INTRODUCTION The San Simon Wash area includes about 2,300 mi2 in the Papago Indian Reservation in south-central Arizona and is characterized by low mountains separated by broad alluvial basins. Most of the basins and mountains trend north and slightly northwest. The basins are underlain by a thick sequence of basin-fill deposits. The mountains are composed of crystalline and consolidated sedimentary rocks, and thin alluvial deposits are present in the narrow mountain valleys and on pediments. The climate is semiarid, and the precipitation pattern is characterized by two distinct types of storms--local summer thunderstorms and regional winter storms. In most of the area the average annual precipitation ranges from 5 to 10 in.; in the Baboquivari Mountains, however, the average annual precipitation is 20 in. (Sellers and Hill, 1974, p. 7). Owing to the small amount of precipitation and the abundant sunshine, the evaporation rate is about 8 to 10 times the average rainfall (Heindl and others, 1962). Storm runoff occurs mainly as sheetflow and floods of short duration. Although some runoff is diverted to catchment tanks for use by livestock, runoff is not known to be diverted for irrigation or public-supply uses. Ground-water development has been slight compared with that in many areas in Arizona. In 1979 about 2,700 acre-ft of ground water was withdrawn, of which 2,200 acre-ft was used for irrigation at Papago Farms, and 500 acre-ft was used for public and livestock supplies. The hydrologic data on which these maps are based are available, for the most part, in computer-printout form and may be consulted at the Arizona Department of Water Resources, 99 East Virginia, Phoenix, and at U.S. Geological Survey offices in: Federal Building, 301 West Congress Street, Tucson, and Valley Center, Suite 1880, Phoenix. Material from which copies can be made at private expense is available at the Tucson and Phoenix offices of the U.S. Geological Survey.