Non-uniform impacts of extreme precipitation on runoff generation across the Yangtze River Basin during 1970–2024

Runoff generation capacity is a critical component of the catchment water cycle and strongly influences water resource stability. However, previous attribution studies in the Yangtze River Basin (China's largest river system in East Asia) have focused on total runoff rather than runoff generation capacity (runoff efficiency) itself, and have not distinguished between mean versus extreme precipitation effects across different reaches. This study examines changes in runoff efficiency (runoff coefficient, Φ n ) during 1970–2024 across the upper, middle and lower reaches of the Yangtze River basin. Results show that Φ n declined slightly in the upper reaches (Δ Φ n ≈ −0.006), remained stable in the middle reaches (Δ Φ n ≈ 0), and increased significantly in the lower reaches (Δ Φ n ≈ +0.056, p < 0.05). Using a two-layer attribution framework, we separate climate contributions (mean and extreme precipitation, potential evapotranspiration) from human impacts (land-use/land-cover change). The rise in Φ n in the lower reaches is mainly climate-driven, with extreme precipitation accounting for ∼67% of the climate-induced Δ Φ n , whereas the decline in the upper reaches stems from combined climatic drying and ecological restoration, with climate and LUCC each accounting for roughly half of the total Δ Φ n . Notably, extreme precipitation impacts runoff efficiency more in wetter basins. Together, these findings highlight strong spatial contrasts in hydrological responses and underscore the need to account for regional differences when attributing runoff changes and formulating basin-wide management strategies.