Drivers of Widespread Flooding in Indian River Basins

Widespread riverine flood events wherein most subbasins simultaneously experience flooding have relatively higher socio-economic implications relative to localized flooding within a river basin. Extreme precipitation covering a large area, favourable antecedent soil moisture conditions and unique atmospheric characteristics are often associated with these rare extreme events. Notwithstanding the huge death toll and economic consequences of flooding, there exists only a couple of studies that explore the causative factors of riverine flooding in India. Here, we identify widespread flooding in seven major river basins in India using streamflow simulations from a well-calibrated Variable Infiltration Capacity (VIC) hydrological model. We use an area-weighted threshold to determine the occurrence of widespread flooding. We estimated the probability of widespread flooding In Indian river basins during the observational period from 1959 to 2020. We find a high probability of widespread flooding  (>10% of high flow events in all subbasins) in the peninsular river basins, while the transboundary rivers of Ganga and Brahmaputra exhibit a low probability. Further, using the VIC simulated top layer soil moisture, gridded precipitation observations from India Meteorological Department (IMD) and ERA5 atmospheric variables; we investigate the antecedent soil moisture conditions and atmospheric characteristics associated with widespread flooding. The study results further our understanding of the causes of widespread flooding in Indian river basins and hence have major implications in managing these extreme events.