|Abstract:||The Willcox basin is an area of interior drainage in the northern part of Sulphur Springs Valley, Cochise and Graham Counties, Ariz. The basin comprises about 1,500 square miles, of which the valley floor occupies about 950 square miles.
The basin probably formed during middle and late Tertiary time, when the area was subjected to large-scale faulting accompanied by the uplift of the mountain ranges that presently border it. During and after faulting, large quantities of alluvium were deposited in the closed basin.
The rocks in the basin are divided into two broad groups--the rocks of the mountain blocks, of Precambrian through Tertiary age, and the rocks of the basin, of Tertiary and Quaternary age. The mountain blocks consist of igneous, metamorphic, and sedimentary rocks; the water-bearing characteristics of these rocks depend primarily on their degree of weathering and fracturing. Even in areas where these rocks are fractured and jointed, only small amounts of water have been developed. The rocks of the basin consist of moderately consolidated alluvium, poorly consolidated alluvium, and unconsolidated alluvium. The water-bearing characteristics of the moderately and poorly consolidated alluvium are not well known. The unconsolidated alluvium underlies most of the valley floor and consists of two facies, stream deposits and lake beds associated with the old playa. The lenticular sand and gravel layers interbedded in silt- and clay-size material of the unconsolidated alluvium constitute the principal aquifer in the basin. The other aquifers, which yield less water, consist of beds of poorly to moderately consolidated sand- and gravel-size material; these beds occur in both the poorly consolidated and moderately consolidated alluvium.
In the Stewart area the median specific capacity of wells per 100 feet of saturated unconsolidated alluvium was 20 gallons per minute, and in the Kansas Settlement area the specific capacity of wells penetrating the poorly and moderately consolidated alluvium, undifferentiated, was only 7.4 gallons per minute per 100 feet of saturated material penetrated. The aquifer in the Kansas Settlement area is much less permeable but more homogeneous than the aquifer in the Stewart area. The coefficient of transmissibility of the aquifers, which was estimated from the specific-capacity data, ranged from 58,000 to 160,000 gal. tons per day per foot.
Prior to extensive ground-water pumpage, the ground-water system probably was in equilibrium, with discharge equaling recharge. At that time, ground water moved toward the playa, where it was discharged by transpiration and evaporation. The estimate of the evapotranspiration in the playa area before large-scale development was about 75,000 acre-feet per year. On the basis of estimates of coefficients of transmissibility of the aquifer and on the basis of the water-table configuration, underflow toward the playa was computed to be about 54,000 acre-feet per year.
By 1963, large-scale pumping had caused marked changes in the shape of the piezometric surface; large cones of depression had developed, and ground-water movement was toward the centers of pumping. The cones of depression caused by large-scale pumping have since expanded, and water-level declines have been measured in the recharge areas along the mountain fronts.
Ground water has been used for irrigation since 1910. In 1928, about 4,000 acre-feet of ground water was pumped, and by 1963 180,000 acre-feet per year was being pumped. An estimated 1,860,000 acre-feet of water has been pumped for irrigation in the Willcox basin through 1963; 680,000 acre-feet from the Stewart area, 990,000 acre-feet from the Kansas Settlement area, and 190,000 acre-feet from the Pearce-Cochise area. In the Sierra Bonita Ranch area and the north playa area, ground-water withdrawal for irrigation through 1963 was small. From the spring of 1952 to the spring of 1964 water-level declines resulting from the