Integrating regionalization and modeling techniques for assessing climate change impacts on flood inundation in ungauged catchments

• Hydrologically similar donor catchment was selected using similarity index. • Regionalization approach provided reference peak flow from hydrological model. • HEC-RAS-2D model simulated peak as well as mapped inundation areas. • Regionalization and modeling approaches were integrated for prediction in ungauged basin. Ungauged basins pose a challenge for hydrological analysis and flood inundation mapping due to lack of reference hydrology for calibrating and validating hydrological/hydraulic models. Integrating regionalization and modeling techniques can address the challenge. This research applied the approach with a case of Reu catchment in central Nepal and assessed climate change impacts on flood inundation. A donor catchment (i.e., East Rapti), hydrologically similar to the target catchment (i.e., Reu) was selected based on similarity index, an aggregate of 13 catchment descriptors. By regionalization of hydrological model parameters from donor catchment to the target catchment, the highest peak flood event of the baseline period for flood inundation modeling was selected as 952 m 3 /s, which served as a reference peak flood for calibrating HEC-RAS-2D hydraulic/inundation model. It resulted baseline inundation area of 53.3 km 2 . With projected wetter and warmer future with climate change in future, areas under different depths of flood inundation are projected to increase for all scenarios and three future periods. For example, flood extent area in near future is projected to rise up to 9.0% and 4.5% under SSP245 and SSP585 scenarios, respectively, and areas under the extremely high (i.e., > 3.0 m depth), high (i.e., 2.0–3.0 m depth) and moderate (i.e., 1.0–2.0 m depth) risks are projected to increase by 7.2 km 2 , 6.6 km 2 and 14.6 km 2 , respectively. With increase in inundation area under SSP245 scenario, inundation depth at the catchment outlet is projected to increase by 1.17 m This reveals the need to prioritize climate resilient strategies, including change mitigation and adaptation solutions, for addressing increased threat in future.