We use the gradient of the aquifer base to investigate the hydrology of mature, shallow karst systems. We first present a 3D geological model of the Gempen plateau (NW Switzerland) that reveals the geometry of aquifers and aquitards and their displacement at faults, then transfer the 3D geological model to a hydrological model. The transformation is based on a conceptual karst model approximating subsurface flow in mature, shallow karst systems to open surface flow on the top of the uppermost aquitard or aquifer base. The gradient of this surface is expected to mainly influence regional groundwater flow patterns. We use the hydrological model to delineate spring catchment areas. The discharge areas and corresponding catchment areas of the model are compared with the occurrence of springs in the study area and with hydraulic links confirmed by tracer tests. We also describe the way in which the hydrological model contributes to identifying flow processes. The proposed aquifer base gradient approach enhances vulnerability assessment in mature, shallow karst regions by (1) localizing catchment areas as a precondition of source protection strategies and (2) indicating dominant flow processes associated with individual springs.