Increased rainfall-runoff drives flood hazard intensification in Central Himalayan river systems

Abstract The Central Himalayan floodplain is one of the most flood-affected regions of the world, but we have limited knowledge of climate-change impacts on fluvial floods. Here, we provide the first large-ensemble, regional climate-change impact assessment of design floods for the Karnali River in Nepal and China, based on high-resolution modelling, considering climatic, hydrological and statistical uncertainties, needed for building local mitigation strategies. Our simulations project increases in the 1% annual exceedance probability flood magnitude of + 40% (P 10/90 : 33/48%) (medium-emissions) and + 79% (P 10/90 : 54/82%) (high-emissions) for 2060–2099, with rainfall-runoff contributing ≥ 90% of the additional flood water. We show that the uncertainty in the projections is linked to the relative differences between the magnitudes of the largest and more common events and argue that it is important to include both atmospheric only general circulation models and earth system Models in ensembles to capture the range of event and model uncertainty.