Climate-Driven Trends in the Occurrence of Major Floods in North America and Europe
Thursday, 18 December 2014
Every year floods cause much damage around the world. It is important to understand historical changes in major floods to help inform how flood frequency may change in the future. To date, however, there is very limited evidence of past changes in major-flood magnitude and occurrence (exceeding 25, 50, and 100 year return-period thresholds). Many studies have analyzed annual-maximum flood trends but they have often not differentiated between trends influenced by human catchment alterations and those caused by climatic changes. Here we present the first intercontinental assessment of historical climate-driven trends in major-flood occurrence using many diverse but minimally altered catchments. There is no compelling evidence for consistent increases in major-flood occurrence across this very large domain. Flood occurrence at 1206 gauges increased from 1961 to 2010 but not significantly, driven primarily by European increases. There also were non-significant increases in flood occurrence at 322 gauges from 1931 to 2010, but this time driven primarily by North American increases. Flood occurrence both increased and decreased for different subgroups of gauges (a few were significant changes) differentiated by catchment size, type of climate, flood threshold, and period of record. Past changes in major-flood occurrence are complex and future changes will be likewise. International hydrologic networks containing minimally altered catchments will play a key role in understanding these complexities for both historical and future climatic conditions.