Abstract
Abstract. Subsurface flow and storage dynamics at hillslope scale are difficult to ascertain, often in part due to a lack of sufficient high-resolution measurements and an incomplete understanding of boundary conditions, soil properties, and other environmental aspects. A continuous and extreme rainfall experiment on an artificial hillslope at Biosphere 2's Landscape Evolution Observatory (LEO) 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. 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 driven by the topography of the bottom boundary, which is comparable to impermeable bedrock in natural environments. 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. The results bring into question the representation of saturation excess overland flow in conceptual rainfall-runoff models and land-surface models, since flow is gravity-driven in 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 of ecological and hydrological processes in determining landscape water dynamics during future experiments in LEO.
Highlights
Understanding hillslope runoff response to extreme rainfall events is an important topic in hydrology, key to correct prediction of extreme streamflow, erosion and/or landslides, and important to integrated studies of landscapes where such processes affect vegetation dynamics, biogeochemical cycling and biosphere–atmosphere exchanges
Soil moisture time series show that the saturation process can be described by a sequential step-wise process rather than by a gradual process at all depths: three relatively stable phases (1–3) were separated by two rather abrupt steps. These steps are visible at individual locations as well as in horizontally averaged data over the whole hillslope, over the convergent area only, and over upslope area only
Saturation is assumed when measured volumetric water contents exceed the porosity determined in the laboratory
Summary
Understanding hillslope runoff response to extreme rainfall events is an important topic in hydrology, key to correct prediction of extreme streamflow, erosion and/or landslides, and important to integrated studies of landscapes where such processes affect vegetation dynamics, biogeochemical cycling and biosphere–atmosphere exchanges. The development of saturated areas in catchments is central to the variable source area concept, which states that the bulk of catchment runoff is generated from a relatively small fraction of the total surface area of the system (Dunne and Black, 1970; Freeze, 1974). This source area is generally concentrated around a stream bed and can expand upslope into dry channels and laterally up hillslopes. The source areas expand and contract with the seasons (Dunne and Black, 1970) as well as during and after an intense rainfall event (Dunne et al, 1975; Bernier, 1985)
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