Water budget and mathematical model of the Coconino Aquifer, southern Navajo County, Arizona

Abstract

The main source of water in the 3,400-square-mile area of southern Navajo County is the large volume of ground water in storage in the Coconino aquifer, which consists of the Coconino Sandstone, the uppermost part of the underlying Supai Formation, and the overlying Kaibab Limestone. The amount of water withdrawn from the aquifer increased from about 13,800 acre-feet in 1960 to 38,400 acre-feet in 1972. As industrial and agricultural development continues, the amount of withdrawal probably will increase greatly. Aquifer tests indicate that the hydraulic conductivity of the aquifer ranges from 8 to 40 feet per day; however, a flow-net analysis indicates that the hydraulic conductivity may be as much as 80 feet per day in the north-central part of the area. In the southern and central parts of the area the aquifer is unconfined, and the storage coefficient is estimated to be about 0.15. In the northern and eastern parts the aquifer is confined, and the storage coefficient ranges from 0.00013 to 0.0014. A mathematical model was developed to simulate the groundwater system and to provide a management tool for estimating the effects of present and future ground-water withdrawals. The model indicates that the inflow to and outflow from the aquifer were about 105,600 acre-feet in 1960 prior to extensive ground-water development and that about 192,000 acre-feet of water was derived from groundwater storage between 1960 and 1972. The mathematical model provides an approximation of the Coconino aquifer and can be used to estimate the future response of the aquifer.