A team of researchers released a model detailing how much groundwater Earth has to offer in response to an increasing demand to know the actual figure.

The University of Victoria (UVIC) stated in a news release that researchers attempted a "back-of-the-envelope calculation" in the 1970s. The new study, published in the journal Nature Geoscience, is the first "data-driven estimate" aimed at quantifying the Earth's stores of groundwater.

"This has never been known before," study lead author Tom Gleeson, a researcher at UVIC, said in the release. "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."

According to BBC News, groundwater is stored in rock and soil beneath the surface and only about six percent of it makes it into humans' daily lives. That is because such a small amount of ground water exists near the Earth's surface, and the bit that does is called "modern" groundwater.

"Intuitively, we expect drier areas to have less modern groundwater and more humid areas to have more, but before this study, all we had was intuition," study co-author Kevin Befus, a post-doctoral fellow at the United States Geological Survey who participated in the study for doctoral research at the University of Texas, said in the release. "Now, we have a quantitative estimate that we compared to geochemical observations."

The researchers also designed a map (above) detailing where in the world groundwater is most prevalent.

"Old water is highly variable," Gleeson told BBC News. "Some places it is quite deep, in some places not. In many places, it can be poor quality.

"It can be more saline even than ocean water and it can have lots of dissolved metals and other chemicals that would need to be treated before it could be used for drinking or agriculture."