Mountains as ‘water towers’ for world’s lowlands

Yangtze river - main
Qutang Gorge along the Yangtze River. The longest river in Asia, it originates from tributaries in the eastern portion of the Tibetan Plateau. Copyright: Tan Wei Liang Byorn (CC BY 3.0).

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  • World’s lowlands increasingly rely on mountain runoffs for water needs
  • 24 per cent of lowland populations may depend on mountain waters by 2050
  • Sustainable development in mountain regions critical for the future

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[NEW YORK] The world’s lowland inhabitants are increasingly dependent on mountain runoffs for their water needs, says a new scientific study on water supply and consumption. 

Published this month (July) in Nature Sustainability, the study analyses river catchments of 10,000 square kilometres or more to show that dependence on mountain runoffs is more widespread than previously thought.

“Ensuring the function of mountains as 'water towers' should be a major concern of the world's lowland populations”

Daniel Viviroli, University of Zurich

Daniel Viviroli, lead author and professor, department of geography, University of Zurich, said while seven per cent of the lowland population was dependent on mountain runoffs in the 1960s, this figure could rise to 24 per cent by mid-21st century.

According to the IPCC special report on the oceans and cryosphere in a changing climate, released in September 2019, widespread cryosphere changes affect physical, biological and human systems in the mountains and surrounding lowlands, with impacts evident even in the oceans.

“Until now, research has focused mainly on river basins that originate in High Mountain Asia. But, in many other regions, irrigated agriculture is heavily dependent on water from mountainous areas, such as in the Middle East and North Africa, as well as parts of North America, South America, and Australia,” Viviroli said in a statement.

The new study focuses on important catchment areas such as the Ganges-Brahmaputra-Meghna, Yangtze, and Indus rivers in Asia, the Nile and Niger in Africa, the Euphrates and Tigris in the Middle East and the Colorado River in North America.

Viviroli explained the study is based on a middle-of-the-road scenario in terms of population growth as well as technological, economic and social development. "Ensuring the function of mountains as 'water towers' should be a major concern of the world's lowland populations," Viviroli said.

Matti Kummu, co-author and associate professor specialising in global water and food issues at Aalto University, Espoo, Finland, tells SciDev.Net: “We first assessed the availability of local water resources, today, and under climate change. If those were not adequate in relation to the demand, then we assessed the role of water originating from mountain regions. It was in this way we were able to, for the first time, map the areas that are dependent on these 'water towers’.”

He adds: “So in a way, we are not seeing mountain water as a 'primary source' to meet the future needs, but just assessing importance of mountains in supplying water downstream.”

Kummu says in cases where water (local and upstream) is insufficient, other sources need to be sought and water use decreased and used more efficiently — such as by resorting to drip irrigation. Desalination can be an option in coastal areas, "but it is a very expensive and energy-intensive way to produce water”.

When water is consumed for irrigation, Kummu says, “It cannot be reused as it evaporates. Most of the domestic water use (showers, etc.) could be purified to be used again. But this is very expensive, and counts for only three per cent of global total water use”.

Looking at the needs of Asia specifically, Kummu says large parts of the Indus and Ganges lowlands are dependent on water from mountains, and the Himalayas are very critical for those regions.

“As we show in our work, it is crucial to acknowledge the importance of mountains to supply water to lowlands. Our work highlights that sustainable development of mountain regions is essential, for example, by preventing agricultural overuse and ensuring the functioning of ecosystems,” Kummu says.

Lian Pin Koh, director, Centre for Nature-based Climate Solutions, National University of Singapore, says lowland communities in Asia are heavily dependent on mountain water resources. “For example, the study projects that by 2050, over half a billion people living in the Ganges-Brahmaputra-Meghna basin, which drains into the Bay of Bengal, may depend on essential mountain contributions for their water supply.”

Sustainable development of mountain regions is critical, Viviroli and his co-authors stress. This includes preventing agricultural overuse and climate change issues. For example, due to the rising temperatures, meltwater peaks from snow-covered mountain regions sometimes already occur several weeks earlier and are thus not as useful for summer agriculture.

For the future, Viviroli said, “It will be crucial that lowland and mountain regions work closely together despite political, cultural, social and economic differences”.
This piece was produced by SciDev.Net’s Asia & Pacific desk.