Quantifying Subsurface Lateral Flow along Sloping Horizon Boundaries in Soil Profiles of a Hummocky Ground Moraine

CORE IDEAS: Hydropedologic factors control subsurface lateral flow in eroded hillslope soils. Depth to C horizon, hydraulic properties, and initial conditions are most sensitive. Virtual 2D model experiment predicts effect of tracer injection on lateral movement. Lateral flow at sloping C horizon surface locally separated from tracer plume. Erosion‐induced feedback on subsurface lateral flow by varying depths to C horizon. Subsurface lateral flow in hillslope soils depends on lower permeability or texture‐contrasting soil horizons. In the arable hummocky soil landscape, erosion processes caused glacial till appearance closer to the soil surface at upslope positions. The objective of this work was to quantify the potential for subsurface lateral flow depending on the erosion‐affected spatial hydropedological complexity. The eroded Haplic Luvisol profile was studied due to the presence of a relatively dense C horizon that varied in depth, thickness, and sloping angle. A two‐dimensional numerical modeling and sensitivity analysis for the saturated hydraulic conductivity (Kₛ) of the C horizon and the depth to C horizon (i.e., soil solum thickness) was performed for rainstorms in 2011 and 2012 using HYDRUS‐2D. A Kₛ value of ≤2.5 cm d⁻¹ for the C horizon was required for lateral flow initiation. Lateral flow was (i) increasing with decreasing solum thickness, indicating an erosion‐induced feedback on subsurface lateral flow, and (ii) dependent on the soil moisture regime prior to rainstorms. The effect of lateral flow on the movement of a conservative tracer was simulated in the form of a “virtual experiment”. Simulation scenarios revealed only a relatively small lateral shift of the tracer plume caused by a local decoupling of water (already lateral) from subsequent tracer movement (still vertical). Longer term simulations suggested that, for the present conditions, lateral flow was limited mostly to occasional summer storm events. Even without considering preferential flow contribution to lateral flow, highly complex hydropedologic interactions are present in erosion‐affected heterogeneous soil profiles.