1Hydrology and Quantitative Water Management Group, Wageningen University, Wageningen, the Netherlands
2Biosphere 2 – College of Science, University of Arizona, Tucson, Arizona, USA
3Department of Ecology and Evolutionary Biology, University of California, Irvine, California, USA
4School of Natural Resources and the Environment, University of Arizona, Tucson, Arizona, USA
5Department of Soil, Water and Environmental Science, University of Arizona, Tucson, Arizona, USA
6Department of Geosciences, University of Arizona, Tucson, Arizona, USA
7Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, Arizona, USA
8Department of Atmospheric Sciences, University of Arizona, Tucson, Arizona, USA
9Department of Hydrology and Water Resources, University of Arizona, Tucson, Arizona, USA
*now at: Department of Earth Sciences, VU University, Amsterdam, the Netherlands
**now at: US Geological Survey, Menlo Park, California, USA
Received: 30 Dec 2013 – Discussion started: 20 Feb 2014
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.
Revised: 19 Aug 2014 – Accepted: 25 Aug 2014 – Published: 24 Sep 2014
Gevaert, A. I., Teuling, A. J., Uijlenhoet, R., DeLong, S. B., Huxman, T. E., Pangle, L. A., Breshears, D. D., Chorover, J., Pelletier, J. D., Saleska, S. R., Zeng, X., and Troch, P. A.: Hillslope-scale experiment demonstrates the role of convergence during two-step saturation, Hydrol. Earth Syst. Sci., 18, 3681-3692, doi:10.5194/hess-18-3681-2014, 2014.