A theoretical analysis of interflow of water through surface soil horizons with implications for movement of chemicals in field runoff

Steady interflow of water in the near‐surface saturated soil horizons, which are the important nonpoint sources of applied agricultural chemicals appearing in field runoff, is investigated theoretically. Analytical solutions are obtained for some relevant cases of interflow in a sloping layered soil having a subsoil of lower permeability. Analysis is made for the effect of the relative permeability of the subsoil and of the depth of the subsoil above an impermeable base varying from zero to infinity, in relation to different ratios of slope length to soil profile depth. Solutions and analyses are also extended to a case where flow through the topsoil is saturated and steady, but where the subsoil is still accepting a constant downward flux of water uniformly over all its area. The solutions are for a rectangularly bounded, tilted flow medium; but for long slopes the details of the flow medium boundaries at the top and bottom ends should have negligible effects on overall flow. The results show that the interflow through each horizon of a layered soil profile can be approximated by a one‐dimensional Darcian flow parallel to the slope, if the ratio of slope length to soil profile depth is greater than about 6 to 10. This extends the finding reported earlier in the literature for a uniform soil. For an extreme case of a sloping soil with a semi‐infinitely deep subsoil on an impermeable base, the interflow through the topsoil, if it is highly conductive compared to the subsoil, can still be treated as a one‐dimensional flow. For such cases, the movement of soil chemicals with interflow can be described as a one‐dimensional miscible displacement process. A uniformly constant downward flux of water into the subsoil decreases interflow through the topsoil. The extent of the decrease depends upon the land slope and the relative conductivity of the subsoil. Of even greater significance for chemical transport is the finding that the cross section of the topsoil through which the interflow traverses and picks up its chemical load is drastically reduced by downward leakage, even when the subsoil conductivity is two orders of magnitude smaller than that of the topsoil. A simplified prediction of this region of interflow is deduced from the results. By representing this wedge‐shaped area by an equivalent rectangle, the chemical movement, for a first‐order estimation in field application, may still be approximated as a one‐dimensional displacement.