Subsurface flow in a soil-mantled subtropical dolomite karst slope: A field rainfall simulation study

Soil and epikarst co-evolve resulting in complex structures, but their coupled structural effects on hydrological processes are poorly understood in karst regions. This study examined the plot-scale subsurface flow characteristics from an integrated soil–epikarst system perspective in a humid subtropical cockpit karst region of Southwest China. A trench was excavated to the epikarst lower boundary for collecting individual subsurface flows in five sections with different soil thicknesses. Four field rainfall simulation experiments were carried out under different initial moisture conditions (dry and wet) and rainfall intensities (114 mm h− 1 (high) and 46 mm h− 1 (low) on average). The soil–epikarst system was characterized by shallow soil overlaying a highly irregular epikarst surface with a near-steady infiltration rate of about 35 mm h− 1. The subsurface flows occurred mainly along the soil–epikarst interface and were dominated by preferential flow. The subsurface flow hydrographs showed strong spatial variability and had high steady-state coefficients (0.52 and 0.36 for high and low rainfall intensity events). Irregular epikarst surface combining with high vertical drainage capacity resulted in high threshold rainfall depths for subsurface flows: 67 mm and 263 mm for initial wet and dry conditions, respectively. The above results evidenced that the irregular and permeable soil–epikarst interface was a crucial component of soil–epikarst architecture and consequently should be taken into account in the hydrological modeling for karst regions.