Coupled vertical and lateral preferential flow on a forested slope

Abstract

Coupling of vertical and lateral preferential flow paths was examined on a forested slope with thin soil cover during artificial irrigations. Point‐scale infiltration was measured at sites with differing soil macroporosities using vertical profiles of time domain reflectometry probes and suction samplers. Lateral fluxes of water and solutes from the slope were determined at a through flow trench. Sites with greater macroporosities tended to exhibit vertical preferential flow, while infiltration at sites with relatively small macroporosities was largely by vertical propagation of a well defined wetting front through the soil profile. Generation of vertical preferential flow at sites with relatively large macroporosities increased with input intensity. Lateral macropores made a minor contribution to slope runoff. Instead, runoff largely occurred in a thin saturated layer at the soil‐bedrock interface, both in a highly conductive zone at the bedrock surface and in the overlying saturated soil matrix. Some assumptions underlying the use of isotopic and geochemical tracers to study runoff generation are called into question by complex mixing of event and pre‐event water in this saturated layer. Soil depth, bedrock topography, and antecedent soil wetness determine the thickness, connectivity, and upslope extent of the pre‐event saturated layer above the bedrock surface. These, in turn, control whether vertical preferential and matrix flow reaching the bedrock surface participate in slope runoff.