Numerical simulation of conduit-type karst groundwater system based on TOPMODEL and MODFLOW-CFP

External water from non-karst areas is a common recharge source for karst groundwater system in Southwest China. The rapid and concentrated recharge of external water can lead to a unique hydrological response in the water-cycle processes of karst groundwater systems. Methods for investigation, monitoring, and numerically simulating for this special external-recharge process in karst groundwater systems remain insufficiently developed. ObjectiveThis study aims to investigate the numerical simulation method for karst groundwater systems recharged by external water. MethodsTaking the Ganxi Yuquan Cave karst groundwater system in Enshi, Hubei as the study area, detailed investigation of the hydrogeological characteristics and recharge patterns of the karst groundwater system was conducted. Underground tracer tests and high-resolution monitoring of rainfall-surface-subsurface runoff dynamics were carried out. The MODFLOW-CFP numerical model was used to represent the dualmedium characteristics of karst fissures and conduits. Focusing on the concentrated inflow of external water from non-karst areas at the entrance of karst conduits, the surface water model TOPMODEL was used to quantitatively characterize the runoff generation and concentration processes of external water from non-karst catchment. This output was then set as the flow boundary condition at the conduit entrances in the MODFLOW-CFP model, achieving the coupling of surface water and groundwater models, and thereby improving the accuracy of representing external water recharge in the MODFLOW-CFP simulations. ResultsThe results show that when the coupled TOPMODEL and MODFLOW-CFP model is used to simulate the discharge at the outlet of the Ganxi Yuquan Cave karst groundwater system, the relative peak errors compared with the measured values range from 0.7% to 19.7%, the peak-time lag is within 3 hours, the correlation coefficient (R2) is 0.93, and the Nash-Sutcliffe efficiency (NSE) is 0.86. During the validating period, the relative peak errors of the simulated outlet discharge of the Ganxi Yuquan Cave karst groundwater system compared with the measured values ranged from 1.1% to 2.5%, with peak-time lag within 2 hours, R2 of 0.91, and NSE of 0.77, indicating good model performance. ConclusionThese findings indicate that the coupled TOPMODEL and MODFLOW-CFP model developed in this study has practical value for simulating the rainfall-runoff and hydrological response processes of karst groundwater systems with external water recharge.