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<br />38
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<br />J.D. RHOADES
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<br />of our water supplies. The blending process limits the maximum practical ben-
<br />efit that can be derived from the total water supply, The return of saline waters
<br />to the water supply, even when sufficient diltjtion occurs to keep the salinity
<br />of the mixture within apparently safe limits, reduces the quantity of the water
<br />supply that can be used in consumptive processes which are limited by salt
<br />concentration, such as the growth of salt-sensitive crops.
<br />Broadly speaking, users of a water supply may be classified into two groups:
<br />(1) those who consume the water in the process of use, and (2) those who use
<br />it without appreciable consumption. Type (1) users suffer disbenefit in the
<br />"blending" philosophy of water quality protection. The purposes of this paper
<br />are to provide evidence - theoretical and conceptual - that the blending ap-
<br />proach typically used for water quality protection is deficient for its intended
<br />purpose and to offer an alternative approach for dealing with the "disposal" of
<br />saline agricultural drainage waters that provides a greater practical benefit
<br />from the total water supply than blending does. While the advocated strategy
<br />is one that is "tailored" to western U.S.A, conditions, the principle has wide
<br />. applicability throughout the irrigated world.
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<br />THEORY AND PROCEDURES
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<br />In considering reuse, it is important to recognize that the total volume of
<br />any natural water supply cannot be beneficially consumed for irrigation and
<br />crop production; the greater its salinity', the less it can be consumed, The
<br />practice of blending or diluting excessively saline waters with good quality water
<br />supplies should only be undertaken after consideration is given to how it af-
<br />fects the volume of consumable water in the total supply.
<br />Plants must have access to water of a quality that permits consumption
<br />without the concentration of salts (individually or totally) becoming excessive
<br />for adequate growth, In the process of transpiration, plants separate nearly
<br />pure water from the salt solutions present in the rootzone and the salts are
<br />concentrated in the remaining unused soil water. This water ultimately be-
<br />comes drainage water, A plant will not grow properly when the salt concentra-
<br />tion in the soil water exceeds some limit specific to it under the given condi-
<br />tions of climate and management (Bernstein, 1975 j, Thus, it is obvious that
<br />not all of the water in a supply can be consumed by a plant, if the water contains
<br />salt,
<br />A series of case examples were simulated to illustrate some of the preceding
<br />conclusions, In these examples, the factor limiting crop growth is assumed to
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<br />.Salinity (a term referring to the total content of soluble inorganic constituents in the water) is
<br />generally limiting in this regard, but certain individual plant-toxic constituents, such as boron,
<br />may be in special cases: The term salinity will be used herein in a general sense to mean the
<br />presence of total dissolved salts and/or individual toxic constituents,like boron.
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