Attribution and projection of runoff variability in a subtropical monsoon basin under changing environments

Study region The Pearl River Basin is a humid subtropical monsoon basin in southern China and an important water source for South China, including the Pearl River Delta and Hong Kong. Its abundant precipitation, high vegetation coverage, and strong seasonal variability make it a representative basin for examining runoff responses to climate and land-surface changes in humid monsoon regions. Study focus This study applies the Budyko–Porporato–Guswa coupling framework to investigate historical runoff variability during 1990–2020 and to project future runoff responses during 2021–2060. Scenario-based attribution, runoff elasticity analysis, and correlation analysis were used to quantify the effects of precipitation, potential evapotranspiration, and LAI-based vegetation change on runoff variability. Bias-corrected GFDL-ESM4 outputs under SSP2–4.5 and SSP5–8.5 were further used to drive future runoff projections. New hydrological insights for the region Historical runoff showed strong interannual variability and spatial heterogeneity, with higher runoff in the Pearl River Delta, Dongjiang Basin, and western Beijiang Basin, and lower runoff in the Nanpanjiang and Beipanjiang basins. Scenario-based attribution indicated that climatic forcing, especially precipitation change, dominated basin-scale runoff variability, whereas LAI-based vegetation effects remained limited under generally humid conditions. However, the dry-hot valley sub-basin suggested that vegetation and land-surface factors may become more important in relatively water-limited areas. Based on the bias-corrected GFDL-ESM4 forcing used in this study, runoff was projected to decline by approximately 8% under SSP2–4.5 and 18.4% under SSP5–8.5 by 2060, with increasing spatial disparities. These findings highlight the need to consider both basin-wide climatic controls and sub-basin heterogeneity in future water-resource management.