Century-scale human land transformation reshapes flood hazards in a tropical catchment

Study region The Masai River catchment (26 km²) in southern Peninsular Malaysia is a rapidly urbanizing tropical catchment that has undergone extensive land transformation over the past century. Study focus This study integrated historical land-use reconstruction, high-frequency hydrological observations, and an observation-constrained Rainfall–Runoff–Inundation model to assess hydrological changes across four reconstructed land-use conditions: 1926, 2002, 2018, and 2024. Identical rainfall forcing was applied to all scenarios to isolate the hydrological effects of urbanization from climatic variability. The model was calibrated against one observed flood event and independently validated against three additional events. New hydrological insights for the region Relative to the 1926 landscape, progressive urbanization increased peak discharge by 3.4–39.6% and total runoff volume by 3.0–54.4%, while maximum inundation depth under the 100-year, 3-h design storm increased from 1.24 to 1.40 m. Urbanization also reorganized flood hazards spatially: inundation expanded from historically vulnerable downstream reaches toward tributary catchments and newly urbanized upstream areas. Remaining grasslands and open spaces within flood-prone areas represent potential locations for nature-based flood mitigation. These findings demonstrate that century-scale land transformation has left a persistent hydrological legacy in the catchment and show that historical land-use trajectories should be incorporated into regional flood-risk assessment and planning.