A karst runoff generation module based on the near-surface critical zone structure and threshold behaviors

Hydrological simulation in karst areas is of great importance and challenge. It is a practical way to enhance the performance of existing hydrological models in karst areas by coupling karst modules that represent hydrological processes in these areas. The near-surface critical zone structure affects runoff generation in karst areas significantly and its complex hydrological processes could be simplified with threshold behaviors. This study proposed a three-thresholds-based karst runoff generation module (3T-KRGM), which used three reservoirs to represent water storage in the soil zone, soil–epikarst interface, and epikarst zone. The 3T-KRGM is coupled with Xinanjiang (XAJ) model to extend the applicability of the model to karst areas. Both the improved XAJ model and the original XAJ model were used in the Shibantang watershed, which is a typical karst watershed located in southwest China. The results indicate that the performance of daily discharge simulations was obviously improved by introducing the 3T-KRGM. In addition, both the parameter sensitivity analysis and baseflow simulation demonstrate that the 3T-KRGM is rational in structure. The 3T-KRGM could also be easily coupled into other hydrological models, thus benefiting the hydrological simulation in karst areas.