Global estimation of bankfull river discharge reveals distinct flood recurrences across climate zones

This study examines bankfull discharge, the maximum flow a river can convey before spilling over its banks, which is central to modelling flood risk and understanding river-channel evolution. Global flood inundation models commonly assume a 2-year return period for bankfull conditions, but this assumption has remained untested globally. The authors used observations and machine learning to estimate bankfull discharge at approximately 1-km resolution along a recently developed global river network. The analysis combined 2,657 bankfull discharge observations, a suite of global datasets and a Random Forest model to estimate bankfull discharge across approximately 2.87 million km of rivers worldwide.

The study shows that the widely used 2-year discharge reasonably approximates bankfull-flow magnitude, but bankfull return periods vary substantially within and across climate zones. Bankfull conditions occur more frequently in tropical and temperate regions, with median return periods of 1.5 and 1.8 years, and less frequently in cold and arid regions, with median return periods of 2.8 and 4.3 years. The authors find that the assumption that bankfull discharge occurs consistently every two years is unlikely to be appropriate everywhere, especially in tropical and arid regions. They recommend representing the spatial variation in bankfull discharge return periods in global flood models and suggest that climate-specific recurrence intervals or reach-scale estimates could improve modelling of global fluvial flood hazard and risk