Hillslope runoff generation influenced by layered subsurface in a headwater catchment in Ore Mountains, Germany

The Ore Mountains were one of the important flood source areas for several heavy floods over the last years. Reducing damages caused by floods demands sufficient information on the runoff generation processes in the catchments. The aim of this study is to provide insights into prevailing flow pathways, retention times and threshold behavior of a representative hillslope catchment with layered subsurface in the Ore Mountains. The study site is a forested headwater with gneiss as bedrock. We used hydrometrical methods, soil temperature data and sprinkler experiments. Results indicate that the hydraulic anisotropic structure of the deepest layer in 0.9–1.7 m depth is the major controlling factor for subsurface water flow paths. On one hand, this layer acts as an aquitard for seeping water because of its high bulk density. On the other hand, water within the layer is able to flow laterally because of the sandy texture and coarse clasts oriented parallel to the slope. Moreover, three pre-moisture controlled types of runoff processes were addressed. With low antecedent soil moisture, saturation overland flow dominates in the spring bog. With intermediate or high pre-moisture, interflow is generated. The measured runoff coefficients increase in a nonlinear manner with rising pre-moisture. A soil water tension threshold value near field capacity is the tipping point for nonlinear runoff response. These findings emphasize the impact of the layered structure of the subsurface and of antecedent soil moisture for runoff generation in low mountain ranges and may be useful for establishing flood warning systems.