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<br />July 1985
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<br />Hydrologic Linkages
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<br />Salinityin the Col~~do River i~ a rath-
<br />er unu~ual nonpoirit"water quality prob-
<br />lem in that the pollution ~ource~ are geo-
<br />graphically far removed from tho~e
<br />damaged by ~alinity. Moot ~alt loading oc-
<br />cllr~ in the Upper Ba~iil, while adver~e ~ac
<br />linity effect~ are regi~tered ~everal
<br />hundred mile~ away."fhe Imperial Valley
<br />experience~ about ninety percent of the.
<br />total agriculturaldamage~in the U.S.pOr-
<br />tion of the ba~in (Kleinman and Brown.
<br />1918). In fact, both the major user~ of the
<br />Colorado River water, the Imperial Valley
<br />and. the Metropolitan Water District of.
<br />~outhern California, lay out~ide the Col-
<br />orado River Ba~in. .
<br />Some hydrologic a~~umptions are need-
<br />ed to compare the benefit~ with the co~t~ .
<br />of thi~ long distance externality. Although
<br />heavily ~tudied, knowledge of the hydrol-
<br />ogy of the Colorado River.is far from per-
<br />fect. The hydrologic ~tate of the art is em-'
<br />bodied inUSBR'~ Colorado. River
<br />Simulation System. (SeeUSBR, 1983:101).
<br />Thi~ analy~i~ is b~ed on the a~~umption~
<br />in that ~imulation model.
<br />The .fir~t major a~umption relate~ ~alt
<br />loading in the Upper Ba~in to Lower Ba-
<br />~in ~linity levek The ~alt loading rela-
<br />tion~hip adopted i~ that 10,000 ton~ added
<br />to the river above ParkerDam equal~l.OI
<br />mg/liter at Imperial Dam (USDI. 1983:
<br />45). (Equivalently. 9,900 tons equah 1 mg/
<br />liter) Each ton of ~alt removed from up-
<br />~tream i~ ~~umed to re~ult in exactly one
<br />ton le~ ~alinity at Imperial Dam. [If ~ub-
<br />~tantial quantities of ~alt are being precip-
<br />itated out of the water in the ~y~tem of
<br />re~ervoir~, ~ the evidence increasingly
<br />~uggests, (Paul~on and Baker; USBR, 198$:
<br />108-10) ~alinity control efforts would be
<br />proportionately Ies~ effective.]
<br />The ~econda:s~umption concern~ the
<br />time it take~salt~ tOPa$ from the Upper
<br />Ba~in through all the re~ervoirsto the Im-
<br />perial Dam. The USDI (1983:45) a~~ume~
<br />a hydraulic retention time of 5 to 1 year~ .
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<br />Western Journal o/lt..grlcultural Economks
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<br />for ~alt~ to pa~~ through Lakes Powell and
<br />Mead to impact Lower Ba~in water user~.
<br />Ninety percent of. ~alinity bene:fit~ i~ ex-
<br />pected to be regi~tered at Imp.!rial Dam .
<br />at the end of that period. Thi~ i~ a ~ignif-
<br />icant point, becau~e it mean~ that dam-
<br />age~ avoided mu~t be discounted over thi~ z:1
<br />period to e~tabli~h a net pre~ent value for
<br />comparison to Salinity control cust~, a point
<br />overlooked hi. previous economic apprais-
<br />al~.
<br />For lack of a preci~e lag function, we
<br />a~ume a hydraulic retention time of six
<br />. year~ to achieve one hundred percent of
<br />. ~alinity benefit~. Fifteen percent of the
<br />benefit~ are assumed to accrue at the end
<br />of each of the fir~t five year~ with the re-
<br />mainingtwenty-five percent comirig in the
<br />~ixth year. (Thi~ a~llmption i~ probably
<br />overly favorable to fe~ibility, ~ the time
<br />di~tribution of impact~ i~ likely to be
<br />~kewed towards later year~.) .
<br />
<br />Agricultural Benefits
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<br />We adapted the procedure fir~tdevel- . 'l:.
<br />oped by Moore, Snyder and Sun to esti- 11
<br />mate damagei-avoided, or the benefits of
<br />~alinity con. trol to agriculture. Two line. ar' . ..
<br />prog. ra. mming modeh of Imperial .Valley ~
<br />agriculture were developed. (Full detaili . t . "'.
<br />and assumption~ are in. Gardner.) One t '1-- ~,
<br />model. reflecting production at 800 mg/. t~ r
<br />. ..l.i.t er., a.p. proX1.'.m.at~. ~.th.e curren. t.~l.'.tuat..ion... ... ..~~~
<br />(The average ~almlty from 1978-82 w~ ~.. \V'
<br />804 mg/liter.) A ~econd model ~imulating. . I 1;.. .
<br />1,100 mg/liter ~alinity conditions repre-. fl.
<br />~en. t~.. m....a xi. mum fu.ture ~alin. ity.conditiOil~.,. .. .~. t ..
<br />~ince current USBR foreca:st~ without ~. . ~
<br />linity control are 1,024 mg/liter in 2,000 ~
<br />and 1,089 mg/liter by 2,010 (USDI, 1983:
<br />. 47). The difference in net farm income
<br />between the two model~ provides our e~-
<br />timate of agricultural damage cau~ed by
<br />a 300 mg/liter incre~e in ~alinity. This
<br />total damage e~timate i~ converted to an
<br />e~timate of .a verage marginal ~a!inity
<br />damage per mg/liter.
<br />The benefite~timate~ deriv",d below are
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