13 Jul 2012 by Jim Fickett.
Depletion of groundwater has probably caused a large fraction of usable aquifers to drop several feet.
Shocking anecdotes about the groundwater level dropping in specific places are common. I've wondered if there is any global overview of the total groundwater loss, but not found anything. Today, however, I read this in a recent issue of Nature magazine:
During the latter half of the twentieth century, global sea level rose by about 1.8 millimetres per year, according to data from tide gauges. The combined contribution from heating of the oceans, which makes the water expand, along with melting of ice caps and glaciers, is estimated to be 1.1 millimetres per year, which leaves some 0.7 millimetres per year unaccounted for. This gap has been considered an important missing piece of the puzzle in estimates for past and current sea-level changes and for projections of future rises.
It now seems that the effects of human water use on land could fill that gap. A team of researchers reports in Nature Geoscience that land-based water storage could account for 0.77 millimetres per year, or 42%, of the observed sea-level rise between 1961 and 2003. Of that amount, the extraction of groundwater for irrigation and home and industrial use, with subsequent run-off to rivers and eventually to the oceans, represents the bulk of the contribution. …
Depletion of groundwater reserves has more than doubled in recent decades as a result of population growth and the increased demand on groundwater reservoirs for drinking water and the irrigation of croplands. Most of the water pumped up from deep pools is not replenished; it evaporates into the air or flows into river channels, feeding into the seas. Artificial reservoirs, such as the Hoover Dam on the Colorado River and the Three Gorges Dam on the Yangtze River in China, have the opposite effect, locking up water that would otherwise flow into the seas. Scientists once speculated that the effects cancelled each other out, but this study and other recent ones have shown that groundwater depletion has a larger net effect.
This is all modeling, and could well be subject to significant correction in the future. But if we assume it is at least roughly correct, we get that the groundwater loss in 50 years was enough to raise sea level by 0.77*50 = 38 millimeters, or about a an inch and a half. Since oceans have about double the surface area of the earth's continents, that means the groundwater lost would cover all of the land surface to a depth of about 3 inches. Since the primary aquifers being depleted are under a small fraction of the land surface, this probably means that a large fraction of aquifers have dropped several feet. That is in accord with the scary anecdotes, but is shocking nonetheless.