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<br />. <br /> <br />ISSP data plot nearly to the natural vegetation plot for 1974, which <br />did not seem reasonable because development of the new land was <br />sure to have caused some salt discharge. Possible causes of the <br />mismatch were (1) the estimate of net subsurface inflow was too high, <br />(2) the net effect indicated by the ISSP data was too low, and (3) <br />the effect of natural vegetation was underestimated. Further examina- <br />tion of present ground-water levels indicated that the amount of <br />subsurface inflow from the river to the Reservation was probably less <br />in 1974 than estimated by Boyle, It is also likely that the amount <br />changed gradually between 1965 and 1977 as agricultural expansion <br />raised water tables and thereby tended to reduce subsurface inflow. <br />After considering numerous factors a lower estimate of net unmeas- <br />ured salt inflow was made, The ISSP data were adjusted with this <br />estimate as shown on Figure 5 as "ISSP data with unmeasured flow," <br />The adjusted plot and the plot of the net effect of natural vegetation <br />and flushing conform reasonably well. <br />Future Salt Budget <br />Under ultimate development of 93,046 acres on the flood plain in <br />Arizona, total water use efficiency will need to be approximately 62 <br />percent for the water supply to suffice. With an assumed conveyance <br />system effiCiency of 88 percent, onfarm efficiency will need to be <br />approximately 71 percent, <br />Natural vegetation will decline to an estimated 18,700 acres, <br />much of which will continue to accumulate salt, The future salt <br />storage caused by vegetation was estimated in the same way as for <br />the recent past. <br />Ground-water flushing was estimated by comparing the ~SIJ <br />the leaching fraction with the average salinity of the underlying <br />ground water. It was estimated that the salt discharge caused by <br />flushing would decrease gradually from 20,000 tons in 1977 to nil in <br />1981 when the salinity of the drainage would be equal to the average <br />salinity of the ground water, From then on, ground-water flushing <br />would cause an apparent retention of salt as improved irrigation <br />efficiency causes the agricultural drainage to become saltier than the <br />ground water, resulting in an estimated net retention of about 41,000 <br />tons of salt by 1994. Farther in the future the ground water would <br />20 <br />