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f\) 00 <:) 0) ,""', ..;...... . .".' .~...~, , ':,,' ". 110 APPROACHES TO DRAINAGE PROBLEMS compared to Strategy I due 10 higher salinity in the irrigation water (both river and drainage waters), but disposal cost is lower since partofthe drainage watcr is reused for irrigation. The ratio between disposal cost and yield price is such that farms in Projects 1 and 2 are more profitable with Strategy II c.omparcd 10 I, whereas the farms in Projects 3 and 4 are less profitable. Strategy!l is also inferior on a regional basis compared to I. Because river quality remains th~ same and no yield losses occur in any project with Strategy Ill, all projects arc more profitable than with Strategies I and II and the regional profit is higher. Results where yield prices are higher (20 percent) and water prices are higher (50 percent) are presented only for the regional level. The regional profit is higher with increased crop price level and the gap between Strategy II and I decreases. For the case where irrigation water price is 50 percent higher, the profitability of reuse is improved. In all cases, reuse is more profitable as disposal costs increase. Note that an additional cost associated with the degradation of water quality in the river for other nonagricultural users was not included in the anal~is. The economic appeal of Strategy III would increase when such costs exist. DISCUSSION These results show that the normal loss of crop yield resulting when drainage water is returned 10 the good-quality water supply and the "blcnd" (the river in this case) is subsequently used 10 irrigate typical field crops can be avoided by using Strategy III. In this strategy the drainage water is interceptcd, isolated from the good-quality water and reused for the irrigation of sall- tolerant crops in the same project. Not only is the loss in crop yield that results under conventional management (Strategy I) avoided but also the salinity of the river is maintained at a uniformly low level (0.5 dS/m in this case) throughout its entire length. All users have water of equal quality whether they irrigate from the upstream or downstream sections ofthe river. Reuse is made of the drainage water for irrigation without any loss of crop yield. The ultimatc volume of drainage water needing disposal (or desalting) is minimized and distributed equally between all projects. In this strategy, all areas have water of equal quality and disposal problems are shared equally rather than progres- sively burdening downstream users as is typically the case (Strategy I). The results of the case study clearly show that adding saline waters to good qualitywatersupplies reduces the volume of the good-quality water supply that can eventually be ronsumed by salt-sensitive crops. The actual amount of such reduction will depend, of course, upon the relative volumes and concentrations of the receiving and waste-waters and upon the tolerances of the crops to be