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"'-% \'~J t,;.:~:., Gardner and Y DUng .... CD " W "average" benefit~ for the range of 800 mgllfter . to l,loo,':ri1gfliter of di~~.alved ~olid~. Thi~ form\!liltion ~erve~ to over~tate benefit~ (to an ';;;known degree) for two rea~on~. The fir~ti~ that becau~e iJf poten- tials for alteration~ in crop mix and water . use technology a~~alinity increa~e~, the damage function i~ mo~t probably nonlin- ear, increa~ing at an increa~ing rate. Sec- .and, the likely average level of ~alinity over the future planning period without the control program i~ le~ than the 950 mgfliter implicitly as~umed in the federal analy~is~ Hence, the appropriate b~e from which tomea~urebenefit~; even before the. . high flow~ which diluted~alinity in 1983 and. 1984, would be in the 800-900 mgf liter range. Thu~.webelieve the marginal agricultural damage~ are actually ~ome- what le~ than reported here; The model. Agriculture in.the Imperial Valley i~an extremely diver~ified, year, round enterpri~e. Morethan one-fourth of the 450,000acre~ of cropland are double- cropped. The extremely arid region di- vert~ OVer 2.5 mHIion acre-feet of water annually from the Colorado River.. Each linear program con~i~ted of 76 crop pro, duction activitie~ ~pread over five field. crop~, nine vegetable crop~. two ~oildrain- age condition~, and two irrigation fre- quencie~. The predominant dOllblecrop.. ping option~ were each combined into ~ingle activitie~. . Cropbudget~ and water u~e e~timates were adapted from the Imperial County Cooperative Exten~ion Service (1982). Harve~t co~t~ were varied with yield. Crop price~ are real 1977-81 averages, ex- pre~~ed in 1982 dollar~. Proportional con~traint~ were impo~ed on the degreeto.which each crop could be produced on well-drained ~oil~. Thi~ added reali~m. by ~imulating heterogene- ity of ~oil di~tribution~ and cropping rigid- itie~ cau.~ed by marketing contract~, The~e con~traint~ limit. the maximum re~pon~e . farmer~ in the model can make to .in- crea~ed ~alinity. Thi~ i~ thought to be more ;ir~'i ,.'.'Ii:: Salinity Control Evaluation reali~tic than .earIier approache~ which al- lowed ~witching the higher valued crop~ entirely to well-drained ~oil~.f(The general format of the model i~ given in Table 1.) Crop yield~. Since the model reflects productivity on two ~oil types and under . two irrigation frequencie~,'publi~hed 1977-81 Di~trict average yield~ were ad- ju~ted according to agronomi~t~' judg- ment~ to derive the expected yield under each of the~e four alternative~. In addition, yield declination~ were e~- thnated for an.increa~e in irrigation water ~aIinity from 800 to 1,100 mgfliter. The . yield declination~. were calculated from the effective ~oil ~aturation extract con- ductivitie~ by ~aIinity level arid ~oil type (~reported byRobin~on,1978:90). They . are "effective" conductivitie~in that they were adju~teddownwardto reflect the ~ignificant amount~ of gyp~um (calcium ~uIfate) in the ~oil and water. (Gyp~um contribute~tonieasured ~Iinity, but is relatively less detrimental to plant growth.) The~e conductivitie~ were used with the expected yield decrements from increa~- ing ~oil salinity estimated by Kleinman and Brown (p. 121). Thi~ method allow~ the e~timation of yield decrement~on poorly drained ~oil at a given ~alinity level, ~ well ~ decre- ment~ fromincrea~ed ~aIinity. Yield~ on each ~oil type were estimated using five- year Imperial County average yields,. to- gether with the . expected yield deCre- ments and the crop di~tribution on~oil type~ reported byRobin~on (p.89). Alfal- fa i~ moderately ~en~itive to ~alinity, while the other field crop~ are more tolerant. Vegetable crop~ generally are more ~alt . ~en~itive. Lettuceyield~, inparticlllar, de- dine rapidly ~ ~oil ~aIinitie~ increa~e. Carrots and onion~ are also ~en~iUve to ~a- Iinity, but are grown exclu~ively on well- drained ~oil~. The model w~ Validated by comparing projected crop acreage and water u~e in the 800 mgfliter model with recent ex- perience. Irrigation water requirement~ 5 ~~~t~g::}t~:.~i?':;;.!:::?o -:-"-,,." .,,:;~:' >7:..:'?: ....-, ...... .~:; ::;.~::-.\. ?~:;_:. t:'/~:;