Dynamics of underground soil loss and its response to rainfall based on karst spring monitoring

Soil erosion represents a critical environmental constraint ecological succession in karst regions, where identifying and accurately quantifying underground soil loss process has long persisted as a major international challenge. To inform regional soil and water conservation strategies, this study innovatively adopts spring outlets as the monitoring scale, building upon prior research at fissure and cave scales to address the fundamental scientific question: “What is the total magnitude of underground soil loss on typical karst slopes?” We deployed high-frequency online monitoring for sediment concentration and water level at two contrasting springs in Guizhou Province, China, and integrated hydrochemical and isotopic analyses to systematically investigate the dynamic processes and driving mechanisms of underground soil loss. The results showed that: (1) Concentrations of sediment, nutrients, and ions in spring water generally increased following rainfall events, while water isotopic compositions remained relatively stable with significantly smaller variations than those in precipitation, indicating that spring discharge is primarily regulated by subsurface mixing and storage effects. (2) Rainfall exhibited only a weak correlation with peak sediment concentration, whereas water level showed a stronger relationship with sediment peaks, suggesting that subsurface convergence conditions are more effective indicators of sediment export than rainfall parameters alone. (3) This catchment-scale underground soil loss rate estimated from spring observation is 0.75 t·km −2 ·yr −1 , contributing merely 0.69% to regional soil erosion. These findings are expected to further advance and expand research on karst underground soil loss, providing references for the development and implementation of soil and water conservation measures in karst regions.