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f;i~. '}>.;""":- tv --J (.0 ...:J REUSE OF AGRICULTIJRAL DRAINAGE 101 salinity increase in a river system used for irrigation supply under different strategies of drainage management. The salinity of the soil water resulting from irrigation was calculated from the irrigation water composition and the leaching fraction (L, the fraction of the infiltrated water nO\ consumed in evapotranspiration and which passes beyond the root zone as drainage) after the method of Rhoades (1984<:,1986). In this method the water-uptake distribution within the irrigated root zone is assumed to be 40:30:20:10 by successive quaner-deplh fractions; steady-state chemistry and "piston-displacemenl-type"water flow are also assumed. Rela- live crop ,ieldwas then calculated from the predicted averagesoil water salinity , from knowledge of the plantlolerance to salinity and the assumption that crops respond to the average salinity within their root zone. Each of these assump- tions is sumciently true for the purposes of this chapter (Ingvalson et al.,l976 , and Rhoades and Merrill, 1976). More specifically, the average level ofsoi! salinity (expressed as the electrical conductivity of the saturation-paste extract, ECJ within the crop root zone resulting from the long-term irrigation with a water ofEC~ was obtained from {IJ EC. = F,' EC.., where ECiw is the electrical conductivity of the irrigation water and F, is the related cono:n<ration [actor appropriate for 1- F, was obtained [mm the calculable relationship which exists between F, and L (Rhoades, 1982 and 1984<:). The relaliveyields achievable as a function of average root zone EC, were obtained lrom the crop salt-tolerance tables of Maas (1986). Analogous values for boron and other specific solutes are also given in this same reference and could be used to evaluate yield losses for such specific solutes. The fraction o[ the irrigation waler that is consumed in evapotranspiration (Y jV..) is related to Las [2J VjViw = (l-L). For the purposes of lhis chapter, the volumes of V.. were normalized by expressing them relative to V ,j.e., for the case where V is taken to be equal ft ft to 1. These relative results (yields and volume ratios) were used to evaluate whether blending drainage water with good quality water would be detrimen- lal, or not, with respect to reducing Ihe volume of Ihe tOlal water supply that could be used for crop growth withoulloss in yield.