Historical increases in flood variability due to changing storm volumes and soil moisture

Changes in flooding have substantial economic consequences. Increases in flooding increase economic losses while decreases in flooding increase water scarcity. Greater extreme rainfalls due to climate change are expected to cause greater flood magnitudes. This is particularly true for urban and developed catchments where there is a lack of impervious surfaces and surface water storage. However, in rural and undeveloped catchments historical trends are mixed, with many catchments experiencing decreases in flood magnitude.

Here we argue the observed increase in flood variability is due to (1) changes in the extreme rainfall patterns and antecedent soil moisture conditions that drive flood response, and (2) the influence of event rarity on the interaction of these flood drivers. To investigate this hypothesis, we use 2776 stations from the Global Runoff Data Centre paired with rainfall and soil moisture from the Global Land Data Assimilation System. Flood events are chosen to isolate the flood driving rainfall volume, rainfall peak, and antecedent soil moisture, and trends are analysed in each of these variables alongside the subsequent flood peak. The analysis is limited to stations with 30 years or more of active record with the majority of stations in North America, Europe, Brazil, Oceania, and southern Africa.

We find that, while peak rainfall magnitudes are increasing globally, storm volumes are not increasing as greatly, resulting in a decrease in storm durations. Antecedent soil moisture on the other hand is consistently drier across the world. The result is a mixed flood response that depends on the local climate and the event rarity. In temperate and cold regions of the world floods are generally increasing in magnitude – these increases are less for more frequent events (those expected to occur on average once per year) and greater for more rare events (those expected to occur once every 10 years). The increases in flood magnitude are consistently less than the increases in the peak rainfall and rainfall volume because of drying soils.

In tropical and arid regions, there is a decline in flood magnitude for frequent flood events, with increases in flood magnitude only for the rarest events. This is because, in these regions, the antecedent moisture decreases outweigh the increase in rainfall. These results point to a worst of both world’s scenario where small floods, responsible for filling our water supplies, are decreasing, while the large flood events which pose a risk to life and infrastructure, are increasing.