LONDON, Nov 17 : The total amount of groundwater on the planet, held in rock and soil below our feet, is estimated to be 23 million cubic km. If this volume is hard to visualise, imagine the Earth's entire land surface covered in a layer some 180m deep.
The new calculation comes from a Canadian-led team and is published in the journal Nature Geoscience. Significantly, little of this water - just 6% - is the kind of bankable resource that is most useful to people. That small fraction is referred to as "modern" groundwater: it is extractable because it is near the surface, and can be used to supplement above-ground resources in rivers and lakes.
"It's the groundwater that is the most quickly renewed - on the scale of human lifetimes," explained study leader Tom Gleeson from the University of Victoria.
"And yet this modern groundwater is also the most sensitive to climate change and to human contamination. So, it's a vital resource that we need to manage better."
To quantify just how much water is stored in the top 2km of the Earth's surface, Dr Gleeson's team has had to combine large data sets with an element of modelling.     ?BBC