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C:'".\ N --.l N ~ CHAPrERIV ECONOMIC IMPACTS OF SELECTED SALINITY CONTROL MEASURES THE GRAND V ALLEY COLORADO CASE STUDY Salinity (dissolved solids) in water supplies causes significant economic damages to agricultural, munici. pal and industrial water users in the Lower Colorado River Basin. Salinity is due to both natural causes (salt springs, surface runoff) and man. made causes (agri. culture and industry). Total salt contributions from irrigation in the Upper Basin have been estimated to account for about 88 percent of the total damages which accrue to downstream water users. The saline irrigation return flow problem in the Upper Basin is unusual, in that substantial amounts of salt are "picked up" from ancient marine deposits beneath the irrigated lands in addition to the more typical ferti1izet leaching and concentration of dissolved solids via evapotranspiration. This report focuses on the economic costs to water users of nonstructural methods of controlling saline irrigation return flows in the Upper Colorado River Basin (Appendix 5). The Grand Valley in western Colorado is used for a case study. The Grand Valley is located in west central Colorado at the confluence of the Gunnison and Colorado Rivers. The elevation is about 4,400 feet, and the normal growing season averages about 190 days. With an annual rainfall seldom exceeding 10 inches, irrigatiort is necessary to maintain a viable commercial agriculture in the valley. Approximately 57,000 acres of land is presently irrigated. Major crops grown include corn, alfalfa, sugar beets, small grains, and permanent pasture. Slightly less than 15 percent of the irrigated acreage is planted to pome and deciduous orchards and other specialty crops. The primary source of salinity comes from extremely saline aquifers (as high as 10,000 mg/l) overlying a marine-deposited Mancos shale formation. Lenses of salts contained in the shale are dissolved by water entering and coming into chemical equilibrium with the shale formation before returning to the river channel. Water enters the aquifers by seepage from delivery canals, laterals and drains (about 55 percent of the total), and from deep percolation from fields associated with application of irrigation water (about 45 percent). Average annual salt pickup attributable to irrigated agriculture in the Grand Valley is estimated at 600,000 tons, or about 10 tons per irrigated acre. Engineering studies have recommended that return flow control programs begin with lining irrigation water conveyance systems. Such structural measures would be effective, but are relatively expensive. The Bureau of Reclamation's proposed canal lining and drainage program may cost in excess of $60 million (1973 prices), or over $1,000 per acre. In the hope that nonstructural measures, involving changes in the institutional system (incentives, constraints, penalties) could do part of the job less expensively, several modifications of present irriga- tion practices were examined. Assumptions Two practices hypothesized which influence the amount of deep percolation (drainage water) and hence salt pickup, are analyzed. First, irrigators may modify traditional irrigation practices by varying the rate of water applied per unit area in the crop season. Previous research by agricultural engineers has revealed that soil infiltration rates in the study area are high in the early part of the irrigation season, but drop to low levels as the season progresses. Hence, if most of the deep percolation is thougbt to occur in the first two irrigations, salt percolation losses can be minimized merely by changing a) the length of time water is allowed to run in each furrow, and/or b) the rate of application by adjusting the size or number of siphon tubes, and/or c) spacing of furrows, and/or d) use of basin irrigation. Crops typically vary as to deep percolation losses, even with similar irrigation practices. A second method of reducing deep percolation can be achieved by cutting back the acreage of crops wbich are high contributors in favor of those which are less of a problem. Both of these alternatives involves increased costs or decreased income to affected farmers.Y Tbe Economic Model Linear programming models of representative farm situations provide the basis for deriving estimates of the economic costs of nonstructural salinity controls. Data for the models were collected 25