FIGURE 26.4 The Ogallala aquifer. The Ogallala aquifer, also called the High Plains aquifer, is the largest in the United States, with a surface area of about 450,000 km2 (174,000 miles2). (a) The change in water level from 1950 to 2005, mostly due to withdrawals for irrigation that have exceeded the aquifer’s rate of recharge. (b) The current thickness of the aquifer.
(Data from http://pubs.usgs.gov/fs/2004/3097/ and http://ne.water.usgs.gov/ogw/hpwlms/)