The High Plains aquifer underlies one of the major agricultural areas in the world, including parts of Colorado, Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas, and Wyoming. Ground-water irrigation expanded rapidly after 1940 in the area underlain by the High Plains aquifer (called the 'High Plains region' in this report): 1949--2.1 million acres; 1959--6.1 million acres; 1969--9.0 million acres; 1978--12.9 million acres; and 1980--13.7 million acres (Gutentag and others, 1984; Thelin and Heimes, 1987). In 1990, 95 percent of the water withdrawn from the High Plains aquifer, 15.7 million acre-feet, was used for irrigation (Marilee Horn, U.S. Geological Survey, written commun., 1996).
Water-level declines appeared in the High Plains aquifer soon after extensive ground-water irrigation development began. By 1980, water levels in the High Plains aquifer in parts of Texas, Oklahoma, and southwestern Kansas had declined more than 100 feet. In response to these declines, the U.S. Geological Survey, in cooperation with numerous Federal, State, and local water-resource agencies, began a ground-water monitoring program in 1988 to assess water-level change in the aquifer annually, using water-level measurements in more than 7,000 wells. The water-level measurements are made in winter or early spring, when water levels generally represent nonpumping conditions.