Runoff reduced by thawing permafrost

Nature : 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. At the same time, 10% of the the permafrost on the Tibetan plateau has degraded over the past 10 years, and alpine wetland has decreased by 37%. Wang Genxu of the Institute of Mountain Hazards and Environment in Chengdu and colleagues investigated whether these changes affected the amount of water runoff from permafrost into Yangtze tributaries. They found runoff increased if the thawing layer was less than 60 cm deep, but decreased if the thaw went deeper. They suspect that when more permafrost thaws, the thicker-than-usual thawed layer soaks up water that would otherwise have run off into the river. The work highlights the role of permafrost in the water cycle and its importance as a headwater source.