Economic feasibility of selective adjustments in use of salvageable waters in the Tucson region, Arizona.

Abstract

Water in the Tucson region is a limited resource. Ground water has been developed in the past as a primary source of water to supply all uses, and the present possibility emerges that the output products of this system of uses can be salvaged and combined with the primary source for further use. A water-salvage industry is conceived in which the outputs of water uses, augmented by storm runoff, become the inputs to the industry; water treatment is the main activity of the industry; and water of improved levels of quality is its principal output. The regional sources of salvageable water are defined as 1) domestic-industrial effluents from the metropolitan sewage collection and disposal system, 2) domestic and industrial effluents from isolated locations, called discrete sources, and 3) storm runoff from both urban and non-urban watersheds. At the 1970 level the available quantity of such waters is estimated to be in excess of 35,000 acre-feet per year and increasing. Historically, only the first of these classes of water has been salvaged and reused in the Tucson region, and then only for irrigation of fiber, field, and forage crops. The current level of treatment technology is adequate to upgrade the quality of any or all of these salvageable waters to the requirements of additional agricultural as well as recreational and industrial uses. What needs to be determined is the degree of economic feasibility of allocating the salvaged waters to these uses under the prevailing institutional constraints. The selected types of potential uses in the water reuse subsystem are industrial uses as represented by power plants and by mining and milling operations, the recreational uses of urban fishing and boating activities and park irrigation, and agricultural uses in the form of irrigation of field and forage crops, cotton, orchard, and produce. Each of the classes of use is embodied in an industry which realizes a net return to the water input. An urban recreation survey indicates, for example, that under maximum intensity of use the net returns to water for fishing and boating might be as much as $500 per acre-foot. The objective relative to all uses is to maximize aggregate net returns to water from the combined supply, and the measure of effectiveness is a net benefit function representing the difference between gross benefit and incremental cost for water in each activity. The available salvaged waters are substituted incrementally for ground water in the total regional water supply function. Calculated numerical examples of allocation of combined supplies, in a linear programming format with restraining institutional conditions, demonstrate that under optimal allocation the treated municipal-industrial effluent would be used to some extent to serve not only agricultural but recreational and industrial uses. This result is attributable in part to the condition that the metropolitan water agencies bear the obligation of primary and secondary treatment at no direct cost to the user. Under benefit maximization this effluent also could serve a significant part of the water needs of the remote mining operations; under existing institutional arrangements, however, this industry is minimizing cost by pumping from the nearest available ground-water source. Preliminary calculations indicate that structural adjustments involving construction of new facilities in the Tucson region for the utilization of salvaged waters could be economically justified, their feasibility being highly sensitive to the extent of use for urban recreational activities.