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'" co <=> 00 112 APPROACHES TO DRAINAGE PROBLEMS tlley are in a suitably salt-tolerant growtll stage (after seedling establisllmcnt); tile "good water" is used at tile otller times. This successive usewaterwitlllow and high salinity levels prevents tile soil from becoming excessively salinc while permilling the substitution, over tile long period, of saline water for con- ventional water lO meet a substantial portion (up lO about 50 percent depend- ing on the crop rotation, etc.) of tile irrigation water needs for tile area wllile also permitting tile growtll of salt-sensitive crops in the same fields. Results of eJCtensive field experiments have demonstrated the credibility and feasibilityof this strategy and these conclusions (Rhoades et aI., 19883 and band 1989). Since continuous recycling, in the sense of a closed loop, is not possiblc, ' reuse efforlS should ideally be designed so that the drainage waters intercepted and isolated from the major part of the project area are redistributed to a dedicated "reuse-area" within the project, or sequentially from areas where crops of lesser lO greater salt lOlerance are grown (often this occurs naturally from upslope to downslope lands); the ultimate minimized volumeofdrainage resulting in the reuse area must eventually be desalted or disposed of. This ultimate disposal should not be accomplislled by discharging the drainage water into good quality water supplies, unless no other means is practical, for the reasons previously discussed. I SUMMARY AND CONCLUSIONS An example was given toshowlhat irrigating salt-sensitive crops with blends ofsaline and good quality water supplies or diluting drainage with good quality water inorder to meet discharge standards may be inappropriate under certain situations. Even though the concentration of the blend may appear to be low enough to be acceptable by conventional standards, the usability of the good quality water supply for growing sa Ii-sensitive crops (or for other salt-sensi tive water uses) is reduced through the process of blending. Each time the salt content of an agricultural water supply is increased, the degree to which it can be consumed before its concentration becomes excessive is decreased. More crop production can usually be achieved from the lOtal water supply by sole use , of the good quality water component. Serious consideration sllould be given to keeping saline drainage waters separate from the good quality water sup- plies,evenwhen the saline waters are to be used for irrigation. Theycan be used more effectively as a substitute for the conventional water supply in tile irrigation of certain crops grown in the rotation after seedling establishment. The feasibility of such reuse for irrigation has been demonstrated in field studies in California. Reuse of drainage water for irrigation of suitably salt- tolerant crops reduces the volume of drainage water needing ultimate disposal and the offsite pollution problems associated wilh the discharge of irrigation