The area discussed in this report is in south-central Arizona and includes two valleys the Rainbow Valley and Waterman Wash areas where ground water is used for irrigation. The two valleys are separated by a low saddle cut on granite that has been partly covered by alluvium. The granite is exposed across the saddle in small hills and in the bottoms of dry washes, and it effectively separates the Rainbow Valley and Waterman Wash areas into two distinct groundwater basins. The valleys, consisting largely of piedmont slopes extending from the foot of the mountains to the center of each valley, are typical of the Basin and Range province in Arizona. Tertiary and Quarternary alluvial material, consisting of lenticular deposits of gravel, sand, silt, and clay of variable thickness, occupies the intermontane troughs. In the Rainbow Valley area ground water is under water-table conditions in the sand and gravel lenses of the alluvial fill and to some extent in lavas near the Gila River south of Gillespie Dam. The general pattern of movement of ground water is southwestward toward the Gila River and from north to south along the river. Water levels in the area are declining in response to the discharge of ground water in excess of replenishment. In the center of the irrigated area, the water table declined more than 100 feet in the 9-year period 1952-61. For the most part, the ground water in the Waterman Wash area is under water-table conditions although some water may be under artesian conditions locally. Medium to high water-yielding materials of the alluvial fill, composed mostly of sand and gravel with lesser amounts of silt and clay, constitute the major aquifer in the area. In 1960, about 60,000 acre-feet of ground water was pumped from the aquifer in the Waterman Wash area. It is estimated that more than 80 percent of this water was removed from storage. Water-level declines for the period 1952-61 ranged from about 20 feet on the edge of the irrigated area to more than 80 feet in the center. Seven million acre-feet of ground water is estimated to be available from storage in the area from a depth of about 300 to 800 feet below the land surface. By definition, this is the amount of water in storage that will drain by gravity to wells. The ability to actually Fl CONTRIBUTIONS TO THE HYDROLOGY OF THE UNITED STATES withdraw the full amount is contingent upon several factors, such as the depth and distribution of wells and operation of the groundwater reservoir at optimum rates and schedules of pumping. The quality of the ground water at depth will also affect the amount of water that can be used.
