Thawing permafrost reduces river runoff
The decrease in runoff into the Yangtze is accompanied by widespread changes in permafrost, says Wang. Across the Tibetan plateau, 10% of the permafrost has degraded in the past decade. The area of alpine wetland and high-vegetation-cover alpine meadow has decreased by 37% and 16%, respectively.
This prompted Wang and his colleagues to assess whether these changes had affected the amount of water running off the land. At a research station near Fenghuoshan Mountain in the northeast of the Tibetan plateau, they studied how the runoff from permafrost into the Zuomaokong River, a Yangtze tributary, is affected by air and soil temperatures, the depth to which permafrost has thawed and the levels of vegetation cover.
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.