Wet antecedent conditions and high baseflow trigger widespread floods in Indian sub-continental river basins

Widespread floods affecting multiple subbasins in a river basin are more disastrous than localized flooding. Understanding the mechanisms, drivers and probability of widespread flooding is pertinent for devising suitable policy measures. Here, we investigate the occurrence and drivers of widespread flooding in seven Indian sub-continental river basins during the observed climate (1959-2020). We use a novel methodology for determining widespread floods and a non-stationary extreme value distribution to identify the mechanisms of widespread flooding. We find that the peninsular river basins have a high probability of widespread flooding, while the transboundary basins of Ganga and Brahmaputra have a low probability. In addition to wet antecedent conditions, the relative rareness of high flows across different subbasins is crucial in explaining the variability of widespread flood probability across different river basins. Our results show that favourable antecedent baseflow and soil moisture conditions, uniform precipitation distribution, and streamflow seasonality determine the seasonality and probability of widespread floods. Further, widespread floods are associated with large atmospheric circulations, resulting in near-uniform precipitation within a river basin. Moreover, we found no significant relation between widespread floods and oceanic circulations. Our findings highlight the prominent drivers and mechanisms of widespread floods with implications for flood mitigation in India.