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<br />"1 ~ .-_ <br />..~ ;:.:~1 <br /> <br />I'\) <br />~ <br />00 <br />II- <br /> <br />1 - <br /> <br />ECiw <br />ECm <br /> <br />[5] <br /> <br />where ECiw is the electrical conductivity (concentration <br />can be used alternatively) of the water supply and ECm <br />is the maximum salinity <electrical conductivity, <br />concentration, etc.) of the water in the root zone (not <br />on an ECe but on an ECw basis; essentially ECdw) the <br />plant can tolerate (i.e., draw water from and still <br />yield about 85-100 percent). Values of ECm vary among <br />the crop species, but typically they are (according to <br />Bernstein, 1975) about 45 for such tolerant crops as <br />cotton, sugar beets, barley, 30 for intermediate crops <br />like. tomatoes, wheat, and alfalfa, and about 15 for <br />sensitive crops, like beans, clovers, and onions. In <br />some cases, it may make economic sense to blend and to <br />bear the consequences of the losses of water usability <br />and of crop yield when the alternative costs of disposal <br />are much more costly. <br />The examples given show that irrigating salt- <br />sensitive crops with blends of saline and pure waters or <br />diluting drainage waters with good quality waters in <br />order to meet discharge standards may be inappropriate <br />under certain situations. Even though the concentration <br />of the blend may appear to be low enough to be <br />acceptable by conventional standards, the usability of <br />the water supply for growing salt-sensitive crops (or <br />for other salt-sensitive water uses) is reduced through <br />the process of blending. Each time the salt content of <br />an agricultural water supply is increased, the degree to <br />'which it can be consumed before its concentration <br />becomes excessive is decreased. More crop production <br />can usually be achieved from the total water supply by <br />just solely using the "good-quality" water component. <br />Serious consideration should be given to keeping saline <br />drainage waters separate from "good-quality" water <br />supplies. even when the saline waters are to be reused <br />for irrigation. Reuse of drainage water for irrigation <br />of suitably salt-tolerant crops reduces the volume of <br />drainage water needing ultimate disposal and the <br />off-site pollution problems associated with the <br />discharge of irrigation return flows (Rhoades, 1984b). <br />Another case example is given elsewhere to further <br />illustrate the effects of blending and of alternative <br />drainage management practices on the usability of water <br />supplies (Rhoades, 1989). This case compares the losses <br />in crop yield and in the volumes of usable water that <br />accompany the increases of salinity in a hypothetical <br />river system being used for irrigation under different <br />strategies of drainage management. Such a comparison <br />shows that the pollution of waters (rivers) that occur <br />through the return of drainage waters can be avoided by <br /> <br />9 <br /> <br />Rhoades <br />