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Last modified
1/26/2010 12:52:12 PM
Creation date
10/11/2006 11:59:08 PM
Metadata
Fields
Template:
Water Supply Protection
File Number
8273.600.30
Description
Colorado River Basin Salinity Control - Federal Agencies - USDA
Basin
Colorado Mainstem
Water Division
5
Date
1/1/1991
Author
JD Rhoades
Title
Blending Saline and Non-Saline Waters Reduces Water Usable for Crop Production
Water Supply Pro - Doc Type
Report/Study
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<br />......., <br />" <br /> <br />. ~ '. <br /> <br />1'\:) <br />-,1 <br />OJ <br />o <br /> <br />beans (see case 1). For this case, Lr was 0.09, Viw was <br />1.099 units, and Vdw was 0.099 units. A comparison of <br />the results of these two cases shows that 127 percent <br />more of the "good-quality" water had tb be used to <br />irrigate the bean crop when it was used in the blend <br />(1.401 units more; 2.50 versus 1.099 units) compared to <br />when it was used solely. This is SO because 1.401 units <br />of the good-quality water was made unavailable for <br />evapotranspiration by the bean crop (with reference to <br />no loss in yield) through the blending process. Also as <br />a result of blending, the volume of required drainage <br />was increased substantially (1.500 versus 0.099 units). <br />Such excessive drainage may cause other problems, such <br />as increase in water logging in the project, in the loss <br />of nutrients through excessive leaching. etc. <br />Another way to illustrate that a loss of usable water <br />in the total supply has occurred as a consequence of <br />blending is to contrast the relative fraction of the <br />good-quality water supply that could be used to grow <br />beans (i.e., could be used for evapotranspiration) with <br />and without blending. For this purpose, assume that the <br />volume of the good-quality water of EC=0.5 dS/m is 100 <br />units. Without blending all but 9 units. i.e., 91 <br />units. (100-Vdw. or (100)-(100)(.09)) can be consumed in <br />ET. However. when saline drainage water of EC=32 dS/m <br />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 <br />102.5 units (for which the Lr is 0.6 and vdw is 61.5 <br />units), only 41 units (102.5 - 61.5) are usable for ET <br />by beans without loss of yield. Thus. 50 units (91-41) <br />of the original 100 units of good-quality water were <br />~ade unusable for the production of beans by adding <br />saline water of EC=32 dS/m to it in the ratio of 1:40. <br /> <br />Conclusions <br /> <br />The results of the three case-studies clearly show <br />that adding saline waters to "good quality" water <br />supplies reduces the volume of the water supply that can <br />be consumed by salt-sensitive crops. The amount of such <br />reduction will depend upon the relative volumes and <br />concentrations of the receiving and waste waters and <br />upon the tolerances of the crops to be irrigated. <br />Therefore the merits of blending should be evaluated on <br />a case by case basis. <br />In the case-studies. it was assumed that the fraction <br />. of water usable for crop production was limited by ECe. <br />Obviously. more water use can be achieved, if loss of <br />yield is permitted. when the growth-limiting factor is <br />salinity. the ultimate fraction of water in a supply <br />that can be used in crop growth is: <br /> <br />8 <br /> <br />Rhoades <br />
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