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Economic Damages to Irrigated Agriculture 25 l\) o ~'A (Ji baseline level below the threshold of salinity damage from the crop in question; and the second, the actual level that can be expected from the USBR simulation of river conditions (projections shown on figures 3-5, page 19). Next, twelve crops were selected that repre- sent the highest value crops in the irrigated farlOS of the Lower Basin, and which have salinity damage thresholds within the range of 200-1200 mg/L, as determined by E.V. Maas of the U.S. Salinity Laboratory. The highest value crops were identified from the most recent (1985) Bureau of Reclamation Summary Statis- tics, Vol. I Water, Land, and Related Data. They are: lettuce, alfalfa hay, cotton lint, wheat, sugar beets, carrots, cantaloupe/melons, oranges/tangerines, lemonsllimes, grapefruit, table grapes, and dates, However, five of these crops were excluded because their salinity damage threshold exceeds 1200 mg/L: cotton lint, wheat, sugar beets, dates, and cantaloupe! melons. Two other salt-sensitive crops of rela- tively high value were added: onions and avocados. Regressions on Yield of Agricultural Crops Data on crop yield of nine salt-sensitive crops in three agricultural counties where Colorado River water is used for irrigation were obtained by year for the 1971-1984 period from Arizona and California agricultural statistics. The counties were Yuma, Arizona; Imperial, California; and Riverside, California. La paz County, Arizona, had been created only in 1983, so data were sparse and therefore not included, The data show average yield of each crop in con- ventional units of yield (e.g., tons or cwt) per acre. Data on salinity levels in the Colorado River at Imperial Dam and at Parker Dam were ob- tained for each of the years from the Bureau of Reclamation's Quality of Waler, Colorado River Basin, Progress Report No. 13, January 1987, Tables 19 and 20. During the 1971-1984 period, TDS values ranged from 675 to 892 mg/L at Imperial Dam and from 611 to 758 mg/L at Parker Dam. These data were combined into data sets (yield vs. TDS) and regression curves and for- mulas generated using the computer regression program. For six of the crops, data from all three counties were available, although Yuma County data on citrus crop yield (i.e., oran- ges/tangerines, grapefruit, and lemonsllimes) were available only for the 1980-1984 period. For carrots, yield data were available only for Imperial and Riverside Counties. For avocados and table grapes, yield data were available only for Riverside County. By combining data from three counties, the implicit hypothesis was made that salinity is the sole factor determining yield, and that other factors such as climate, local rainfall and temperature variations, soil quality, and farming techniques are uniform among the counties. Such simplification is known to be untrue, so the yield data being regressed against salinity are believed to provide only an approximation of the true yield/salinity relationship. Nevertheless, no better data are known to be available for use in this study, The use of an empirical TDS/yield relation- ship based on actual crops grown and on such profit maximizing adjustments in agricultural techniques as were made in the Lower Basin during the 1971-85 period parallels the reasoning of Richard Gardner, who calculated annual salinity damages "as the difference in net income between the two salinity levels, that is, the amount by which profits are reduced after all profit maximizing adjustments have been made to the higher salinity level."lO For some crops, inspection of the data shows that crop yields from the desert portion of River- side County are lower than those in Imperial and Yuma Counties, even though Riverside County receives Colorado River water from below Parker Dam which is less saline than water from below tmperial Dam. Such an inversion causes iUogical regression results, e.g., that carrot yield is directly (not inversely) proportional to salinity l~chard L Gardner, "Economirs and Cost Sharing of Salinity Control ~n the Colorado River Basin," Ph.D. disserta- tion. Agricultunl and Natural Resource Economics Department, Colondo State Univt:~ity, Fort Collins, Falll98J, p. 124.