LONDON-In a unique study, scientists have assessed and ranked the importance of Earth’s great “water towers”.

These are the 78 mountainous regions that are able to generate and then store vast quantities of water. They deliver it in a controlled way to major populations living downstream.

The Dutch-led team finds Asia’s Indus basin - fed by the Himalayan, Karakoram, Hindu-Kush, and Ladakh ranges - to be the most important storage unit on the planet.

Its waters, produced at high elevation from rain and snow, and draining from lakes and glaciers, support more than 200 million people settled across parts of Afghanistan, China, India and Pakistan.

But the Indus water tower, the researchers point out, is also the most vulnerable on their list of 78.

It’s subject to a range of current and future pressures, from ever greater demand - for more drinking water, for increased irrigation and industry, etc - to issues that could severely curtail supply. The latter will include geopolitical tensions, given the Indus intersects national boundaries; but the most obvious threat is climate change. A warming world will disrupt precipitation patterns and denude glaciers of their storage capacity.

“If, basically, the demand is higher but the supply decreases, then we really have a problem,” said research team-member Dr Tobias Bolch from the University of St Andrews, UK.

“And this is, I think, one of the major strengths of our study - that we have looked closely at both sides, so the supply index and the demand index,” he told BBC News.

Dr Bolch is speaking here at the American Geophysical Union’s Fall Meeting, the largest annual gathering of Earth scientists. His and colleagues’ work is being published simultaneously in the journal Nature.

The team conducted the assessment across the major continents, identifying what it regarded as the five most relied-upon, natural water tower systems in those regions:

North America: Fraser, Columbia and Northwest United States, Pacific and Arctic Coast, Saskatchewan-Nelson, North America-Colorado

South America: South Chile, South Argentina, Negro, La Puna region, North Chile Africa does not appear in this listing, principally because it is devoid of major glacier systems. Ice bodies do exist on the continent, in places such as on Mount Kilimanjaro and Mount Kenya, but their contribution to downstream catchments is limited.

And one of the defining aspects of the towers is the way they are able to maintain essential water supplies to populations even in drought years through the steady melt of high-elevation ice in summer months.