Mechanisms of surface and subsurface runoff generation in subtropical soil-epikarst systems: Implications of rainfall simulation experiments on karst slope

Southwest China receives abundant rainfall with a mean annual precipitation of 1450 mm (1960–2013) but surface runoff is small, whereas subsurface runoff is relatively large on karst hillslopes. However, not enough studies have been done to investigate the mechanisms of surface and subsurface runoff generation in subtropical karst landscapes. Here we report the dynamics of soil water content (SWC), instantaneous water levels at the soil-epikarst interface (SEI), and runoff characteristics related to the mechanisms of near-surface runoff generation at the slope scale (5 m × 20 m). Four field rainfall simulation experiments were conducted with rainfall intensities ranging from 35 to 136 mm h-1. Subsurface saturation started first at the relatively flat lower slope, and then extended up slope. Subsurface runoff began after subsurface saturated areas connected to each other, representing a “fill-and-spill” mechanism. Surface runoff, which mainly developed after instantaneous water levels reached near the surface, represents an “infiltration-excess and saturation-excess” runoff mechanism, where two thresholds must be attained: rainfall amount and intensity. The rainfall amount threshold is dependent on soil water deficit, water capacity of the epikarst-surface depression at the SEI, and deep percolation from SEI. The rainfall intensity threshold must be larger than the steady infiltration rate of SEI, which is the prerequisite for the saturation of the epikarst-surface depression and soil layer. Steady SEI infiltration rate was estimated (40 mm h-1) according to the surface runoff generation mechanism. This parameter is important as it represents the lower boundary condition in modeling hillslope hydrological processes. Rainfall-runoff thresholds for surface and subsurface runoff decrease with increasing rainfall intensity. Overall, our results show that epikarst permeability along karst hillslopes is relatively high, being the main factor controlling surface and subsurface runoff generation. Therefore, epikarst permeability significantly affects near-surface hydrological processes in karst landscapes. Our data contribute to a more comprehensive understanding of runoff generation processes and water cycle in the critical zone.