Shallow lateral flow from a forested hillslope: Influence of antecedent wetness

A 5-m-wide soil pit at the base of a forested hillslope in coastal British Columbia was instrumented for studying subsurface flow processes during rainstorms. Three typical, low-intensity autumn rainfall events with different antecedent moisture conditions are assessed. Outflow from the organic horizon was captured and measured by a single trough, and outflow from the mineral horizon (above compact glacial till and bedrock) was measured separately for three adjacent sections of the soil pit. For two storms that were preceded by dry conditions, lateral outflow from the organic horizon occurred, although the water table did not rise up to the organic horizon. However, the calculated effective contributing area was small (<0.7 m2) and the effective contributing slope length was short (<0.15 m). Furthermore, volumes of outflow from the organic horizon during these storms were more than 400 times less than during a later storm with wet antecedent conditions. During this later storm, a portion of the outflow from the organic horizon may have been generated as saturated overland flow due to the rising water table. The calculated effective contributing area (>170 m2) and the effective contributing slope length (>36 m) were substantially greater for the “wet” antecedent storm compared to the “dry” storm events. Shallow lateral flow over unsaturated soil is therefore unlikely to be a significant contributor to storm runoff at such forested sites. Flow at the organic horizon–mineral soil interface may contribute to interconnected preferential flow pathways during wetter antecedent conditions.