A continuous intense rainfall experiment on an artificial hillslope resulted in saturation excess overland flow in the convergent area of the hillslope. The data showed a two-step saturation process in which convergence played an important role.
Hillslope experiment demonstrates the role of convergence during two-step saturation
A continuous and intense rainfall experiment on an artificial hillslope at the Landscape Evolution Observatory in Biosphere 2 resulted in saturation excess overland flow and gully erosion in the convergent hillslope area. An array of 496 soil moisture sensors revealed a two-step saturation process. First, the downward movement of the wetting front brought soils to a relatively constant but still unsaturated moisture content. This wetting front moved at a relatively constant speed throughout the hillslope. Second, soils were brought to saturated conditions from below in response to rising water tables. Convergent areas responded faster than upslope areas due to contributions from lateral subsurface flow. This led to the formation of a groundwater ridge in the convergent area, triggering saturation excess runoff generation. This unique experiment demonstrates, at very high spatial and temporal resolution, the role of convergence on subsurface storage and flow dynamics, providing observational insight to advance our understanding and prediction of flash floods, landslides and debris flows.