On the importance of soil moisture dynamics for flood generation in Africa

In the context of an ever-increasing vulnerability to floods in Africa, it is important to encourage innovative research on the understanding of these extreme events, notably to improve warning systems and increase the resilience of socio-economic systems to global changes. A recently developed database with discharge data covering most regions of the African continent makes possible a continental-scale assessment of the main flood drivers. Two complementary analyses were performed; the first based on directional statistics to compare the timing of floods with annual maximum rainfall and soil moisture and the second one relying on a process-based classification of flood drivers. The results indicate that the annual maximum floods are more strongly related to the timing of the annual peak of soil moisture than of annual maximum precipitation. Furthermore, temporal changes in flood magnitudes are better explained by the variability of annual maximum soil moisture than by the variability in the annual maximum precipitation. About the dominant flood drivers, excess rainfall over saturated soils and long rainfall events are the main flood generating mechanisms, accounting for more than 75% of floods across the African continent. The relative contribution of the different flood’s drivers across a range of different catchments being strongly related to aridity conditions. The main implication of these results is that precipitation extremes should not be analyzed alone to explain the occurrence and temporal evolution of floods.