CONTRIBUTION OF SUBSURFACE FLOW FROM UPPER SLOPES OF FORESTED WATERSHEDS TO CHANNEL FLOW

Overland flow and subsurface flow from 0.054 and 0.089 ha plots on the upper slopes of forested watersheds were intercepted and measured in trenches located at the bases of the plots. Channel flow was measured in flumes at the watershed outlets and intensity, duration and amount of rainfall were measured by gages on the watersheds. Over a 3 yr period, 115 storms produced little overland and subsurface flow above the B-horizon. Subsurface flow over an impermeable clay layer was as much as 90% of rainfall during a calendar quarter. Thirty-six of the storms produced subsurface flow on one or both plots. For these storms subsurface flow from the upper third of the watersheds was equivalent to 26% of the channel flow on 1 watershed and 27% on the other. For 27 of the 36 storms subsurface flow from the plots peaked sooner than channel flow. Twelve storms produced subsurface flow within 20 min of the onset of rainfall and 15 storms produced channel flow within the same time. Subsurface flow often began shortly after rainfall began even when there was neither saturation at the point of outflow nor high antecedent soil moisture. Interconnected channels through the soil formed by decayed roots and animal burrows are the best explanation for the quick subsurface flow responses. The similarity in timing of subsurface and channel flows indicated that subsurface flow from upper slopes contributed significantly to storm hydrographs in forested areas where permeable surface soils overlaid an impermeable stratum.