2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 7
Presentation Time: 10:00 AM

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

MCDONNELL, Jeffrey J. and GRAHAM, Chris, Department of Forest Engineering, Oregon State Univ, Corvallis, OR 97331-5706, Jeff.McDonnell@orst.edu

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.