Subsurface flow and storage dynamics at hillslope scale are difficult toascertain, often in part due to a lack of sufficient high-resolutionmeasurements and an incomplete understanding of boundary conditions, soilproperties, and other environmental aspects. A continuous and extremerainfall experiment on an artificial hillslope at Biosphere 2's LandscapeEvolution Observatory (LEO) resulted in saturation excess overland flow andgully erosion in the convergent hillslope area. An array of 496 soil moisturesensors revealed a two-step saturation process. First, the downward movementof the wetting front brought soils to a relatively constant but stillunsaturated moisture content. Second, soils were brought to saturatedconditions from below in response to rising water tables. Convergent areasresponded faster than upslope areas, due to contributions from lateralsubsurface flow driven by the topography of the bottom boundary, which iscomparable to impermeable bedrock in natural environments. This led to theformation of a groundwater ridge in the convergent area, triggeringsaturation excess runoff generation. This unique experiment demonstrates, atvery high spatial and temporal resolution, the role of convergence onsubsurface storage and flow dynamics. The results bring into question therepresentation of saturation excess overland flow in conceptualrainfall-runoff models and land-surface models, since flow is gravity-drivenin many of these models and upper layers cannot become saturated from below.The results also provide a baseline to study the role of the co-evolution ofecological and hydrological processes in determining landscape water dynamicsduring future experiments in LEO.
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