When interflow also percolates: downslope travel distances and hillslope process zones

Abstract

Copyright © 2014 John Wiley & Sons, Ltd. Introduction In hillslopes with soils characterized by deep regoliths, such as Ultisols, Oxisols, and Alfisols, interflow occurs episodically over impeding layers near and parallel to the soil surface such as low-conductivity B horizons (e.g. Newman et al., 1998; Buttle andMcDonald, 2002; Du et al., In Review), till layers (McGlynn et al., 1999; Bishop et al., 2004), hardpans (McDaniel et al., 2008), C horizons (Detty and McGuire, 2010), and permeable bedrock (Tromp van Meerveld et al., 2007). As perched saturation develops within and above these impeding but permeable horizons, flow moves laterally downslope, but the perched water also continues to percolate through the impeding horizon to the unsaturated soils and saprolite below. Perched water and solutes will eventually traverse the zone of perched saturation above the impeding horizon and then enter and percolate through the impeding horizon. In such flow situations, only lower hillslope segments with sufficient downslope travel distance will deliver water to the riparian zone within the time scale of a storm. Farther up the slope, lateral flow within the zone of perched saturation will act mainly to shift the point of percolation (location where a water packet leaves the downslope flow zone in the upper soil layer and enters the impeding layer) down the hillslope from the point of infiltration. In flatter parts of the hillslope or in areas with little contrast between the conductivities of the upper and impeding soil layers, lateral flow distances will be negligible. From Darcy’s law, we can estimate the downslope travel distance for quasi-steady-state conditions within the saturated layer assuming Boussinesq (slope parallel) flow above the restrictive layer and normal flow (perpendicular to the slope) through the restrictive layer to unsaturated soils below (Figure 1). Using a downslope flow vector, we must employ an orthogonal normal vector and avoid Harr’s (1977) mistake of adding non-orthogonal and non-independent gradient vectors in calculating flow directions. Weassume that each layer is isotropic, and the impeding layer is saturated or nearly so and drains freely to an unsaturated layer below (pressure head equals zero at the base of the impeding layer). We assume diffuse porosity within the impeding layer so percolation is not restricted to randomly spaced discontinuities. With these assumptions generally applicable to hillslopes with deep regoliths and a low conductivity layer paralleling the soil surface, we will calculate downslope travel distances for reasonable combinations of slopes, conductivities, and impeding layer thicknesses; evaluate these distances with respect to typical slope lengths;map the downslope travel distances on watersheds with soil and slope conditions matching these assumptions; and discuss how couplingBoussinesq downslope flow and percolation affect our understanding of the potential zonation of lateral flow processes.