The vulnerability of water towers

The ideal natural fresh water supply system is constant and reliable: capable of storing water during times of plenty and releasing it when it’s needed.

Cue the world’s “water towers” – the term used to describe the role that high mountain ranges play in water storage and supply to sustain environmental and human water demands downstream.

Water towers seasonally generate higher runoff than their downstream areas as a result of orographic precipitation – the falls of snow and rain that result from moist air lifting over mountains.

The water’s release is delayed because it’s stored in snow, glaciers and lake reserves, which means that water towers have a buffering capacity. By supplying glacial melt water during hot and dry seasons, they provide a relatively constant supply to downstream areas.

Now, a global scientific team has assessed the planet’s 78 water tower systems and, for the first time, ranked them in order of importance to adjacent lowland communities. The team also reviewed their vulnerability to future environmental and socioeconomic changes.

These systems supply invaluable water resources to 1.9 billion people globally – roughly a quarter of the world’s population.

Published in the journal Nature, the research provides evidence that global water towers are at risk, in many cases critically, due to climate change, growing populations, water-resource mismanagement and other geopolitical factors.

The authors conclude that it’s essential to develop international, mountain-specific conservation and climate-change adaptation policies and strategies to safeguard both ecosystems and people downstream.

The world’s most relied-upon mountain system – and also one of the most vulnerable – is the Indus water tower in Asia, made up of vast areas of the Himalayan mountain range and covering portions of Afghanistan, China, India and Pakistan.

Other high-ranking water tower systems are the southern Andes, in Latin America; the Rocky Mountains, in North America; and the European Alps.

To determine their importance, researchers analysed the various factors that determine how reliant downstream communities are upon the supplies of water from the 78 water towers.

Predictions of future climate and socioeconomic changes were used to assess the vulnerability of each water tower’s resources, and that of the people and ecosystems that depend on it.

The study – authored by 32 scientists from around the world – was led by Walter Immerzeel and Arthur Lutz of Utrecht University in The Netherlands, both long-time researchers of water and climate change in the high mountains of Asia.

“What is unique about our study is that we have assessed the water towers’ importance, not only by looking at how much water they store and provide, but also how much mountain water is needed downstream and how vulnerable these systems and communities are to a number of likely changes in the next few decades,” says Immerzeel.

“By assessing all glacial water towers on Earth, we identified the key basins that should be on top of regional and global political agendas,” Lutz adds.