Seasonal events such as snowmelt affect when, and if, waterways overflow their banks
By Laurel Hamers 2:28pm, August 10, 2017
HIGH WATER In August 2005, severe floods hit alpine communities like this village in Tirol, Austria.
Across Europe, rivers aren’t flooding when they used to.
Long-term changes in temperature and precipitation are making some rivers flood days, weeks or even months earlier than they did 50 years ago, and pushing flooding in other areas much later, researchers report August 11 in Science. Those changes could impact people, wildlife and farms near rivers.
Previous studies have shown that climate change is likely to increase the severity and frequency of coastal floods, but it can be tricky to concretely link river flooding to climate change, says Günter Blöschl, a hydrologist at the Vienna University of Technology who led the study.
Coastal flooding is worsened largely by one overriding variable that can be tracked: sea level rise. But river flooding is affected by a complex set of factors, says Rob Moore, a policy analyst at the Natural Resources Defense Council in Chicago who specializes in water issues. Both the timing and quantity of precipitation matter, as does the type of soil and whether it’s dry or already waterlogged when rain hits. What’s more, changes in land use around a river or engineering projects such as dams that change river flow can also affect flood risk — but aren’t necessarily related to the climate.
Many rivers across Europe now typically flood at different times in the year than they did 50 years ago. The shift is as small as a few days in some areas and as big as several months in others. This map shows those changes, which vary widely across Europe. In red regions, floods are happening earlier; blue regions are flooding later.
G. Blöschl et al/Science 2017
So instead of tracking the size or frequency of river floods, the researchers examined the seasonal timing of those floods. That measurement is less impacted by factors that have nothing to do with climate. Blöschl worked with researchers from 38 countries to analyze hydrological data collected at 4,262 sites across Europe from 1960 to 2010.
Flood season shifted as much as 13 days earlier or nine days later per decade, the researchers found. Over the entire study period, that shift added up to floods in some regions occurring, in the most extreme cases, as much as 65 days earlier or 45 days later. The biggest changes were in Western Europe, where a quarter of the monitoring sites recorded flood timing shifts of more than 36 days over the 50-year period. Elsewhere, effects were more moderate, though still measurable: In northeastern Europe and the area around the North Sea, for instance, more than half of the stations showed shifts of more than 8 days.