Document Type

Report

Publication Date

January 1983

Abstract

The need for more efficient water management is gaining recognition due to the increased cost of water supply, the growth in the demand for water, and greater environmental and social impacts of water programs. "Conjunctive use" of surfact and groundwater resources provides opportunities for increasing net benefits to the water users. Past "conjunctive use" studies, however, have usually not included water quality constraints. In Tooele valley, Utah, spatial variation of groundwater qualtity (total dissolved solids) is significant. The areas of good (400-500 mg/1), fair (500-1,000 mg/1), and poor (1,000-3,000 mg/1) quality groundwaters were identified in an earlier study by the USGS. The water quality dimension was incorporated into the conjunctive use planning to account for crop yield changes due to changes in salinity levels in irrigation water. The possibilities for increasing total net benefits by blending surface and groundwater of different qualities were examined by developing a linear programming optimization model. The optimization model provides for mixing the different qualities of water available for the cops to maximize benefits. It applies linear programming to the Tooele Valley water supply system and optimizes over three locations, four coprs, and five qualities of water of differing costs. The groundwater withdrawals at the locations dictated by the optimization model were input to the Tooele Valley groundwater simulation model developed by USGS to study the effects on the valley's principal artesian aquifer. Economic analyses of the probable scenarios of future agricultural development in Tooele Valley did not suggest that extensive increases in groundwater with drawals will occur. Economic infeasibility of major increases in groundwater extraction is a limiting factor for agricultural development in most parts of the valley. Groundwater mining therefore does not seem like a major future problem. The areas where new wells can be drilled without interference causing technological diseconomies are indicated. Profitable application of blending technology to irrigated agriculture in Tooele Valley is not possible without making a drastic shift to some higher valued crop such as fruit trees. All surface water sources should be fully utilized before developing additional and expensive groundwater. Even though an additional 20,000 to 25,000 ac-ft of groundwater can be extracted without mining, there would be a high risk of destroying natural phreatophyte habitats and degradation of water quality in at least some parts of the artesian aquifer.

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