This study by the USGS is summarized here:
Estimated groundwater depletion in the United States during 1900–2008 totals approximately 1,000 cubic kilometers (km3 ). Furthermore, the rate of groundwater depletion has increased markedly since about 1950, with maximum rates occurring during the most recent period (2000–2008) when the depletion rate averaged almost 25 km3 per year (compared to 9.2 km3 per year averaged over the 1900–2008 timeframe).
The purposes of this report are (1) to document the magnitude and trends in longterm groundwater depletion in the United States, and (2) to provide additional background information and details that support the methods and calculations used to estimate groundwater depletion for the 40 areas and 1 land use category of depletion in the United States—information that underlies the assessments in Konikow (2011). Because no substantial volumetric groundwater depletion is evident in Alaska, that area is excluded from the maps in this report.
Discussion of Results The 41 separate assessments (table 1) provide evidence that the long-term (1900–2008) cumulative depletion of groundwater in the United States is about 1,000 km3 —about twice that of the volume of water contained in Lake Erie (about 480 km3 ). The depletion volume had increased from about 800 km3 in 2000—an increase of 25 percent in just 8 years. This large volume of depletion represents a serious problem in the United States because much of this storage loss cannot be easily or quickly recovered and affects the sustainability of some critical water supplies and base flow to streams, among other effects.
Conclusions This study assessed long-term groundwater depletion in 40 separate aquifer systems or subareas, and one land use category. The cumulative volume of groundwater depletion in the United States during the 20th century is large—totaling about 800 cubic kilometers (km3 ) and increasing by an additional 25 percent during 2001–2008 (to a total volume of approximately 1,000 km3 ).
Cumulative total groundwater depletion in the United States accelerated in the late 1940s and continued at an almost steady linear rate through the end of the century. In addition to widely recognized environmental consequences, groundwater depletion also adversely impacts the long-term sustainability of groundwater supplies to help meet the Nation’s water needs. Groundwater depletion also is a small contributor to global sea-level rise, but sufficiently large that it needs to be recognized as a contributing factor and accounted for when explaining long-term global sea-level rise.
In general, unconfined aquifers exhibit greater volumetric depletion than do confined aquifers, although the latter tend to have greater water-level declines. Depletion in confined aquifers is derived primarily from leakage and storage depletion in adjacent low-permeability confining units. Depletion is also greater in the semiarid to arid western States than in the humid eastern States because of the greater potential for recharge to offset or balance withdrawals in humid areas.