Thawing permafrost reduces runoff into the Yangtze River

Jane Qiu in Nature: Chinese researchers have revealed that the amount of water entering the Yangtze River near its source on the Tibetan plateau has fallen by 15% over the past four decades, despite a 15% increase in glacial melt and increased rainfall over the same period.

Wang Genxu, an ecologist at the Chengdu-based Institute of Mountain Hazards and Environment, part of the Chinese Academy of Sciences (CAS), says that the findings came as a surprise. “It is in contrast to results from the Arctic where global warming has generally caused increased river discharge,” he says.

Wang and his collaborators at the Cold and Arid Regions Environment and Engineering Research Institute (CAREERI) in Lanzhou, also part of the CAS, have just completed a five-year project to document changes in glaciers, snow and permafrost and to assess their impact on water resources in western China.

Ding Yongjiang, CAREERI’s deputy director, notes that by contrast, many other river systems in western China have seen more water input as glacial retreat and rainfall have increased with a warming climate. The runoff into the Tarim River headwaters, for instance, has increased by 13% since 1961, mainly as a result of increased glacial melt, which has risen by 26%. “But the Yangtze headwaters are an exception,” says Ding.

...The researchers found that the depth of the 'active' ground layer — the part that freezes and thaws every year — is crucial for water passage. Runoff increased if the thawing layer was less than 60 centimetres deep, but decreased if the thaw went deeper. The reasons are unclear but the researchers suspect that when more of the permafrost thaws, the thickened active layer may act like a sponge, soaking up water that would otherwise have run off into the river. Alternatively, more water may leak deep into the ground, also reducing surface discharge....

The upper reaches of the Yangzi River bearing north, having just emerged from the worlds deepest gorge in Yunnan (the Tiger Leaping gorge, SW China. Shot by Peter Morgan, Wikimedia Commons via Flickr, under the Creative Commons Attribution 2.0 Generic license