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<br />DRAINAGE WATER REUSE STRATEGY
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<br />39
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<br />be the presence of excessive total dissolved salts, but an analogous case could
<br />also be made for boron or any other constituent that is toxic to plants, Calcu-
<br />lations of the salinity of the soil water within the rootzone are made from
<br />knowledge of the salinity of the irrigation water (ECiw) and leaching fraction
<br />(L, the ratio between the volume of drain water and the volume of infiltrated
<br />irrigation water) after the method of Rhoades (1984c, 1986). Relative crop
<br />yield is calculated from the predicted average soil water salinity, knowledge of
<br />the plant tolerance to salinity and the assumption that crops respond to the
<br />average salinity within their rootzone (Ingvalson et aI., 1976). The water-up-
<br />take distribution within the irrigated rootzone is assumed to be. 40: 30: 20: 10
<br />by successive quarter-depth fractions; steady-state chemistry and "piston-dis-
<br />placement-type" water flow are also assumed. Each of these assumptions is
<br />sufficiently true that the results are reasonable (Rhoades and Merrill, 1976).
<br />The required leaching fraction, Leo is taken to be that value of L required to
<br />keep the average salinity of the rootzone from exceeding the tolerance level of
<br />the crop (the maximum level that the crop can tolerate without loss of yield,
<br />EC;; a lower value can be used, if loss of yield can be tolerated).
<br />The average level of soil salinity (expressed as the electrical conductivity of
<br />the saturation-paste extract, ECe) within the crop rootzone resulting from the
<br />long-term irrigation with a water of ECiw is predicted from
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<br />Eee = Fe' ECiw
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<br />(1)
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<br />where EC,w is the electrical conductivity of the irrigation water and Fe is the
<br />relating concentration factor appropriate for L (see Fig, 1), A calculable rela-
<br />tionship exists between Fe and L; it is the same as that existing between F;
<br />and L" which is depicted in Fig. 1 (after Rhoades, 1982, 1984c). Assuming
<br />conventional irrigation management, an ECiw of 0.5 dS/m and L equal to 0.15,
<br />Fe is 1.51 and the average level of soil salinity within the active rootzone (ECe
<br />basis) is predicted to be 0.75 dS/m (=0.5X 1.51).
<br />. The leaching requirement is calculated when EC; is used in eq. (1) and Fig.
<br />1. EC; is taken as the maximum tolerable level of ECe in all cases herein, The
<br />maximum degree to which the irrigation water can be concentrated before sal-
<br />inity begins to reduce crop yield is given by F;:
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<br />maximum permissible salinity, EC;
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<br />electrical conductivity of irrigation water, ECiw'
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<br />(2)
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<br />The values of EC; used were those given in the crop tolerance tables of Maas
<br />(1986)*.
<br />The fraction of the irrigation water that can be consumed in evapotranspir-
<br />ation without yield loss is given by Vet! Viw, which is related to L, as
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<br />. Analogous values for B~ are also given in this same reference.
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