|
<br />, .
<br />
<br />l'\)
<br />GO
<br />W
<br />to
<br />
<br />-~-: " .'
<br />
<br />. ~ :: :": .." :"'.
<br />;"'..'. ~: :' ;.
<br />~~~:.~:r.:, ~:~.,.. <:..-:.
<br />
<br />.." ".'
<br />- ..,
<br />
<br />50
<br />
<br />J.D. RHOADES
<br />
<br />of the time. The levels of salinity will be especially lower during the critical
<br />periods of germination and seedling establishment - relative to using the saline
<br />water solely or in a blend. Whatever excessive salt build-up occurs in the root-
<br />zone from irrigating the "tolerant" crops in the rotation with the saline water
<br />are alleviated in a subsequent cropping period when a more sensitive crop is
<br />grown using the low-salinity water for irrigation, (It should be noted that a
<br />soil will not generally become unduly saline from use of a saline water for just
<br />a part of a single irrigation season and often not for several seasons.) The yield
<br />of the salt-sensitive crop will not be reduced when grown on land where irri-
<br />gation with saline water was previously used, if the irrigation of this crop is
<br />carried out with low salinity water, if proper preplant irrigations and seedbed
<br />management are used during germination and seedling establishment periods
<br />to prevent excessive salt accumulation in the seed area and shallow soil depths,
<br />and if subsequent "in season" irrigations are sufficient to leach these salts.
<br />farther down in the profile and ahead of the advancing root system. The kind
<br />of conventional management practiced by good farmers is generally adequate
<br />in this regard. Such management will "reclaim" the soil sufficiently by the end
<br />of this cropping period so that the cycle can be repeated over and over, i.e., the
<br />saline water can be used again in the production of the tolerant crop (s) in the.
<br />next cycle of the rotation, etc. This successive use of low and high. salinity
<br />waters will prevent the soil from becoming excessively saline while permitting,
<br />over the long period, substitution of a saline water for the conventional water
<br />for a substantial fraction (up to about 50% depending on the crop rotation,
<br />etc.) of the irrigation water needs of the area and the growth of salt-sensitive
<br />crops in the same fields, Results of extensive field experiments have demon-
<br />strated the credibility and feasability of this strategy and these conclusions
<br />(Rhoades et aI., 1988a,b).
<br />Since continuous recycling, in the sense of a closed loop, is not possible, reuse
<br />efforts should ideally be designed so that the drainage waters intercepted and
<br />isolated from the major part of the project area are redistributed to a dedicated
<br />"reuse-area" within the project, or sequentially from areas where crops of lesser.
<br />to greater salt-tolerance are grown (often this occurs naturally from upslope
<br />to downslope lying lands); the ultimate minimized volume of drainage result-
<br />ing in the reuse area must eventually be desalted or else disposed of, This ul-
<br />timate disposal should not be accomplished by discharging the drainage water
<br />into good-quality water supplies, unless no other means is practical, for the
<br />reasons previously discussed.
<br />
<br />SUMMARY AND CONCLUSIONS
<br />
<br />Examples were given to show that irrigating salt-sensitive crops with blends
<br />of saline and pure waters or diluting drainage waters with good quality waters
<br />in order to meet discharge standards may be inappropriate under certain sit-
<br />
<br />.~<-~_:-:;:;. '-':': ::~~~-:/,": ~;, ':::i~;::Y":~2:i
<br />
<br />'.,
<br />
<br />. "." '.. ...... .".
<br />
<br />~~t~~~5~~
<br />
<br />
<br />......- ". "." . .
<br />......-'.'......,_... ... . . ,
<br />{:, ~:~. .~/;... /:.:~ '.,:' ...."-;:~ ~ -:-.:;:~~;:. .<.~:-\~<.
<br />:.~.:/:~~ .~::~~.:~'/:. .:<~;~~~;.-<:\~:. /.:':
<br />".- : ,", . :'.' r.::~.;....~; ..... . ~...'
<br />~""'. ':-.:;.'~' ':'>.::".'\;
<br />:",-,,::::::,,::-":'::.~,\"~:~.. ".: "'";" :.;.,". .
<br />~'::.:~:;<:}'\~;;.l':;'~~'~"".';:';:'~:;'-"'..i.:/.'.:;
<br />
<br />, .~ '-~;' '.. ..... ~
<br />;~::~..:::...~:..:;>,.::,.\,;/?:t;
<br />
<br />}
<br />
<br />. "./ ~'.'
<br />:...
<br />
<br />
<br />:f;;~ ~:;:.,::~,/~,>,
<br />
<br />" ," -.~ . " . ::., . .:. ",..
<br />
<br />..' .
<br />
<br />..
<br />....~. ,
<br />
<br />)..
<br />
<br />".. ......".
<br />.;-~., .::..>~ '/><'::.;>:'"
<br />
<br />-.- ,.-..."
<br />...:':....-..,
<br />
<br />,
<br />
<br />, ,'""
<br />.......
<br />
<br />r':.~'. .
<br />
<br />..'
<br />
<br />.... .-.
<br />
<br />"." :.:~: r._ .. ~_"
<br />
<br />'::;':~ ::;':"':.\. '" ....
<br />
<br />....-.."':l,;.-;.>:... '.
<br />
<br />",
<br />
<br />.
<br />"':,.r',:
<br />
<br />
<br />
<br />:'.-, "
<br />
<br />,.....
<br />. ~.:.
<br />
<br />~.-. r'
<br />
<br />..;..
<br />
|