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Abstract
Water infiltration and unsaturated flow through heterogeneous soil control the distribution of soil moisture in the vadose zone and the dynamics of groundwater recharge, providing the link between climate, biogeochemical soil processes and vegetation dynamics. Infiltration into dry soil is hydrodynamically unstable, leading to preferential flow through narrow wet regions (fingers). In this paper we use numerical simulation to study the interplay between fingering instabilities and soil heterogeneity during water infiltration. We consider soil with heterogeneous intrinsic permeability. Permeabilities are random, with point Gaussian statistics, and vary smoothly in space due to spatial correlation. The key research question is whether the presence of moderate or strong heterogeneity overwhelms the fingering instability, recovering the simple stable displacement patterns predicted by most simplified model of infiltration currently used in hydrological models from the Darcy to the basin scales. We perform detailed simulations of constant-rate infiltration into soils with isotropic and anisotropic intrinsic permeability fields. Our results demonstrate that soil heterogeneity does not suppress fingering instabilities, but it rather enhances its effect of preferential flow and channeling. Fingering patterns strongly depend on soil structure, in particular the correlation length and anisotropy of the permeability field. While the finger size and flow dynamics are only slightly controlled by correlation length in isotropic fields, layering leads to significant finger meandering and bulging, changing arrival times and wetting efficiencies. Fingering and soil heterogeneity need to be considered when upscaling the constitutive relationships of multiphase flow in porous media (relative permeability and water retention curve) from the finger to field and basin scales. While relative permeabilities remain unchanged upon upscaling for stable displacements, the inefficient wetting due to fingering leads to relative permeabilities at the field scale that are significantly different from those at the Darcy scale. These effective relative permeability functions also depend, although less strongly, on heterogeneity and soil structure.
Highlights
Unsaturated flow through heterogeneous porous media controls the distribution of soil moisture in the vadose zone and the dynamics of groundwater recharge, providing the link between climate, biogeochemical soil processes, and vegetation dynamics [1]
From the evolution of these maps of water saturation, it is already apparent that larger correlation lengths lead to less efficient wetting during infiltration, in the sense of smaller areal coverage of the flow paths and larger distance between fingers
Note that the flux is different for each simulation. This is due to the impact of the correlation length on the actual mean saturated hydraulic conductivity at the top boundary, which leads to slightly different infiltrating fluxes
Summary
Unsaturated flow through heterogeneous porous media controls the distribution of soil moisture in the vadose zone and the dynamics of groundwater recharge, providing the link between climate, biogeochemical soil processes, and vegetation dynamics [1]. Fingering due to wetting front instabilities has been identified as an important transport process in infiltration into snow [12] and has been proposed as a mechanism to explain the origin of columnar structures in arenitic caves [13]. Work on unsaturated flow in coarse-textured soils has demonstrated the importance of fingered flow at the field scale [10,23,31,35,36,37,38,39], including the role of water repellency as an enhancing mechanism [19,40,41,42,43,44]. Preferential flow due to fingering may significantly influence the transport of contaminants to surface and ground waters [45]
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