Runoff processes on a forested slope on the Canadian shield

Throughfall reaching the ground surface on forested slopes may follow a var iety of pathways to receiving water bodies, and travel times and chemical i nteractions between water and soil may differ between these pathways. We ex amine the controls that throughfall and pre-event soil water characteristic s impose on: 1) separation of incident throughfall between flow over/throug h the soil's organic mat and subsurface flow; and 2) separation of subsurfa ce runoff between bypassing flow via macropores and translatory flow throug h the soil matrix. Runoff response to summer and fall rainstorms in 1997 wa s examined for a forested slope in south central Ontario. Flow over and thr ough 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 effectiv e during drought conditions which promoted hydrophobicity of the organic la yer. Vertical bypassing flow was directly related to throughfall intensity, but independent of pre-event soil water content on the slope. Subsurface r unoff properties were also not related to antecedent soil wetness; however, strong association between throughfall intensity and slope runoff suggeste d that coupled vertical and lateral macropore flow controlled runoff genera tion during small-to-medium size events. Translatory flow displacement of p re-event soil water on the slope may increase in importance for larger even ts and greater antecedent wetness conditions than were observed in this stu dy.