Perilous Times
Groundwater Depletion Rate Accelerating Worldwide
ScienceDaily (Sep. 25, 2010) — In recent decades, the rate at which
humans worldwide are pumping dry the vast underground stores of water
that billions depend on has more than doubled, say scientists who have
conducted an unusual, global assessment of groundwater use.
These fast-shrinking subterranean reservoirs are essential to daily
life and agriculture in many regions, while also sustaining streams,
wetlands, and ecosystems and resisting land subsidence and salt water
intrusion into fresh water supplies. Today, people are drawing so much
water from below that they are adding enough of it to the oceans
(mainly by evaporation, then precipitation) to account for about 25
percent of the annual sea level rise across the planet, the researchers
find.
Soaring global groundwater depletion bodes a potential disaster for an
increasingly globalized agricultural system, says Marc Bierkens of
Utrecht University in Utrecht, the Netherlands, and leader of the new
study.
"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," Bierkens warns. "That is something that you can see
coming for miles."
He and his colleagues will publish their new findings in an upcoming
issue of Geophysical Research Letters, a journal of the American
Geophysical Union.
In the new study, which compares estimates of groundwater added by rain
and other sources to the amounts being removed for agriculture and
other uses, the team taps a database of global groundwater information
including maps of groundwater regions and water demand. The researchers
also use 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added to aquifers and withdrawn.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.
Applying these techniques worldwide to regions ranging from arid areas
to those with the wetness of grasslands, the team finds that the rate
at which global groundwater stocks are shrinking has more than doubled
between 1960 and 2000, increasing the amount lost from 126 to 283 cubic
kilometers (30 to 68 cubic miles) of water per year. Because the total
amount of groundwater in the world is unknown, it's hard to say how
fast the global supply would vanish at this rate. But, if water was
siphoned as rapidly from the Great Lakes, they would go bone-dry in
around 80 years.
Groundwater represents about 30 percent of the available fresh water on
the planet, with surface water accounting for only one percent. The
rest of the potable, agriculture friendly supply is locked up in
glaciers or the polar ice caps. This means that any reduction in the
availability of groundwater supplies could have profound effects for a
growing human population.
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.
Most water extracted from underground stocks ends up in the ocean, the
researchers note. The team estimates the contribution of groundwater
depletion to sea level rise to be 0.8 millimeters per year, which is
about a quarter of the current total rate of sea level rise of 3.1
millimeters per year. That's about as much sea-level rise as caused by
the melting of glaciers and icecaps outside of Greenland and
Antarctica, and it exceeds or falls into the high end of previous
estimates of groundwater depletion's contribution to sea level rise,
the researchers add.