Runoff Processes on a Forested Slope on the Canadian Shield

Throughfall reaching the ground surface on forested slopes may follow a variety of pathways to receiving water bodies, and travel times and chemical interactions between water and soil may differ between these pathways. We examine the controls that throughfall and pre-event soil water characteristics impose on: 1) separation of incident throughfall between flow over/through the soil's organic mat and subsurface flow; and 2) separation of subsurface runoff between bypassing flow via macropores and translatory flow through the soil matrix. Runoff response to summer and fall rainstorms in 1997 was examined for a forested slope in south central Ontario. Flow over and through the thin podzol soil cover was measured at a throughflow trench, while vertical profiles of soil water content were measured at various sites on the slope. Overland flow increased with throughfall intensity and decreased with antecedent soil wetness, implying that this pathway was most effective during drought conditions which promoted hydrophobicity of the organic layer. Vertical bypassing flow was directly related to throughfall intensity, but independent of pre-event soil water content on the slope. Subsurface runoff properties were also not related to antecedent soil wetness; however, strong association between throughfall intensity and slope runoff suggested that coupled vertical and lateral macropore flow controlled runoff generation during small-to-medium size events. Translatory flow displacement of pre-event soil water on the slope may increase in importance for larger events and greater antecedent wetness conditions than were observed in this study.