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.. ,,~. '. ~~:~~.;'::.,?-{;?i:<:;-> .~::~::, .~. .~. DRAINAGE WATER REUSE STRATEGY 49 , '. -'.- .-'........ ~ ,~ W 00 EC. TUF=I-~ ECm where EC;w is the electrical conductivity (concentration can be used alterna- tively) of the water supply and ECm is the maximum salinity (electrical con- ductivity, concentration, etc.) of the water in the rootzone (not on an EC. but on an ECw basis; essentially ECdw) the plant can tolerate (i.e., draw water from and still yield about 85-100%). Values of ECm (dSjm) vary among the crop species, but typically they are (according to Bernstein, 1975) about 45 for such tolerant crops as cotton, sugar beets, barley, 30 for intermediate crops like, tomatoes, wheat, and alfalfa, and about 15 for sensitive crops, like beans, clo- vers, and onions, In some cases, it may make economic sense to blend and to bear the consequences of the losses of water usability and of crop yield when the alternative costs of disposal are much higher, (4) tt~i~ti~~ , '~'.~' A DRAINAGE WATER REUSE STRATEGY Minimizing leaching and deep percolation always minimizes the volume and salt load ofthe drainage water and usually minimizes pollution ofthe receiving water (Van Schilfgaarde et a!., 1974; Rhoades et aI., 1974; Rhoades and Suarez, 1977), For this reason, minimizing leaching and deep percolation should be the goal of irrigation management, For those situations where the waters cannot be, or have not been, fully utilized in their first "passage" through the rootzone, the drainage water should be intercepted before its discharge to water supplies of better quality and reused for irrigation (Rhoades, 1984d), The preceding case examples illustrated the merits of this management strategy. While con- centrations of salts in drainage waters are higher than those of the correspond- ing irrigation water supply, they are often within acceptable limits for growing suitably salt-tolerant crops (Rhoades, 1977, 1986). A reuse strategy that avoids blending and is superior to that described in strategy III has been proposed and demonstrated in field projects to be viable and advantageous in well managed irrigation projects (Rhoades,1984a,b, 1987; Rhoades et a!., 1988a,b), In this reuse strategy, the two water supplies (good- . quality water and saline drainage water) are kept separate and are used with- out blending. The saline drainage water is intercepted, isolated and substituted for the conventional "good water" in suitable locations in the project when irrigating certain salt-tolerant crops grown in the rotation when they are in a suitably salt-tolerant growth stage (after seedling establishment); the "good water" is used at the other times. The appropriate timing and amount of sub- . stitution of the saline water will, of course, vary with the quality of the two waters, the cropping pattern, the climate, the irrigation system, etc. The max- imum soil salinity in the rootzone that would result from the sole use of saline water for irrigation will not occur when such water is used for only a fraction .: '. ,." . ."..>:;...-:.::-:.: .. - ';:" ~.:'..'. :"::.~_:..: :;~:(~<:.~; ':':"~/";':'::'::,',,~,,':' . ....".- . ':'~:/""',,: ~ . ,'- :i. '. .... ~7;~',~:: ~: .... -; .:;:.:...., :~-?;~.':>:=. ,:.,",' ~~:-.. -~. .,. ...".,." < '-' .'. ;: ~ ,'.'., . . . . .... :-..............: ......._.-... .-;:.::';~. --::..>.:,~':~''-:;::.::..-:.. .," '.; ';' -";';~;. '. ;,., ...... '. . . ~.., "';.--;;';'.' '. ":;':.,-. . <,.::....... . . '. '" /~.... ~ .. .-_</:;~.,.:<~~.. ;:.. .", '.:' J,'.::' ,.~:, . .~ ........ .. .",-' .;. , "-"'.' ..- '. .... - .~ . ,. <