Evaluation of agricultural adjustment to irrigation water salinity : a case study for Pinal County, Arizona

Abstract

The Central Arizona Project (CAP) is a billion-dollar-plus project to construct an aqueduct to transport water from Lake Havasu on the Colorado River into the Maricopa County-Phoenix area and then through Pinal County to Tucson. Upon completion of CAP in 1986, some of the Colorado River water will be delivered to Pinal County for agricultural use. Water available to Pinal County farmers in the initial years of the project is estimated at 659,000 acre-feet annually. Any new importation of water to an established irrigated agricultural area implies adjustments in the organization of the economy of the area. For irrigated agriculture, adjustments will occur in input mix, output mix, acreage farmed, and in gross and net incomes. A complicating factor associated with importation of Colorado River water is that the imported water will contain different dissolvedsalt concentrations than will the groundwater and surface water currently being used. Dissolved salts in irrigation water (salinity) decrease crop yields, i.e., as the salinity of water applied to a crop increases, yield per acre decreases. The magnitude of yield reduction due to salinity is dependent on the level of salinity of the irrigation water and on the crop's salt sensitivity. In areas of Pinal County where local water supplies have a lower average salinity than CAP water, average crop yields will decrease if CAP water is added to the crop-water mix. On the other hand, in those areas where the salinity of local water is higher than that of CAP water, higher crop yields will be realized by using CAP water in the crop-water mix. Thus, the optimal CAP-local water mix is determined in order to evaluate the economic adjustments of Pinal County farmers to the new water source. Pinal County is divided into seven irrigation districts, each of which has filed a letter of intent to purchase CAP water. Representative farm data for each district are stratified by farm size and pumping depth. Farms are divided into four size classes in order to reflect economies due to farm size. Because the cost of local pumped water varies with the pumping lift, the farms are also stratified by three depth-to-water classes. Thus, a total of 12 representative farms are necessary to describe the agricultural activities in each irrigation district, and 84 representative farms are needed for the county. Mathematical programming models of representative irrigated farms in Final County project adjustments implied under several assumptions as to the availability, cost and salinity of irrigation water from various sources. Results show that (1) most monetary benefits of the project will be captured by the Indian farmers of the county, (2) groundwater conservation will be minimal unless farmers are forced to purchase large quantities of CAP water, (3) provision of CAP water will not affect cotton acreage but will significantly increase the acreage of small grains and alfalfa, (4) the possibility of increased salinity from CAP water should not concern farmers in the county since projected decreases in net income occurring because of increased salinity average only 61 cents per acre per year, and (5) increased income to non-Indian farmers resulting from provision of CAP water at the currently proposed price will not be sufficient to pay the additional fixed costs for distribution systems.