Spatial synchronization of river floods growing beyond the basin boundaries in Peninsular India.

Floods often affect large areas and cause serious societal consequences. Despite their importance to society, the simultaneous occurrence of floods in multiple catchments is poorly understood in Peninsular India. This study aims to map regional variations in the spatial synchronization of river floods, assess changes in regional flood magnitudes, and identify regions with similar flood behaviour. The results indicate that regional flood trends exhibit a significant decline in flood magnitudes. However, the spatial dependence of floods varies regionally and has been increasing in recent times, often extending beyond the individual river basin boundaries. Five flood similarity clusters where the catchments within a cluster are likely to experience floods in closer time intervals are identified using the combination of F-madogram and Partitioning Around Medoids (PAM) clustering algorithm. Clustering results provide a basis for defining regional hydrological modeling boundaries. The synchrony of inter-basin flooding is often overlooked as the risks are measured at individual basins. The results of the study highlight strong connections of floods across space beyond the individual river basin boundaries in Peninsular India. We conclude that the regional and temporal variations in spatial flood dependencies must be considered for accurate regional flood hazard risk assessment.