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<br />DRAINAGE WATER REUSE STRATEGY
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<br />47
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<br />Comparison of simulation results for the three different strategies of irrigation and drainage man-
<br />agement (I. II, III)
<br />
<br />TABLE 4
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<br />2:Vetll.
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<br />IV1....b
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<br />2:VdwC
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<br />Yield IOBSd
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<br />ECrwr
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<br />Strategy
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<br />Project 3
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<br />Project 4
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<br />I
<br />II
<br />III
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<br />417.6
<br />417.6
<br />417.6
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<br />495.5
<br />427.8
<br />427.8
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<br />77.9
<br />10.2
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<br />4
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<br />20
<br />45
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<br />82.4
<br />82.4
<br />72.2
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<br />3.0
<br />3.0
<br />0.5
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<br />..2: Vel ==cumulative volume (units) of water used by crops in evapotranspiration in all four projects.
<br />b2: Vjw=cumulative volume (units) of water diverted (pumped) from river for irrigation in all four
<br />projects.
<br />c2: Vdw==curnulative volume (units) of drainage water discharged (pumped) back to river.
<br />dCumulative loss of crop yield within Projects 3 and 4.
<br />.V rw== Volume (units) of river remaining in stream below Project 4.
<br />rEC~=EC of river below Project 4 (dS/m).
<br />
<br />but also the ultimate volume of drainage water needing disposal (or desalting)
<br />can be minimized and distributed equally between all projects. In this strategy,
<br />all areas have water of equal quality to use and each shares equally in the
<br />disposal problems and not progressively the downstream users as is typically
<br />the case (I).
<br />The preceding case studies illustrate the potential disbenefits of blending
<br />saline and good-quality waters and the need to consider the consequences of
<br />diluting drainage waters to meet water quality standards on the supply of plant-
<br />available water. The advantages of the "interception, isolation and reuse"
<br />strategy (III) relative to the conventional blending strategy are obviously many,
<br />as was shown. In these simulations, conservation of salt was assumed in the
<br />calculations. In the real world, salt- loading of the river would probably be greater
<br />than that shown, and more so for strategy I compared to II and III; hence the
<br />benefits of the latter strategy is likely underpredicted in these simulations.
<br />More realistic calculations of the salt-loading processes could .be made, as has
<br />b~en done'elsewhere (Rhoades and Suarez, 1977). .
<br />The results of the four case-studies clearly show that adding saline waters
<br />to good quality water supplies reduces the volume of the good-quality water
<br />supply that can be consumed by salt-sensitive crops, The amount of such re-
<br />duction will depend upon the relative volumes and concentrations of the re-
<br />ceiving and waste waters and upon the tolerances of the crops to be irrigated.
<br />In the previously discussed case-studies, it was assumed that the fraction of
<br />water usable for crop production was limited by EC;. Obviously, more water
<br />use can be achieved, if loss of yield is permitted, When the growth-limiting
<br />factor is salinity, the ultimate fraction of water in a supply that can be used in
<br />crop growth is:
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