HYDROLOGY OF A FORESTED HILLSLOPE DURING STORM EVENTS

Subsurface flow from a 0.47 ha hillslope subcatchment at the upper most reach of the West Fork (38 ha) of Walker Branch Watershed was monitored throughout 1988. Stormflow in the first-order stream draining the West Fork responded simultaneously with the initiation of subsurface flow through 0 to 2.5 m depth from the hillslope. Subsurface flow from the upper hillslope was only 1.3% of total precipitation for 1988, however, individual storms yielded as much as 12.5% of precipitation as subsurface flow. The West Fork had 10.4% of precipitation observed as storm flow with an additional 29% as baseflow. Individual storms exhibited up to 71% of precipitation as storm flow in the West Fork, illustrating the significant increase in percent rain converted to subsurface flow in lower landscape positions as compared to upper hillslopes. Peak subsurface flow preceded peak streamflow and occurred primarily through the 1.0-2.5 m depth (Bt2 horizons) due to perched water tables. Preferential flow from hillslopes through macro- and mesopores was concluded to be the predominant stormflow mechanism. The contributions of the various size pores was reasoned to be dependent upon the position within the soil profile and the stage of the hydrograph. Preferential flow occurred at extreme soil water conditions with the response due to perched water tables under wet conditions and believed to be due to hydrophobic conditions under extremely dry conditions. Intermediate soil water conditions resulted in infiltration into soil matrix pores precluding subsurface stormflow. © 1990.