Effect of subsoil conductivity and thickness on interflow pathways, rates and source areas for chemicals in a sloping layered soil with seepage face

Abstract

The effects of relative hydraulic conductivities and thicknesses of topsoil and subsoil layers and leakage rate of a compact base material on the saturated steady water flow through a sloping soil to an outflow face (interflow) were investigated theoretically. The water was assumed to be supplied from the surface, at a rate adequate to keep the entire soil saturated. Simplified descriptions of the interflow characteristics were sought wherever possible. A Gram-Schmidt orthonormalization procedure of W.L. Powers, D. Kirkham and G. Snowden provided a potential-theory solution of the problem, even though the outflow seepage face existed over two soil layers. As reported earlier by the present authors for a nonlayered soil, two regions of water entry and flow could be distinguished when the base material was not leaky: (1) flow in the main body of soil (region 1); and (2) additional infiltration and flow near the seepage face (region 2). For a slope length)/(soil depth) ratio of 20 or greater, the flow in region 1 could be approximated as a one-dimensional flow downslope in each soil layer. The length of slope that contributed to flow in region 2 was linearly related to the ratio of the hydraulic conductivity of the subsoil to that of the topsoil. A subsoil of higher conductivity than that of the topsoil increased several fold the rate of infiltration and interflow and the soil (both topsoil and subsoil) volume that was traversed by interflow near the seepage face. It also increased the area and rates of water entry and flow through the topsoil near the upper end of the slope. A significant finding of this analysis is that the transport of topsoil chemicals to field runoff will be speeded up from certain, now predictable, portions of the slope length. When the underlying base material was leaky, some water entering the soil surface seeped into the base material and the soil volume traversed by the interflow was greatly reduced. Amounts of chemicals carried to the outflow face would also be reduced. There were several situations of base leakage rates, soil conductivities and thicknesses considered. For leakage rates of practical interest, the source areas and the soil volume traversed by the interflow could be predicted adequately by a simple procedure. The total rates of water interflow in both the nonleaky- and leaky-base cases could be predicted adequately from known reference values of a nonlayered soil by using a weighted average hydraulic conductivity of the layered profile. However, for prediction of chemicals transported with interflow, knowledge of the flow directions and rate variation along the slope length will be required.