Until now, scientists assumed that most plants suffer from water stress during droughts: they close their stomata to retain water, stop growing and, in the worst case, wither. As a result, there is a decrease in evaporation and transpiration of water from vegetation, soil and water surfaces – a process that experts call evapotranspiration. “But despite dry and warm conditions, droughts are not occurring at higher altitudes in, say, forested mountain areas,” says Simone Fatichi, senior assistant at the ETH Zurich Institute of Environmental Engineering.
Analyses of observations and computer model simulations from the heatwave of summer 2003 (and recent hot and dry summers) indicate that, during droughts, mountain forests and grasslands at higher elevations release even more water into the air than in “normal” periods of growth with average temperatures and sufficient precipitation.
This is because warmth and abundant sunshine promote vegetation growth. But at the same time, the vegetation has a higher metabolism, and so it essentially sucks every last drop of water from the ground in order to grow. For that reason, evapotranspiration was much greater than expected at higher altitudes during the droughts studied.
Green water predominates in dry and warm summers
Fatichi and his colleagues have now investigated this phenomenon across large areas in the European Alps for the first time, with the help of a computer model. This enabled them to quantify the share of “green” water, i.e. water that reaches the air through evapotranspiration, in proportion to that of “blue” water, the water that runs off into streams, rivers and lakes.
The researchers populated their model with data recorded at more than 1,200 stations throughout the Alpine region that measure, among other things, meteorological parameters and river runoff.
On the basis of their simulation, Fatichi and his doctoral student Theodoros Mastrotheodoros calculated that in forested mountain areas 1,300–3,000 metres above sea level, evapotranspiration rates were above average in large parts of the Alps during the heatwave of 2003.