Scaling up preferential flow in unsaturated undulating terrain due to anisotropic soil hydraulic conductivity and other potential mechanisms

Authors: Green, Timothy; FREYBERG, D.L. - STANFORD UNIVERSITY; CONSTANZ, J.E. - USGS; Ahuja, Lajpat

Submitted to: International Conference on Preferential Flow and Transport Processes in Soil
Publication Type: Abstract Only
Publication Acceptance Date: 8/7/2006
Publication Date: 11/7/2006
Citation: Green, T.R., Freyberg, D., Constanz, J., Ahuja, L.R. 2006. Scaling up preferential flow in unsaturated undulating terrain due to anisotropic soil hydraulic conductivity and other potential mechanisms. International Conference on Preferential Flow and Transport Processes in Soil. Zurich, Switzerland. November 7,2006.

Interpretive Summary: Preferential flow occurs over a range of spatial scales, where flow along certain paths may be much greater than the mean flow rate and orders of magnitude greater than along the lowest flow paths. A combination of topographic slope and soil layering results in subsurface preferential flow particularly in a lateral direction. Here, soil hydraulic properties were upscaled over many layers to derive state-dependent anisotropy in the effective unsaturated hydraulic conductivity functions and water retention characteristics. A finite element model was modified and used to simulate two-dimensional flow in vertical planes beneath hypothetical, periodic undulating landforms with less than 5 m of relief and gravity drainage at a mean depth of 10 m. Landform curvature caused flow-path convergence and soil-water accumulation beneath swales. These accumulation zones acted as preferential drainage areas. Different combinations of landscape topography, anisotropy, and uniform infiltration rates determined the spatial distributions of upscaled flux and soil-water content. Simulated flow patterns have important ramifications for leaching of chemicals, transport to groundwater, and pedological processes related to observed spatial variability in soil development. The present work did not consider potential preferential flow through macropores. However, soil-water accumulation due to the simulated processes could trigger macropore-type flow in wet zones that would not be predicted without the state-dependent anisotropy invoked here. Therefore, scaling of soil hydraulic properties and processes over undulating terrain may involve further nonlinear interactions in space and time. Such topics of future research will be discussed in light of the innovative and multifaceted contributions of Professor Flühler and colleagues.

Technical Abstract: Preferential flow occurs over a range of spatial scales, where flow along certain paths may be much greater than the mean flow rate and orders of magnitude greater than along the lowest flow paths. At hillslope scales, fine-scaling soil layers may be oriented with the ground surface slope due to deposition and weathering, such that water moves downslope along such layers under gravity (unit vertical gradient). Thus, a combination of topographic slope and soil layering results in subsurface preferential flow particularly in a lateral direction. Here, soil hydraulic properties were upscaled over many layers to derive state-dependent anisotropy in the effective unsaturated hydraulic conductivity functions [Green and Freyberg, 1995; Yeh et al., 1985] and water retention characteristics [Green et al., 1996]. A finite element model [Yeh, 1987] was modified and used to simulate two-dimensional flow in vertical planes beneath hypothetical, periodic undulating landforms with less than 5 m of relief and gravity drainage at a mean depth of 10 m. Landform curvature caused flow-path convergence and soil-water accumulation beneath swales. These accumulation zones acted as preferential drainage areas. Different combinations of landscape topography, anisotropy, and uniform infiltration rates determined the spatial distributions of upscaled flux and soil-water content. For steady state simulations, Darcy fluxes varied in space by four orders of magnitude from a minimum beneath the summit position to a maximum beneath a swale. Vertical flux at a depth of 2 m was correlated strongly with topographic curvature (R2 = 0.96) for relatively complex landforms. For transient simulations, temporal variations were greatest in zones of topographically focused drainage. However, the results did not support the hypothesis that undulating terrain would increase the net recharge. Soil hydraulic properties, rather than landscape topography, had a dominant affect on the areal average recharge. Simulated flow patterns have important ramifications for leaching of chemicals, transport to groundwater, and pedological processes related to observed spatial variability in soil development. The present work did not consider potential preferential flow through macropores at core scales nor pipes at hillslope scales, because these features are typically associated with more humid environments. However, soil-water accumulation due to the simulated processes could trigger macropore-type flow in wet zones that would not be predicted without the state-dependent anisotropy invoked here. Therefore, scaling of soil hydraulic properties and processes over undulating terrain may involve further nonlinear interactions in space and time. Such topics of future research will be discussed in light of the innovative and multifaceted contributions of Professor Flühler and colleagues.