Summary appraisals of the nation's ground-water resources – Lower Colorado region

Abstract

This report summarizes ground-water availability in the lower Colorado region and discusses the potential for greater ground-water development and increased efficiency of water use. The climate in the most highly developed southwestern part of the region is warm and dry and that in the northeastern part is cool and moist to dry. Although the regional average annual precipitation is only about 14 inches and most streambeds are dry during most of the year, about 1.5 billion acre-feet of ground water of moderate to good chemical quality is stored in aquifers of the region. Much of the water use is founded on pumped withdrawal of ground water. However, in most of the southwestern part of the region pumpage and consumptive use are in excess of replenishment, resulting in declining water levels. In the southwestern part of the region, water levels generally are from 200-500 feet below land surface and in large areas are less than 200 feet. Large-diameter water wells in this part of the region commonly produce 500-1 ,500 gallons per minute of water. In the northeastern part of the region, water levels generally are more than 500 feet below land surface, but in some large areas water levels are from 200-500 feet, and locally are less than 200 feet below the surface. In the northeastern part of the region, water wells generally have lower yields than those to the southwest. The yields range from a few to 2,000 gallons per minute, but most wells yield from 10 to less than 500 gallons per minute. In the southwestern part of the region about 1 billion acre-feet of ground water is recoverable from storage from the water table to a depth of 700 feet below the land surface. In the northeast about 150 million acre-feet can be recovered by dewatering a 100-foot-thick section of a typical aquifer (16 million-acre area). The estimated current annual rate of ground-water depletion, occurring almost entirely in the southwestern part of the region, is 2.4-3.2 million acre-feet per year. Almost 6 million acre-feet is pumped each year-about 90 percent for agriculture, 6 percent for public supply, and 3 percent for industrial use. The percentage of use for public supply and industry is increasing each year. The potential for greater development of ground water in the southwestern part of the region is constrained by land subsidence, earth cracks, increasing costs of pumping and transportation, and moderate to poor chemical quality of water. More ground water can be developed in the northeastern part of the region, where the major constraint is pumping cost owing to low to moderate well yields and depth to water. Some benefits can be realized everywhere in the region through changes in current use and greater efficiencies of use. Additional supplies may be made available by capture of natural evapotranspiration. Increasing the efficiency of use is possible hydrologically but, in the near term, is more expensive than increasing groundwater development. Decrease of irrigation, change to water-saving methods of irrigation, use of saline water, decrease of per capita public- supply use, and more reuse of water in almost every type of use could help extend the supply and thereby reduce the current rate of ground-water depletion. Financial problems have not yet caused an overall decrease in pumpage, but, locally, operating costs or partial dewatering of the aquifer has eliminated or decreased withdrawal. Current water laws in all States of the region, except Arizona, control or allocate the use of ground water.