Box 1 – Regional Unconfined Aquifer System: The High Plains Aquifer
The High Plains aquifer in the central United States provides a good example of applying the approach of estimating depletion volume by integrating head changes and storage coefficient estimates. Water-level changes in this regionally extensive unconfined aquifer have been estimated from many water-level measurements in a large number of observation wells over many years, producing maps of water-level changes since predevelopment times (Figure Box 1-1).
The area-weighted average specific yield varies by State from 0.081 in Wyoming to 0.185 in Oklahoma and is 0.151 overall for the aquifer (McGuire, 2017; Gutentag et al., 1984; McGuire et al., 2012). McGuire (2017) accounts for this spatial variation and estimates that the total cumulative groundwater storage depletion in the High Plains aquifer since about 1950 is approximately 273.2 million acre-feet (337 km3).