Changes in Continental Water Storage Caused by Groundwater Depletion Since 1900

Development of 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, and pumping costs have increased. These impacts tend to reduce the efficiency and sustainability of groundwater development. Much groundwater pumped from confined aquifers is derived from storage losses in adjacent low-permeability confining layers. Depletion in confining layers is difficult to estimate and rarely monitored, but it can greatly exceed the depletion from the confined aquifer itself and groundwater drainage from confining layers is partly irreversible. For example, in the confined Dakota Aquifer, about 98 percent of the water removed from storage was derived from depletion in adjacent confining units. If cumulative long-term regional and global groundwater depletion is large, it will represent a substantial net transfer of water mass from land to the oceans, thereby contributing to sea-level rise. A U.S. national groundwater depletion census was made primarily using direct volumetric approaches; results indicate that about 800 km3 of water was depleted from groundwater systems in the U.S. during the 20th century-equivalent to a sea-level rise of approximately 2.2 mm-and 1,000 km3 through 2008. Cumulative global groundwater depletion since 1900 totals about 3,400 km3 through 2000 and 4,500 km3 through 2008 (equivalent to a sea-level rise of 12.6 mm). The rate of annual depletion has increased markedly since about 1950, with maximum rates occurring during the most recent period (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 recent period). Overall, the volume and rate of estimated long-term global groundwater depletion balances 6 to 7 percent of the observed sea-level rise since 1900. The recent acceleration in groundwater depletion is greater outside the U.S. Although groundwater depletion rates will ultimately be self-limiting, data show that we have not yet reached that point either nationally or globally.