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., .~; Y') . ~~J ..... ...., '" c-:> o -.J REUSE OF AGRICULTIJRAL DRAINAGE 111 irrigated. When the growth-limiting [actor is salinity, the ullimate fraction o[ water in a supply that can be used in crop growth is: [51 l-ECfiCm where EC", is the electrical conductivity (concentration can be used allema- lively) of the water supply and EC. is the maximum salinity (electrical conduc-' livity, concentration, etc.) o[ the water in the root zone (on an EC. not EC. basis; essentially EC..) the plant can tolerate (i.e., draw water from and still yield about 85-100 percent). Values of ECm vary among the crop species, but typically they are (according to Bernstein, 1975) about 45 [or such tolerant crops as collon, sugar beets, and barley; 30 [or intermediate crops like toma- toes, wheat, and alfalfa;and about 15 [or sensitive crops like beans, clovers, and onions. Minimizing leaching and deep percolation always reduces the volume and sa1l10ad o[ the drainage water and usually minimizes pollution o[ the receiving water (van Schilfgaarde et al., 1974; Rhoades et a!., 1974; and Rhoades and Suarez, 1977). For this reason, minimizing leaching and deep percolation should be the major goal of irrigation management. Except in situations where the waters cannot be, or have not been, fully utilized in their first "passage"through the roOt zone, the drainage water should be intercepted be [ore it is discharged to water supplies of beller quality and reused for irrigation (Rhoades, 19S4d). While concentrations of salts in drainage waters are higher than those of the corresponding irrigation water supply, they are often within acceptable limits [or growing suitably salt-tolerant crops (Rhoades, 1977 and 1986). The results ofthe case study here illustrate the merits of this management strategy. Under other circumstances, it might 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 extreme. In thesesim ulations, conservation of salt was assumed in the calculations. In the real world, salt loading o[ the river from drainage return would probably be . greater than that shown, and more So [or Strategy 1 compared to Il and lll; hence, the benefits of the laller strategy is likely underpredicted in these simulations. More realistic calculations of the salt-loading processes could be made, as has been done by Rhoades and Suarez (1977). A reuse strategy that avoids blcnding 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 irrigalion projects (Rhoades, 1984a and b and 1987 and Rhoadcseta!., 1988a and b). In thisreusestrategy,the two water supplies (good quality water and saline drainage water) arc kepI separate and used without blending. The saline drainage water is intercepted, isolated, and substituted ror the conventional "good water" in suilable locations in the project when irrigating certain sail-tolerant crops grown in the rotation when