More green and less blue water in the Alps during warmer summers

A recent study at ETH Zurich, co-authored by Branco Weiss Fellow Gabriele Manoli, shows that climate change can reduce surface-water supply by enhancing evapotranspiration in forested mountains, especially during heatwaves. Evapotranspiration is the sum of evaporation (the transfer of water from soil and waterbodies to the atmosphere) and transpiration (the movement of water within plants and the subsequent evaporation from leaves).

To investigate this ‘drought paradox’ the researchers used a 1,212-station database and hyper-resolution ecohydrological simulations to quantify blue (runoff) and green (evapotranspiration) water fluxes throughout the Alps. Their findings show that a 3 °C air temperature increase could enhance annual evapotranspiration by up to 100 mm, which would reduce annual runoff at a rate similar to a 3% precipitation decrease. This suggests that green-water feedbacks pose an additional threat to water resources, especially in dry summers. Up to now, these mechanisms were often poorly represented in large-scale model simulations. The new approach will permit more realistic predictions of water availability in mountain regions.

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Read the paper on Nature Climate Change