Change in Water in Storage in the High Plains Aquifer, Predevelopment to 2007

The High Plains aquifer underlies about 174,000 square miles in parts of eight States—Colorado, Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas, and Wyoming.  The aquifer is the principal source of water for irrigation and public supply in this area, which is one of the major agricultural areas in the United States.  However, soon after groundwater irrigation began, water-level declines occurred in some parts of the aquifer.  In response, the U.S. Geological Survey, in cooperation with numerous Federal, State, and local water-resource agencies, began monitoring groundwater levels in the aquifer.  Water levels are measured primarily in irrigation wells. The wells are measured annually in winter to early spring (generally January to May, depending on location), when water levels generally have recovered from groundwater pumping for irrigation in the previous growing season and before the current year's irrigation season.  Water-level elevation for predevelopment conditions (about 1950) was determined using water-level measurements from more than 20,000 wells.  Water-level elevation for 2007 was measured in more than 9,000 wells.  The water-level measurements were analyzed and interpolated to map discrete intervals of water-level changes from predevelopment to the year 2007.  The change in the volume of drainable water stored in the aquifer was calculated using the mapped area of each water-level-change interval, the average water-level change within each mapped interval, and the estimated average specific yield for the aquifer.  
 Water-level changes in the aquifer from predevelopment to 2007 ranged between a rise of +84 feet for a well in Nebraska and a decline of -234 feet for a well in Texas. Area-weighted, average water-level change in the aquifer was a decline of -14 feet from predevelopment to 2007. Total water in storage in the aquifer in 2007 was about 2.9 billion acre-feet, a decline of about 270 million acre-feet (or 9 percent) since predevelopment.