Evaluating inland flood mitigation capacity under extreme rainfall conditions

Study Region Henan Province, central China, is a representative inland monsoon region in the Yellow River–Huai River transition zone, characterized by densely urbanized plains, agricultural lowlands, and vegetated western mountains. The region experienced severe pluvial flooding during the July 2021 Zhengzhou extreme rainfall event. Study Focus Pluvial flooding risks are increasing under climate change and urbanization, yet assessments of flood mitigation capacity (FMC) under extreme rainfall remain limited in inland regions. This study evaluates land-surface FMC, expressed as the runoff-retention ratio, under 24-h rainfall scenarios from 50 to 550 mm using a spatially explicit SCS-CN framework integrating land use, hydrological soil groups, and slope-adjusted curve numbers. Pixel-scale FMC degradation, spatial convergence under extreme rainfall, and urban core–fringe contrasts were analyzed. New Hydrological Insights for the Region FMC decreases nonlinearly with increasing rainfall. The strongest degradation occurs at 0–150 mm, followed by an operational saturation-transition interval at 150–300 mm. Spatial differences in runoff retention converge markedly beyond ~300 mm, reflecting saturation of finite land-surface storage in the SCS-CN framework. Under 0–300 mm rainfall, urban built-up areas showed roughly 0.20 lower R value than non-urban areas, while urban cores exhibited roughly 0.25 lower R value urban fringes. High-FMC areas were concentrated in mountainous, vegetated, and peri-urban regions, whereas low-FMC areas coincided with dense urban cores and low-lying river-adjacent zones.