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Groundwater overdraft is a specific condition in which extraction greatly exceeds the influxes of water (mainly recharge), to produce an unsustainable condition characterized by sustained declining water levels (see accompanying op-ed from Science magazine by M. McNutt). Much like overdraft of a bank account, groundwater overdraft is not a desirable state of affairs. Not only is it unsustainable in terms of management of the groundwater resource, but it also leads to long-lasting damages (a lot like what happens to your credit rating if your bank account is overdrafted!).
Depressurization of the aquifer, if large enough, may cause irreversible collapse and compaction. This reduces both storage (porosity) and hydraulic conductivity. It can also lead to land subsidence, especially in cases where the magnitude of overdraft is large, and where the aquifer units are thick and highly compressible, as is common for unconsolidated or uncemented sedimentary aquifers. One well-known example of groundwater overdraft is the Central Valley of California (Figure 41). Another is the Ogallala aquifer, a major groundwater system spanning across eight states in the American Midwest (Figure 43; see The High Plains Aquifer section). Substantial overdraft and subsidence also occur in widespread areas of the southeastern U.S., the Gulf Coast, and parts of Arizona and Las Vegas (Figure 44).
Source: NASA/JPL-Caltech/University of California, Irvine
Source: Image on bottom from USGS Multimedia Gallery
Source: U.S. Geological Survey Circular 1186
Figure 44. (Top) Areas of land subsidence in the U.S. associated with groundwater overdraft (from USGS Fact Sheet-165-00). (Bottom) Satellite-based estimates of groundwater stored in the subsurface on July 7, 2014, relative to the average from 1948 to 2009. Blue regions have more groundwater for this time of year than the average for this time of year, whereas red areas have less than the average. Note the correlation between areas of groundwater deficit and land subsidence.
Source: Map from NASA Earth Observatory