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<br />2368 <br /> <br />of salt contained in the new land developed each year. The resulting <br />salt discharge from new land developed between 1965 and 1976 was <br />estimated to reach a peak of 97,000 tons in year 1970 and then to <br />decline to about 71,000 tons in 1976, as shown by Plot C on Figure 5. <br />It was assumed that salt flushed from new lands prior to 1965 was <br />already in the ground water and was accounted for in the ground- <br />water sampling program conducted by the Geological Survey. <br />Historic Salt Budget <br />The combined effects of water use by natural vegetation, <br />flushing, and leaching are illustrated on Figure 5 by the "net effect" <br />plot. It was constructed by combining the numerical values of Plots <br />A, B, and C. <br />The results of this analysis indicated a net salt retention of <br />approximately 80,000 tons for 1974, which was adopted as the estima- <br />ted salt retention for purpose of this report. The significance of the <br />tonnage is that an 8,000-ton variation in salt discharge or retention <br />causes a change in Colorado River salinity of approximately 1 milli- <br />gram per liter at Imperial Dam. <br />The net effect plot indicates that the amount of salt retention <br />has varied in the past. During the few years immediately prior to <br />1965 natural vegetation caused a substantial annual salt retention and <br />little flushing occurred. Beginning in 1965, a 5-year period of <br />steady agricultural expansion occurred during which natural vegeta- <br />tion declined and the flushing of salt from new land rose. In 1971 <br />the rate of new land development decreased, ending the rise in initial <br />salt flushing and allowing it to decline somewhat during the mid- <br />1970's. <br />Reconciliation With ISSP Data <br />In order to compare the results of this exercise with the results <br />of the ISSP program, the net salt discharge based on ISSP data <br />(bottom plot of Figure 3) was plotted on Figure 5. Since the ISSP <br />data included only measured flows, an adjustment was needed to <br />reflect unmeasured flows. According to the estimate in the Boyle <br />Report the net unmeasured salt inflow was 93,471 tons, and a down- <br />ward adjustment of the ISSP plot by that amount would ostensibly <br />produce the true salt retention. However, doing that would lower the <br />19 <br />