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<br />42
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<br />.1 .D. RHOADES
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<br />gation of beans. The blend is made up of 40 units of the good quality water and
<br />1 unit of the very saline drainage water; the ECiw of this blend is 1.5 dS/m.
<br />It is concluded that this blended water could be used to grow beans without
<br />yield loss (however a large penalty must be paid in doing so, as will be shown
<br />later), since the predicted resulting level of average rootzone salinity can be
<br />kept less than EC; (1.0 dS/m) but only by irrigating at a very high and gener-
<br />ally impractical level of leaching (L,=0.6, as obtained from Fig. 1). This very
<br />hign level of required leaching makes the use of such water impractical for the
<br />irrigation of beans, except in very sandy soils. Even if used in such soils, the
<br />process of blending has reduced the volume of water in the total supply that
<br />can be used by the bean crop (or. any other salt-sensitive crop) for evapotran-
<br />spiration, as shown in the following paragraphs.
<br />The relative volume of irrigation water required to meet ET and to achieve
<br />L, in this case is 2.500 units [1/ (1- L,) ], Of this volume, 1.500 units will pass
<br />through the rootzone to become drainage water (Vdw= Viw- V.,). Ofthe 2,500
<br />units of blended irrigation water, 2.439 units (40/41X2,500) consist of the
<br />good quality water of EC =0.5 dS/m and 0.061 units (1/41 X 2.500) consist of
<br />the very saline drainage water OfEC=32 dS/m. Thus, at best, only 0.061 u'nits
<br />of the 1.50 units of volume of the drainage water that resulted from irrigating
<br />this bean crop with the blended water could possibly have come from the drain-
<br />age water that was put into this blend. Therefore, the rest (1.439 units) must
<br />have come from the good-quality water put into the blend. This amount of
<br />drainage water is much higher than that for the case where only the good qual-
<br />ity water of Ec=O,5 dS/m was used to grow the beans (see Case 1). For this
<br />case, L, was 0.09, Viw was 1.099 units, and Vdw was 0.099 units. A comparison
<br />of the results of these two cases shows that 127% more of the good-quality
<br />water had to be used to irrigate the bean crop when it was used in the blend
<br />(1.401 units more; 2,50 versus 1.099 units) compared to when it was used solely.
<br />This is so because 1.401 units of the good-quality water was made unavailable
<br />for evapotranspiration by the bean crop without loss in yield, through the
<br />blendingprocess. Also as a result of blending, the volume of required drainage
<br />was increased substantially (1.500 versus 0.099 units). Such excessive drain-
<br />age may cause other problems, such as increase in water logging In the project;
<br />in tne loss of nutrients through excessive leacning, etc.
<br />Anotner way to illustrate that a loss of usable water in the total supply has
<br />occurred as a consequence of this blending is to contrast the relative fraction
<br />of the good-quality water supply that could be used to grow beans (i.e., could
<br />be used for evapotranspiration, ET) with and without blending. For this pur-
<br />pose, assume that the volume of the good-quality water OfEC=0.5 dS/m is 100
<br />units. Without blending all but 9 units, i.e., 91 units, [100- Vdw, or
<br />100- (l00XO.09)] can be consumed in ET. However, when saline drainage
<br />water of Ec=32 dS/m is blended with this 100 units of good-quality water in
<br />the ratio of 40 to 1 to give a larger total supply of 102.5 units (for which the L,
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