CHANGE IN WATER IN STORAGE IN THE HIGH PLAINS AQUIFER, 2000

The High Plains aquifer, which underlies about 174,000 square miles in parts of eight States-Colorado, Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas, and Wyoming-is the principal source of water in one of the major agricultural areas in the United States. Water-level declines began soon after extensive ground-water irrigation. The U.S. Geological Survey, in cooperation with numerous Federal, State, and local water-resource agencies, monitors ground-water levels in the High Plains aquifer using periodic and continuous water-level measurements from wells. Water levels are measured primarily in irrigation wells in winter to early spring, when water levels have generally stabilized. The water-level elevation for predevelopment conditions was determined using water-level measurements from more than 20,000 wells. The water-level elevation in the year 2000 was determined using water-level measurements from more than 7,000 wells; 127 of these wells are instrumented with continuous recorders.

The water-level measurements were analyzed to determine the change in water-level elevation from predevelopment to the year 2000. The change in the volume of water in the aquifer was calculated using the mapped area in each water-level change interval and an average specific yield value for the aquifer in each State. The results indicate that the water in storage in the aquifer decreased about 200 million acre-feet from predevelopment to the year 2000. This decline is about 6 percent of the predevelopment volume in the aquifer. In the 26,000 square-mile area with greater than 25 feet of water-level declines, the water in storage in the aquifer decreased about 190 million acre-feet from predevelopment to 2000. This represents a decline of about 34 percent of the predevelopment volume in that part of the aquifer.

The States' ground-water-management approaches for the High Plains aquifer are designed to prevent aquifer depletion, manage aquifer development, or to attempt to insure the availability of aquifer resources for a specified period. In some States, the management approaches are also designed to limit water-level declines to maintain an acceptable amount of ground-water discharge to rivers and streams.