Searching for the Dominant Subsurface Flowpath at the Hillslope Scale

Abstract:

The processes responsible for the catchment scale hydrological response are still poorly understood. Many questions remain, such as: What are the dominant flow pathways? Where is water stored? Which runoff processes are triggered under different wetness conditions? Our objective is to relate the complexity of the subsurface processes at the plot scale to emergent behavior of the system at the catchment scale. A trenched hillslope in the forested Weierbach catchment (Luxembourg) was instrumented and observed during sprinkling events in order to investigate subsurface processes at the plot-scale. We simulated rainfall and observed conditions necessary to generate trench flow. Quantities and timing of trench flow relative to inputs were used to generate hypotheses on system behavior. Detection of saturation at depths greater than the shallow trench, which extended 1.30 meters below the surface, indicated the presence of preferential flowpaths at the site. Fractures in the schist, which underlies the entire catchment, are aligned parallel to the surface and extend outward from the trench. Roots or connected fragments of schist stones oriented diagonally could direct water to deeper layers more quickly than matric transfer through the soil. We hypothesize that this intermittent system of roots and stones acts as a preferential flow mechanism and quickly drives water through the shallow subsurface. Such behavior could persist in the deep subsurface (the percentage of weathered schist fragments is increasing with depth) and cause the “fill and spill” response, retaining and releasing water according to the orientation of the deeper schist system. The hypothesis is currently being tested within the MIPs (Multiple Interacting Pathways) modelling framework. We show that a simple comparative approach can improve our understanding of the subsurface of this location.