Increasingly frequent heat waves are reducing the supply of surface water in the European Alps, according to a study involving UdeM geographer Christoforos Pappas.
Increasingly frequent heat waves in Europe are benefiting the vegetation at high altitudes in the Alps. But this unusual plant growth - which could become more common with climate change - may deplete the water reserves that percolate from mountain streams to the rivers below.
Indeed, a large-scale modeling study involving postdoctoral fellow Christoforos Pappas of UdeM's geography department confirms this "drought paradox," namely that during periods of drought, plants use more water despite low rainfall.
Written by researcher Theodoros Mastrotheodoros, the study was conducted under the supervision of Simone Fatichi of the Swiss Federal Institute of Technology in Zurich. The results are published in the journal Nature Climate Change.
Water for 170 million people
Spanning an area of more than 260,000 square klometres, the Alps supply water to rivers that flow to some 170 million people in Europe.
But the water runoff the mountain range provides could be significantly reduced if the number and frequency of heat waves in Europe increase and if vegetation continues to use more water, the researchers showed.
This is because heat waves benefit vegetation at high altitudes: by evaporating more water than usual as vegetation grows, they reduce the flow of runoff water that usually percolates down to rivers.
And by capturing more water and moisture from the soil and air, the vegetation itself increases evapotranspiration, the amount of water transferred in gaseous form to the atmosphere through evaporation at ground level and through plant transpiration.
The researchers knew that evapotranspiration was more pronounced in specific locations in the Alps, but Fatichi's team was able to actually quantify the phenomenon for the entire Alpine region.
Data from over 1,212 weather stations
To do this, they took data from 1,212 meteorological stations in the Alps covering an area of 257,000 square kilometres and combined them with aquifer flows in downstream rivers recorded between 2001 and 2003.
"The advanced numerical model we used enabled us to generate highly accurate and high-resolution spatial modelling of water flows over the entire Alpine region," Pappas said.
For example, the simulation results indicate that during the heat wave of 2003 in forested mountainous areas of between 1,300 and 3,000 metres above sea level, evapotranspiration rates were above average in large parts of the Alps.
Similarly, the increase in evapotranspiration observed at altitudes above 1,300 metres resulted in a 32-per-cent decrease in water runoff to rivers compared to the average for other summer seasons.
"By analyzing the simulation results, we estimated that the effect on runoff of a 3°C increase in temperature – which is likely to occur by the end of the century – was equivalent to a 3-per-cent decrease in precipitation, meaning that a slight decrease in precipitation can have significant impacts on water resources," said Pappas.
In the summer of 2003 alone, water runoff in the Alps was 50-per-cent lower than average. At the same time, the amount of precipitation was the lowest recorded between 1992 to 2008.
'Temperature rising at an accelerated rate'
"In fact, the temperature in the Alps is rising at an accelerated rate: humidity is decreasing, evapotranspiration is increasing, glaciers are melting and the distribution of snow is shifting to higher altitudes, while extreme climates are becoming more and more frequent," said Pappas.
Over the years, extreme heat events have been more numerous and their frequency has accelerated: after 2003, Europe experienced heat waves in 2010, 2015 and again in 2018, he noted.
"Our study is important because, for the first time, large-scale modelling is demonstrating the potential effect of climate change on all the water resources in the Alps," Pappas concluded. "In the near future, events such as the drought of 2003 may no longer be classified as extreme."