Freshwater flow dominated by monsoon rains rather than glacier run-off.
Richard A. Lovett
Although global warming is expected to shrink glaciers in the Himalayas and other high mountains in Central Asia, the declining ice will have less overall impact on the region’s water supplies than previously believed, a study concludes.
It’s an important finding, says Richard Armstrong, a climatologist at the US National Snow and Ice Data Center in Boulder, Colorado, who notes that the Intergovernmental Panel on Climate Change had previously predicted dire restrictions on water supplies in Asia. “There clearly were some misunderstandings,” he says.
The researchers behind the latest study began by calculating the importance of meltwater in the overall hydrology of five rivers: the Indus, the Ganges and the Brahmaputra in India, Pakistan and Bangladesh, and the Yellow River and the Yangtze in China1. The authors found that meltwater is most important to the Indus, with a contribution roughly 1.5 times that from lowland rains. In the Brahmaputra, meltwater flow is equivalent to only one-quarter of the volume supplied by lowland rainfall, and, in the other rivers, it forms no more than one-tenth of the input.
Furthermore, the study found that in the Indus and Ganges basins, glacial ice contributes only about 40% of the total meltwater, with the rest coming from seasonal snows. In the other three rivers its contribution is even lower.
High and dry?
That’s important, says Walter Immerzeel, a hydrologist at FutureWater in Wageningen, The Netherlands, and lead author of the study1, because Asian rivers are fed by three sources: rain, snow melt and melting glaciers.
The first two are driven by current weather patterns, because rains fall either as water or as snow that will later melt. The last is a carry-over from the build-up of glaciers in prior centuries. As the glaciers shrink, their contribution will also decline until the glaciers have either melted entirely, or stabilized at smaller sizes.