Observed and projected increase of extreme precipitation events on dry soils

Dry soils are associated with low infiltration capacity and increased runoff due to surface crust formation. Therefore, the occurrence of heavy rainfall on dry soils poses a higher risk of flooding. In recent years, abrupt changes from extremely dry to extremely wet conditions have attracted the attention of researchers, and terms such as precipitation whiplash or precipitation volatility have gained currency to refer to these phenomena. Most studies have focused on investigating these episodes on seasonal or annual scales, i.e. changes from very dry to very wet seasons or years. Here, we focus on analysing these events on a daily scale, i.e. the change from very dry to very wet conditions from one day to the next. For this purpose, dry conditions are detected using a threshold in soil moisture and not the rainfall deficit, which would be meaningless on a daily scale. We argue that this approach is more closely related to flash flood risk. Our results based on reanalysis data show that the global frequency of extreme precipitation events on dry soils has increased dramatically in recent decades, at a rate higher than predicted by historical climate model simulations. Furthermore, we show that this trend will continue to increase based on future projections. Specifically, we estimate that the global probability of such an event will more than double by the end of the present century compared to the pre-industrial era under a high-emissions scenario. Finally, we shed light on whether this trend is dominated by an increase in the probability of occurrence of extreme precipitation and dry soils independently, or rather is related to an increase in the probability of concurrence of both, which could be indicative of a negative soil moisture–precipitation feedback.