The first map showing the groundwater hidden beneath Earth's surface has been developed by scientists – the first step in establishing how much there is and, importantly, when it will run out. The map shows there are 23 million cubic kilometres of total groundwater, and that 0.35 million cubic kilometres of this is less than 50 years old.

An international team of scientists published its findings in the journal Nature Geoscience. The team used multiple data sets and more than 40,000 groundwater models to provide a data-driven estimate of the Earth's total supply of groundwater.

Tom Gleeson of the University of Victoria in Canada said: "This has never been known before. 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."

The team said there is an important distinction between new and old groundwater. Old groundwater is found deeper and is normally used in agriculture and industry. It can contain arsenic or uranium and can be very salty – Gleeson said much of it should be seen as non-renewable. Young groundwater on the other hand is faster moving, closer to the surface and a more renewable resource – but it is also more vulnerable to climate change, contamination and other human activities.

Researchers found less than 6% of the groundwater in the upper two kilometres of the planet is renewable in a human lifetime. Most of the modern groundwater was found in tropical and mountain regions, with the biggest deposits in areas like the Congo, Indonesia, the Amazon Basin and North and Central America. The least was found in dry areas like the Sahara.

Because of the growing demand for water increasing, the team says its map and findings will help policymakers and water managers better manage groundwater resources. It will also help scientists build a clearer picture of how modern and old groundwater is being depleted – and when it will be gone. "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," Gleeson said.

In an accompanying News & Views article, Ying Fan, from the Department of Earth and Planetary Sciences, Rutgers University, added: "This global view of groundwater will, hopefully, raise awareness that our youngest groundwater resources – those that are the most sensitive to anthropogenic and natural environmental changes – are finite."

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