|
<br />.
<br />
<br />,.
<br />
<br />DRAINAGE WATER REUSE STRATEGY
<br />
<br />43
<br />
<br />l\)
<br />Crl
<br />tv
<br />IV
<br />
<br />is 0.6 and Vdw is 61.5 units), only 41 units (102.5-61.5) are usable for ET by
<br />beans without loss of yield. Thus, 50 units (91- 41) of the original 100 units
<br />of good-quality water were made unusable for the production of beans by add-
<br />ing saline water of Ec=32 dS/m to it in the ratio of 1:40.
<br />Some people advocate that drainage waters should be diluted with a good-
<br />quality water (to meet some specified discharge standard; sayan EC of 1.5 dS/
<br />m, as resulted in Case 3) and then returned to a good-quality water supply. For
<br />example (as in Case 3) 1 unit of drainage water of Ec=32 dS/m could be
<br />blended with 40 units of water OfEC = 0,5 dS/m and then the 41 units of blended
<br />water of EC = 1.5 dS / m returned to the major water supply of good quality, The
<br />above results show that even when a relatively small volume of such blended
<br />water is incorporated into the larger good-quality water supply, the net result
<br />is that a fraction of this latter water is made unusable for evapotranspiration
<br />by salt-sensitive crops (such as beans) without loss of yield. In the case de-
<br />scribed above 50 units out of every 100 units in the large supply will be made
<br />unusable for each 1 unit of drainage volume added to it,
<br />The significance of such losses of usable water through blending will depend
<br />upon a number of factors, especially upon the salt sensitivity of the crop to be
<br />grown with the blended water. and the relative concentrations and volumes of
<br />the drainage and receiving waters. Therefore the merits of blending should be
<br />evaluated on a case by case basis.
<br />
<br />Case 4
<br />
<br />To further illustrate' the effects of blending and of alternative drainage man-
<br />agement practices on the usability of water supplies, this case compares the
<br />losses in crop yield and in the volumes of usable water that accompany the
<br />increases of salinity in a hypothetical river system being used for irrigation
<br />under different strategies of drainage management. Such a comparison shows
<br />that the pollution of waters (rivers) that occur through the return of drainage
<br />waters can be avoided by intercepting, isolating and reusing the drainage waters
<br />for irrigation.
<br />The conditions: a river of 500 units of volume and of Ec=0.5 dS/m which
<br />passes through four successive identical irrigation projects, In each project water
<br />is diverted from the river and used to irrigate four crops, Some of the water is
<br />consumed by the crops in evapotranspiration. The amounts consumed in each
<br />project are as follows for each crop: beans (25 units), alfalfa (40 units), wheat
<br />(25 units) and cotton (14.4 units). The volumes of water applied to each crop
<br />are sufficient to meet these levels of ET and the following leaching fractions:
<br />beans, L=O,2; alfalfa, L=O.l; wheat, L=O,2; and cotton, L=0.15, Three dif-
<br />ferent irrigation/drainage management strategies are simulated, In strategy I,
<br />each crop is irrigated with the river water (of salinity level as it enters each
<br />project) and all of the percolated drainage water from each crop is collected
<br />
<br />
<br />..:..,."......,..:-.-....
<br />
<br />. ::'. " ,......', '. .; :..;' '.' '. . ,.,.~ ,-:.... _.'!.~ .
<br />
<br />. . ....:"::... .~...:\(i~;%sj;~. .
<br />
<br />
<br />- . . . '..'~ "", -: ,".:;. '.'. -."":,..
<br />
<br />.' "
<br />.--',:.D:::,'~-,:,/;r:.:: .'
<br />
<br />.~':(..-. .~.>.~? "'~'-"".._'._....'
<br />. . ..;.../:'~.,.:::;. . ";" ': .
<br />.' .";','
<br />
<br />", ''-~ .
<br />
<br />':..,.~;"'::"".~'.>':.; :,:. -
<br />
<br />'.::.:.-'
<br />
<br />. .
<br />:'.-;.::" "" /;
<br />. .- ~:. .,..,.....
<br />
<br />.-.,.,','
<br />
<br />.:.. ~.
<br />
<br />,'.-- <.<.' . .~.
<br />
<br />>~~...~ ~ :;;-:.~~.~:: ",:.
<br />
<br />..".... -'.
<br />
<br />..
<br />'.".' :",
<br />
<br />'. ,"to
<br />
<br />ii;;~i;.:;,~r;;:i.;>>~I:~':\~;:;~'t:~.~
<br />
<br />
<br />.",-. .'
<br />
<br />"';.:.,~::. . -'.
<br />'.. '-'.
<br />
<br />'.'
<br />. ..
<br />
<br />:...........
<br />
<br />. . ". .
<br />. ." >"- ..;'.
<br />
<br />....;....
<br />
<br />....: '.:
<br />
<br />..:. .:.~ '.'
<br />
<br />.-.:-,.
<br />
<br />....... .
<br />,..-.......
<br />
<br />...~...:;:-':<'.~..(
<br />
<br />. - ::~- -~.:' ".
<br />
<br />. .~." ::'. .:.: , .' - ,
<br />
<br />';::.' ....
<br />
<br />.'- ....-..
<br />......-..
<br />.-.,..:.-..'
<br />
<br />::;. -:;.
<br />
<br />" .'.' ..;.
<br />. .' ..;. .' ~... .' -,':' -
<br />:.;:::~~<:':~..:~.....::~::".. . "
<br />:>5~::,;~.,~;);,;;
<br />~~~;~~~~
<br />
<br />
<br />.' ....-,..~-...:
<br />
<br />
<br />
<br />_.
<br />
<br />
<br />-"':.;:.-
<br />
<br />
<br />
<br />.. ' . .-.
<br />"~.' . -'
<br />
<br />.-. -'.
<br />
<br />.; .... ,.... ::-:".-..:' .' .'. ". .-. -;~.,
<br />L':--"-.- .... :.:; '::.,",:
<br />:(~'~:.,:.>"<:" '.': .~.\~: :-...~.:_\::t-: <>.'.~:',
<br />..!":...-.,
<br />". '.' .-.:. -, ;':'.. . :., ~
<br />"-:.:'/:.:'-. ':;~:".:.: -~:':'<.'::':?':~':"> ;--/:< :;'.:; ,
<br />'.. ~'. '.'
<br />
<br />
<br />.:'-;'
<br />
<br />'..'-.-',Y
<br />.:~,-
<br />
<br />" -^...::.
<br />
|