Mind the gap: Variably-saturated lateral flow at the soil-bedrock interface

Numerous hillslope scale hydrological investigations have suggested that the soil-bedrock interface is the key zone for rapid flow and transport in response to storm rainfall. These studies have reported that the development of transient saturation at the soil-bedrock interface and the connectivity of saturated patches are the causal links to threshold hillslope-scale flow generation. Quantifying the details of these processes has been difficult because of the limited number of trenched hillslopes and general inaccessibility of the soil-bedrock interface. Here we present results from a whole-hillslope excavation at the well-studied Maimai watershed in New Zealand to expose and characterize the soil-bedrock interface and its control on lateral flow and transport. In association with our excavations we performed two irrigation experiments on a 6 x 8 m sub-section of hillslope. Results of this work showed that the gap between soil and bedrock was a key feature for controlling the filling small depressions in the bedrock surface and then subsequent spilling of water and tracer downslope. We used the mapped soil-bedrock surface information and fill and spill behavior gained from the line irrigation-based tracer velocity measurements to build a low dimensional model of hillslope soil-bedrock interface flow. We first evaluated the model with extensive field data collected at the site from earlier field campaigns. We then used the tested model as a virtual experimental tool to assess how different ratios of soil to bedrock hydraulic conductivity influence variably-saturated interface flow and transport.