LONG-TERM GROUNDWATER DEPLETION

Development of global groundwater resources for agricultural, industrial, and municipal purposes greatly expanded in the last century, and economic gains from groundwater use have been dramatic. In many places, however, groundwater reserves have been depleted to the extent that water levels have declined tens to hundreds of meters, well yields have decreased, pumping costs have increased, and detrimental environmental impacts have become evident. These impacts increase the cost and reduce the sustainability of groundwater development. Estimates of depletion in individual aquifer systems based primarily on direct volumetric approaches indicate that cumulative global groundwater depletion since 1900 totals about 4,500 km³ through 2008. This large volume represents a substantial net transfer of water mass from continents to oceans, thereby contributing to sea-level rise (equivalent to a sea-level rise of 12.6 mm—approximately 6% of the observed total rise). The rate of annual depletion has increased markedly since about 1950, with maximum rates occurring during 2000-2008, when they averaged about 145 km³/yr (equivalent to 0.40 mm/yr of sea-level rise, or 13% of the reported rate of 3.1 mm/yr during this time). For the U.S. as a whole during 1950-2005, about 15% of total pumpage was derived from a reduction of storage of groundwater—a depletion fraction of 0.15. But depletion fractions vary widely within the U.S. and even within any given large aquifer system. For example, the fraction of long-term (1950-2000) pumpage derived from storage depletion in the High Plains aquifer is about 0.27, but ranges from 0.0 in Nebraska (where there is a slight water-table rise) to 0.42 in Texas. Although the High Plains aquifer has the greatest volume of depletion of any single aquifer in the U.S., during 2000-2008 the Central Valley of California had the largest depletion intensity (a measure that accounts for areal extent of aquifer and time) at an average rate of 0.075 m/yr. Groundwater depletion must be confronted on local and regional scales, where water managers will necessarily have to take actions to reduce demand (primarily in irrigated agriculture) and/or increase supply through managed aquifer recharge, desalination, and/or developing alternative sources.