Spatial differentiation characteristics and controlling factors of the epikarst thickness in Southwest China

The epikarst, a critical link between surface and subsurface in karst regions, plays a vital role in regulating groundwater cycle, sustaining biogeochemical cycles, and securing water resources. This study investigates the spatial heterogeneity and controlling factors of epikarst thickness across Southwest China’s karst area by integrating data from 3,774 hydrogeological boreholes and multi-source datasets (climate, geology, geomorphology, and soil). Machine learning models reveal that epikarst thickness ranges from 0.3 to 23.2 m (mean: 6.3 ± 3.4 m), with 80 % of the region below 10 m, showing a general southeast-to-northwest thinning trend (excluding southwestern Yunnan). Key drivers include climate (effective runoff during the rainy season (from April to September) and annual temperature), soil properties (thickness and bulk density), and topography (surface cutting depth), with rainy-season effective runoff being the dominant factor. Basin-scale analysis highlights the Pearl River Basin as having the thickest epikarst (8.6 ± 4.0 m), while the Yangtze River Basin has the thinnest (5.1 ± 2.4 m). Geomorphologically, thicker epikarst occurs in karst peak-forest plains and karst peak-cluster, whereas karst middle and high mountains and karst graben basins exhibit thinner epikarst. Lithologically, impure carbonate rocks host the thickest epikarst (7.1 ± 3.5 m), contrasting with thinner layers in dolomite-dominated areas (5.2 ± 2.0 m). This study provides foundational data for regional karst ecohydrological research and establishes a methodological framework for global-scale quantitative assessment of epikarst thickness.