Runoff production on forest roads in a steep, mountain catchment

[1] This study investigated how roads interact with hillslope flow in a steep, forested landscape dominated by subsurface flow and how road interactions with hillslope flow paths influence hydrologic response during storms in a second-order catchment. Runoff was measured continuously from 12 subcatchments draining to road segments and covering 14% of a 101-ha, second-order catchment (WS3) in the Andrews Forest, Oregon. Observed runoff over the 1996 water year was compared to predictions for runoff timing and interception of a hillslope water table based on a simple model of kinematic subsurface storm flow. Observed runoff behavior was consistent with model estimates, a finding that underscores the utility of this simple approach for predicting and explaining runoff dynamics on forest roads constructed on steep hillslopes. Road segments in the study area interacted in at least four distinct ways with complex landforms, potentially producing very different effects depending on landform characteristics. Hillslope length, soil depth, and cutbank depth explained much of the variation in road runoff production among subcatchments and among storm events. Especially during large storm events, a majority of instrumented road segments intercepted subsurface flow and routed it to ditches and thence directly to streams with a timing that contributed to the rising limb of the catchment-wide hydrograph. The approach used in this study may be useful for model development and for targeting road segments for removal or restoration.