue in <br />gradually declining amounts until the excess salt is gone, a process <br />which will take well over a hundred years, although most of it may be <br />gone within 60 years. <br />The rate of salt discharge is theoretically proportional to the <br />amount of subsurface drainage, so an improvement in water use effi- <br />ciency would result in a reduction in annual salt discharge. The <br />present onfarm irrigation efficiency in the Palo Verde Subarea is <br />estimated to be approximately 42 percent, and the unlined water dis- <br />tribution system also contributes to subsur~ace drainage. <br />Several alternative methods of reducing the salt discharge to the <br />river from the Palo Verde Subarea were examined at a preliminary <br /> <br />002841 <br /> <br />3 <br />