Evaluating the Hydrological Function of Vegetation Restoration in Fragile Karst Area: Insights from the Continuous Surface and Subsurface Runoff Monitoring

Satellite remote sensing of vegetation has indicated that vegetation greenness has increased globally. However, the feedback mechanisms of vegetation restoration on eco-hydrological functions remain unclear, especially in fragile ecosystems. In this study, twelve three-dimensional hillslope plots with different vegetation restoration strategies were selected and their surface and subsurface runoff (SR and SSR, respectively) were monitored in the karst region of southwest China. These strategies were classified as either natural restoration (NR) or artificial management methods (PF, planted forest; GL, grassland), using cropland (CL) as a reference. The results showed that vegetation restoration effectively reduced both SR and SSR. Runoff from the three restoration strategies (59-66 mm) was significantly lower than that from the CL (119 mm). All the strategies had lower runoff than that of CL, except for PF, which exhibited higher SR because of its sparse canopy. Vegetation restoration reduced SR by increasing vegetation cover, which can intercept rainwater and weak the contribution (2.9-12.9%) of rainfall intensity to runoff, thereby reducing runoff generated by "infiltration-excess" mechanisms. This decrease in SSR may be due to an increased water storage capacity in the soil-root zone, causing the soil-bedrock interface to require more rainwater to "fill" and then "spill" to form lateral SSR during the rainfall events. These findings demonstrate that both canopy interception and root-soil interactions contribute to the improvement in hydro-logical function after vegetation restoration. Among the environmental factors contributing to runoff variation, soil depth was identified as the most crucial (51% of the variance) and showed a positive and negative corre-lations with SR and SSR, respectively. Consequently, restoration strategies with higher rainwater retention ca-pacity can be selected to reduce SR losses, especially in areas with deeper soil layers. Accordingly, this study reveals that vegetation restoration reduces runoff and highlights the influence of soil depth in karst areas of southwest China.