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ScienceWater

Groundwater Disappearing at Accelerating Rate

Over the past fifty years the amount of water that has been pumped out of the subterranean reservoirs that sustain many billions of people has more than doubled, according to a new global assessment of groundwater use.

People are drawing so much water from underground that they are adding enough of it to the oceans to account for about 25% of the annual sea level rise across the planet.

“If you let the population grow by extending the irrigated areas using groundwater that is not being recharged, then you will run into a wall at a certain point in time, and you will have hunger and social unrest to go with it,” warns Marc Bierkens of Utrecht University in Utrecht, the Netherlands, and leader of the new study. “That is something that you can see coming for miles.”

Studying Water Depletion

The study will be published in an upcoming issue of the journal Geophysical Research Letters, and they point out that soaring global groundwater depletion bodes a potential disaster.

The new study compares estimates of groundwater added by rain and other sources to the amounts of water being removed for agriculture and other uses. Using a database of global groundwater information the researchers used models to estimate the rates at which groundwater is both added to aquifers and withdrawn.

For instance, to determine groundwater recharging rates, they simulate a groundwater layer beneath two soil layers, exposed at the top to rainfall, evaporation, and other effects, and use 44 years’ worth of precipitation, temperature, and evaporation data (1958-2001) to drive the model.

These techniques, used worldwide, show that the rate at which global groundwater stocks are shrinking has more than doubled between 1960 and 2000, and the total amount lost has grown from 126 to 283 cubic kilometres (30 to 68 cubic miles) of water per year.

Unknown Quantity

The most unsettling part of that figure is that we don’t have any hard data as to how much water is there to begin with, and thus no way to determine how fast the global supply of subterranean water would vanish at this rate.

What we do know is that if water was taken from the Great Lakes at this rate, they would be depleted in around 80 years.

Furthermore, groundwater represents approximately 30% of the available fresh water on the planet, with surface water accounting for only one percent. The rest is locked up in glaciers and the polar ice caps, which means depleting the groundwater leaves us … bone dry.

The new assessment shows the highest rates of depletion in some of the world’s major agricultural centers, including northwest India, northeastern China, northeast Pakistan, California’s central valley, and the midwestern United States.

“The rate of depletion increased almost linearly from the 1960s to the early 1990s,” says Bierkens. “But then you see a sharp increase which is related to the increase of upcoming economies and population numbers; mainly in India and China.”

As groundwater is increasingly withdrawn, the remaining water “will eventually be at a level so low that a regular farmer with his technology cannot reach it anymore,” says Bierkens. He adds that some nations will be able to use expensive technologies to get fresh water for food production through alternative means like desalinization plants or artificial groundwater recharge, but many won’t.

Source: American Geophysical Union

Image Source: walknboston




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