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I"\) -J Q 0'> . . ~'." :"-:<) '. ..,', 100 APPROACHES TO DRAINAGE PROBLEMS Broadly spe.aking, users ofa water supply may be classified inlO two groups: (1) Those who concentrate the water in the process of use, and (2) those who do not. The firstgroup of users suffer pOlential disbenefit under Ihe "blending" philosophy of waler quality protection. The blending process may limilthe maximum practical benefit that can be derived from the tOlalwatersupply. For example,the return of excessively saline waters to the water supply, even when sufficient dilution occurs to keep the salinity of the mixture within acceptable limits, reduces the quantity of the watersupply Ihatcan be used productively 10 growsalt.sensitive crops (Rhoades, 1989). In this chapter, the factor limiting crop growth is assumlid to be the presence of excessive total dissolved salts in Ihewater (salinity), but an analogous case could also be made for boron or any other constituent that is toxic to plants. The ultimate objective of water quality protection should be to permit the maximum practical benefit (use) from the available water supply. The pur- poses of this chapter are (1) to provide evidence--conceptual and empirical-- that the blending approach typically used forwaterqualityprOlection can result in economic losses to the agricultural community as a whole and (2) to suggest a me.ans to de.al with the "disposal" of saline agricultural drainage which provides gre.ater practical benefit from the tOlal water supply than blending does. The theme of this chapter is that different water qualities may have relative advantages that can be re.alized by application 10 different crops at different periods in the growing se.ason. CONCEPTIJAL FRAMEWORK In considering acceptable salinity levels for irrigation water, it is important to recognize that the ,higher its salinity', the lower the percentage of IOtal volume that can be beneficially consumed for crop production. Plants must have access to water ofa quality that can be consumed without the concentration of salts (individually or totally) becoming excessive for adequate growth. In the process of transpiration, plants essentially perform reverse osmosis; the salts in the irrigation water are concentrated in the remaining unused soil water, which will ultimately become drainage. A plant will not grOw properly when the salt concentration in the soil water exceeds its specific limit under the given conditions of climate and management (Bern. stein, 1975), Thus, not all of the water can be consumed by a plant if a water supply contains salt. A hypothetical case study was made to illustrate the effects of blending and of alternative drainage management practices on the usability of water supplies and the subsequent economic ramifications. The case study compares the losses in crop yield and in the volumes of consumable water resulting from ~