The Papago Farms-Great Plain and upper Rio Sonoyta study area includes about 490 square miles in south-central Arizona and north-central Sonora, Mexico. The area is characterized by a broad, deep, sediment-filled basin bounded by low, jagged fault-block mountains. The climate is arid to semiarid. The climate and abundant ground water provide favorable conditions for irrigated agriculture. Annual precipitation averages 5 to 8 inches per year on the desert floor. Runoff, which occurs as intermittent streamflow and sheetflow, is too short lived and too laden with suspended sediment to be a reliable source for irrigation or public supply. Nearly all the water used to irrigate more than 5,000 cultivated acres in the study area is withdrawn from the unconsolidated to partly consolidated basin fill. The ground water occurs in the deposits under unconfined (water-table) conditions with a saturated thickness that ranges from zero along the mountain fronts to more than 8,000 feet in the center of the basin. The amount of recoverable ground water in storage to a depth of 400 feet below the 1978-80 water table is estimated to be about 10 million acre-feet. Depths to water range from about 500 feet near the southern boundary of the study area to about 150 feet in the center of the study area. Ground water enters the area principally as underflow beneath the San Simon and Chukut Kuk Washes and as recharge along the mountain fronts. On the basis of model results, annual inflow to the ground-water system is estimated to be about 4,390 acre-feet. Ground water moves through the study area along paths that encircle a virtually impermeable unit in the basin center, termed 'the lakebed-clay deposits,' and moves westward to an outflow point beneath the Rio Sonoyta south of Cerro La Nariz. Rates of water movement range from less than I foot per year in clays to about 160 feet per year in well-sorted, coarse stream-channel deposits. Transmissivities along the basin margins range from 10,000 to 40,000 feet squared per day, whereas transmissivities in the basin-center lakebed-clay deposits are estimated to be less than 100 feet squared per day. Most Wells that are located along the basin margin and tap more than 300 feet of saturated basin fill in the upper1,000 feet of the aquifer should yield from 500 to 3,000 gallons per minute to properly constructed and developed wells. Specific capacities should range from 10 to 50 gallons per minute per foot of drawdown. The water in the aquifer is moderate to poor in chemical quality for irrigation and public-supply use. The ground water is mainly a sodium bicarbonate type with dissolved-solids concentrations that range from about 250 to 5,000 milligrams per liter and average about 530 milligrams per liter. The poorest quality water is associated with the basin-center lakebed-clay deposits. In most of the basin, the water contains fluoride concentrations that exceed the maximum contaminant levels acceptable for drinking water. Waters from the basin-center lakebed-clay deposits are also anomalously high in dissolved arsenic and unacceptable for public supply. High concentrations of sodium and bicarbonate in the ground water of the study area present potential hazards to most crops, and the use of this type of water requires careful farm-management practices. In 1981 outflow resulting from withdrawals of water from the aquifer was about 23,2'00 acre-feet. Storage is being depleted at a rate of about 19,000 acre-feet per year. On the basis of a mathematical simulation of the groundwater system and withdrawal rates in 1981, storage depletion and drawdown of the water table were projected to 1991. Water-level declines in 199t were estimated to be as much as 20 feet at Papago Farms and more than 40 feet in the area south of the basin-center lakebed-clay deposits. The estimated amount of depletion in 1991 of ground water stored in the upper 400 feet of the aquifer is less than 3.0 percent of the total amou