HESS Hydrology and Earth System Sciences HESS Hydrol. Earth Syst. Sci. 1607-7938 Copernicus Publications Göttingen, Germany 10.5194/hess-18-3681-2014 Hillslope-scale experiment demonstrates the role of convergence during two-step saturation Gevaert A. I. 1 2 10 Teuling A. J. https://orcid.org/0000-0003-4302-2835 1 Uijlenhoet R. https://orcid.org/0000-0001-7418-4445 1 DeLong S. B. 2 11 Huxman T. E. 2 3 Pangle L. A. 2 Breshears D. D. 2 4 Chorover J. 2 5 Pelletier J. D. 2 6 Saleska S. R. 2 7 Zeng X. https://orcid.org/0000-0001-7352-2764 2 8 Troch P. A. 2 9 Hydrology and Quantitative Water Management Group, Wageningen University, Wageningen, the Netherlands Biosphere 2 – College of Science, University of Arizona, Tucson, Arizona, USA Department of Ecology and Evolutionary Biology, University of California, Irvine, California, USA School of Natural Resources and the Environment, University of Arizona, Tucson, Arizona, USA Department of Soil, Water and Environmental Science, University of Arizona, Tucson, Arizona, USA Department of Geosciences, University of Arizona, Tucson, Arizona, USA Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, Arizona, USA Department of Atmospheric Sciences, University of Arizona, Tucson, Arizona, USA Department 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 24 09 2014 18 9 3681 3692 Copyright: © 2014 A. I. Gevaert et al. 2014 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://hess.copernicus.org/articles/18/3681/2014/hess-18-3681-2014.html The full text article is available as a PDF file from https://hess.copernicus.org/articles/18/3681/2014/hess-18-3681-2014.pdf

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.

 References Abdul, A. S. and Gillham, R. W.: Laboratory studies of the effects of the capillary fringe on streamflow generation, Water Resour. Res., 20, 691–698, 1984. Abdul, A. S. and Gillham, R. W.: Field studies of the effects of the capillary fringe on streamflow generation, J. Hydrol., 112, 1–18, 1989. Anderson, M. G. and Burt, T. P.: The role of topography in controlling throughflow generation, Earth Surf. Process., 3, 331–344, 1978. Bernier, P. Y.: Variable source areas and storm-flow generation: An update of the concept and a simulation effort, J. Hydrol., 79, 195–213, 1985. Beven, K.: Hillslope hydrographs by the finite element method, Earth Surf. Process., 2, 13–28, 1977. Beven, K. and Germann, P.: Macropores and water flow in soils revisited, Water Resour. Res., 49, 3071–3092, 2013. Beven, K. and Kirkby, M. J.: A physically based, variable contributing area model of basin hydrology, Hydrolog. Sci. Bull., 24, 43–69, 1979. Brauer, C. C., Teuling, A. J., Overeem, A., van der Velde, Y., Hazenberg, P., Warmerdam, P. M. M., and Uijlenhoet, R.: Anatomy of extraordinary rainfall and flash flood in a Dutch lowland catchment, Hydrol. Earth Syst. Sci., 15, 1991–2005, <a href="http://dx.doi.org/10.5194/hess-15-1991-2011">https://doi.org/10.5194/hess-15-1991-2011</a>, 2011. Burnash, R. J., Ferral, R. L., and McGuire, R. A.: A generalized streamflow simulation system: conceptual modeling for digital computers, US Department of Commerce National Weather Service and State of California Department of Water Resources, Sacramento, California, USA, 1973. Buttle, J. M. and Sami, K.: Testing the groundwater ridging hypothesis of streamflow generation during snowmelt in a forested catchment, J. Hydrol., 135, 53–72, 1992. Cloke, H. L., Anderson, M. G., McDonnell, J. J., and Renaud, J.-P.: Using numerical modeling to evaluate the capillary fringe groundwater ridging hypothesis of streamflow generation, J. Hydrol., 316, 141–162, 2006. Culligan, P. J., Barry, D. A., Parlange, J. Y., Steenhuis, T. S., and Haverkamp, R.: Infiltration with controlled air escape, Water Resour. Res., 36, 781–785, 2000. Dixon, R. M. and Linden, D. R.: Soil air pressure and water infiltration under border irrigation, Soil Sci. Soc. Am. J., 36, 948–953, 1972. Duan, Q., Schaake, J., and Koren, V.: A priori estimation of land surface model parameters, in: Land Surface Hydrology, Meteorology, and Climate: Observation and Modeling, Water Sci. and Appl. 3, edited by: Lakshmi, V., Albertson, J., and Schaake, J., AGU, Washington, D.C., 77–94, 2001. Dunne, T.: Field studies of hillslope flow processes, in: Hillslope Hydrology, edited by: Kirkby, M. J. and Chorley, R. J., Wiley, Chichester, 227–293, 1978. Dunne, T. and Black, R. D.: Partial area contributions to storm runoff in a small New England watershed, Water Resour. Res., 6, 1296–1311, 1970. Dunne, T., Moore, T. R., and Taylor, C. H.: Recognition and prediction of runoff-producing zones in humid regions, Hydrolog. Sci. Bull., 20, 305–327, 1975. Fiori, A., Romanelli, M., Cavalli, D. J., and Russo, D.: Numerical experiments of streamflow generation in steep catchments, J. Hydrol., 339, 183–192, 2007. Freeze, R. A.: Role of subsurface flow in generating surface runoff: 2. Upstream source areas, Water Resour. Res., 8, 1272–1283, 1972. Freeze, R. A.: Streamflow generation, Rev. Geophys., 12, 627–647, 1974. Gillham, R. W.: The capillary fringe and its effect on water table response, J. Hydrol., 67, 307–324, 1984. Hofer, M., Lehmann, P., Stähli, M., Seifert, S., and Krafczyk, M.: Two approaches to modeling the initiation and development of rills in a man-made catchment, Water Resour. Res., 48, W01531, <a href="http://dx.doi.org/10.1029/2011WR010719">https://doi.org/10.1029/2011WR010719</a>, 2012. Hopp, L., Harman, C., Desilets, S. L. E., Graham, C. B., McDonnell, J. J., and Troch, P. A.: Hillslope hydrology under glass: confronting fundamental questions of soil-water-biota co-evolution at Biosphere 2, Hydrol. Earth Syst. Sci., 13, 2105–2118, <a href="http://dx.doi.org/10.5194/hess-13-2105-2009">https://doi.org/10.5194/hess-13-2105-2009</a>, 2009. Huxman, T., Troch, P. A., Chorover, J., Breshears, D. D., Saleska, S., Pelletier, J., and Zeng, X.: The Hills are Alive: Earth Science in a Controlled Environment, EOS Trans. AGU, 90, 120–120, <a href="http://dx.doi.org/10.1029/2009EO140003">https://doi.org/10.1029/2009EO140003</a>, 2009. Nguyen, H. T., Wiatr, T., Fernández-Steeger, T. M., Reicherter, K., Rodrigues, D. M. M., and Azzam, R.: Landslide hazard and cascading effects following the extreme rainfall event on Madeira Island (February 2010), Nat. Hazards, 65, 635–652, 2013. Niu, G.-Y., Pasetto, D., Scudeler, C., Paniconi, C., Putti, M., Troch, P. A., DeLong, S. B., Dontsova, K., Pangle, L., Breshears, D. D., Chorover, J., Huxman, T. E., Pelletier, J., Saleska, S. R., and Zeng, X.: Incipient subsurface heterogeneity and its effect on overland flow generation – insight from a modeling study of the first experiment at the Biosphere 2 Landscape Evolution Observatory, Hydrol. Earth Syst. Sci., 18, 1873–1883, <a href="http://dx.doi.org/10.5194/hess-18-1873-2014">https://doi.org/10.5194/hess-18-1873-2014</a>, 2014. Novakowski, K.-S. and Gillham, R. W.: Field investigations of the nature of water-table response to precipitation in shallow water-table environments, J. Hydrol., 97, 23–32, 1988. O'Loughlin, E. M.: Saturation regions in catchments and their relations to soil and topographic properties, J. Hydrol., 53, 229–246, 1981. Phi, S., Clarke, W., and Li, L.: Laboratory and numerical investigations of hillslope soil saturation development and runoff generation over rainfall events, J. Hydrol., 493, 1–15, 2013. Robinson, J. S. and Sivapalan, M.: Instantaneous response functions of overland flow and subsurface stormflow for catchment models, Hydrol. Process., 10, 845–862, 1996. Sklash, M. G. and Farvolden, R. N.: The role of groundwater in storm runoff, J. Hydrol, 43, 45–65, 1979. Starr, J. L., DeRoo, H. C., Frink, C. R., and Parlange, J.-Y.: Leaching characteristics of a layered field soil, Soil Sci. Soc. Am. J., 42, 386–391, 1978. Troch, P. A., Paniconi, C., and van Loon, E. E.: The hillslope-storage Boussinesq model for subsurface flow and variable source areas along complex hillslopes: 1. Formulation and characteristic response, Water Resour. Res., 39, 1316, <a href="http://dx.doi.org/10.1029/2002WR001728">https://doi.org/10.1029/2002WR001728</a>, 2003. Turner, T. R., Duke, S. D., Fransen, B. R., Reiter, M. L., Kroll, A. J., Ward, J. W., Bach, J. L., Justice, T. E., and Bilby, R. E.: Landslide densities associated with rainfall, stand age, and topography on forested landscapes, southwestern Washington, USA, J. Hydrol., 259, 2233–2247, 2010. van Esse, W. R., Perrin, C., Booij, M. J., Augustijn, D. C. M., Fenicia, F., Kavetski, D., and Lobligeois, F.: The influence of conceptual model structure on model performance: a comparative study for 237 French catchments, Hydrol. Earth Syst. Sci., 17, 4227–4239, <a href="http://dx.doi.org/10.5194/hess-17-4227-2013">https://doi.org/10.5194/hess-17-4227-2013</a>, 2013. Ward, R. C.: On the response to precipitation of headwater streams in humid areas, J. Hydrol., 74, 171–189, 1984. Waswa, G. W., Clulow, A. D., Freese, C., Le Roux, P. A. L., and Lorentz, S. A.: Transient pressure waves in the vadose zone and the rapid water table response, Vadose Zone J., 12, <a href="http://dx.doi.org/10.2136/vzj2012.0054">https://doi.org/10.2136/vzj2012.0054</a>, 2013. Wilson, C. J., and Dietrich, W. E.: The contribution of bedrock groundwater flow to storm runoff and high pore pressure development in hollows, Erosion and Sedimentation in the Pacific Rim, Int. Assoc. Hydrol. Sciences Pub. No. 165, IAHS Press, Wallingford, UK, 49–59, 1987. Zhao, R. J.: The Xinanjiang model applied to China, J. Hydrol., 135, 371–381, 1984.