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Groundwater: it's one of the planet's most exploited, most precious natural resources. It ranges in age from months to millions of years old. Around the world, there's increasing demand to know how much we have and how long before it's tapped out.

For the first time since a back-of-the-envelope calculation of the global volume of groundwater was attempted in the 1970s, an international group of hydrologists has produced the first data-driven estimate of the Earth's total supply of groundwater.

The bigger part of the study is the "modern" groundwater story. The report shows that less than six per cent of groundwater in the upper two kilometres of the Earth's landmass is renewable within a human lifetime.

"This has never been known before," says Gleeson. "We already know that water levels in lots of aquifers are dropping. We're using our groundwater resources too fast—faster than they're being renewed."

With the growing global demand for water—especially in light of climate change—this study provides important information to water managers and policy developers as well as scientists from fields such as hydrology, atmospheric science, geochemistry and oceanography to better manage groundwater resources in a sustainable way

"Since we now know how much groundwater is being depleted and how much there is, we will be able to estimate how long until we run out," says Gleeson. To do this, he will be leading a further study using a global scale model.

-- http://phys.org/news/2015-11-earth-hidden-groundwater.html / Nov 2015

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http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo2590.html

http://www.nature.com/ngeo/journal/vaop/ncurrent/extref/ngeo2590-s1.pdf