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_:,:J,;......_ :)-:-"-.'. " .:: ~: '-~: -:':'.' -', . l\) ....} -..J C.1I 1 = ECa ECb [11 where the EC values refer to the allowable salinities (expressed in electrical conductivity) in the drainage water for the first crop, a, and the second crop, b. It should be recognized that extremely high irrigation efficiences are needed to completely utilize most common irrigation waters in a single use. For example, for an irrigation water of EC=1.0 dSm-' , leaching fractions of 1/45 to 1/15 would be needed for the most salt-tolerant and salt-sensitive crops, respectively. With such efficiencies, 67 percent of the drainage water from the most sensitive crops would be usable for tne most tolerant crops. But return of such saline waters to a common water supply reduces the fraction of that supply which could potentially be used by salt-sensitive crops for transpiration. In conclusion it follows that, if a water is so saline that its use for crop production is already spent, then diluting it with purer water and using the mix for the irrigation of crops of the same or lesser salt-tolerance does not add to or contribute to the usable water supply for crop production. One has, in this process of mixing, simply mixed the usable and unusable waters into one blend which must be separated again during the use by the plant. Procedures A series of case examples are given to illustrate the preceeding conclusion. The results of the case examples also illustrate how one can evaluate whether a saline water is suitable for irrigation, or not, and whether blending a saline water with "good quality" water is likely to be detrimental, or not, with respect to reducing the volume of a fixed water supply that can be used for crop growth without loss in yield. In these examples, the factor limiting crop growth is assumed to be the presence of excessive total dissolved salts, but an analogous case could also be made for boron or any other constituent that is toxic to plants. calculations of the salinity of the soil water resulting within the root zone from irrigation are predicted from knowledge of the salinity of the irrigation water (ECiw) and leaching fraction (L. the volume of drain water/the volume of infiltrated irrigation water) after the method of Rhoades (1984a, 1986). Relative crop yield is determined from the predicted average soil water salinity, knowledge of the plant tolerance to salinity and the assumption that crops respond to the average salinity within their 3 Rhoades