Understanding changes in flood magnitude and timing

Small shifts in the climate can have large consequences on flood timing and magnitude. For example, changes of only 1-2 degrees Celsius and 10–20% in annual precipitation can cause order of magnitude changes in flooding. Changes in flood magnitude and timing can affect flood risk, farming productivity, the health of ecological systems, and impact water supply reliability.

Here we examine changes in both flood magnitude and timing across the world with a focus on Mediterranean and semi-arid climates, such as those in south-west Australia, which are dominated by dry summers and wet winters. Relative to other climates across the world, we find semi-arid regions are particularly susceptible to drying antecedent moisture conditions, with the largest decrease in antecedent soil moisture of any climate zone across the world. 

Drying antecedent soil moisture conditions in semi-arid regions are causing a shift to a later onset of flooding in the wet season. Despite increases in extreme rainfall magnitudes, frequent floods, such as those expected to occur on average once per year, are decreasing in magnitude due to the impact of drying antecedent moisture conditions more than offsetting the impact of increasing extreme rainfall. Only rarer events, such as those expected to occur once every 10 years, are increasing in response to increases in extreme rainfalls, where the increase in extreme rainfall outweighs the decrease in soil moisture. These results point to smaller floods, such as responsible for filling our water supplies, decreasing, while the larger flood events that pose a risk to life and infrastructure are increasing, with semi-arid regions particularly susceptible to these changes.