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2371 one-fourth at a specific conductance of 35 millimhos/cm2 [5]. Extra- polation of the data suggests that water use would cease entirely at 50 millimhos/cm2. It is difficult to relate this to actual field condi- tions in which there are other plant species and root zone depths are not well known. However, it appears that the natural vegetation would thrive for well over 100 years with a gradually declining con- sumptive use rate. Salinity Control Potential In order to develop the ultimate irrigable acreage, most or all of the canals and laterals will need to be lined and onfarm efficiency will need to be increased from the present 56 percent to approximately 71 percent. There is strong economic incentive to develop additional land and the Bureau of Indian Affairs proposes to construct the necessary system improvements within the next decade. Improvements in onfarm efficiency are anticipated under the IMS program and possi- bly other programs. Consequently, a relatively high degree of effi- ciency is expected to be attained which will have a beneficial effect on salt returns to the river. Assuming that conditions without a Federal salinity project are those projected on Figure 5, additional potential for reducing the net salt discharge to the river lies primarily in increasing onfarm efficien- cy above 71 percent. With increased onfarm efficiency salt retention would be greater until the ground water reached the same salinity as the leaching fraction. The feasibility of attaining onfarm efficiencies greater than 71 percent is not known. It is questionable whether they would be attained on the Reservation under the present conditions of water supply and acreage projections. A remaining means for reducing salt discharge from the Reservation is the potential use of agricultural drainage for power- plant cooling or other industrial use. No definite proposals of this nature have yet been made. 22