Subsurface flow velocities in a hillslope with lateral preferential flow

Our understanding of hillslope subsurface flow relies on assumptions about how storm characteristics affect the hillslope runoff response. Experiments in hillslopes dominated by preferential flow features often show that runoff is dynamic and is affected by antecedent conditions, rainfall conditions, and position of the slope. We applied tracers to a hillslope under natural and steady state flow boundary conditions to determine the relationship between lateral tracer velocities and various hillslope lengths and storm indicators. Tracer velocities were similar to the fastest velocities measured in other similar experiments. The velocities were dependent on the boundary conditions and slope length, and the subsurface flow velocity was most closely related to the 1-h rainfall intensity. Unlike some studies, there was little correlation between our measured flow velocities and storm volume or antecedent conditions. We attributed this to the hillslope characteristics and the relatively consistent wet antecedent conditions during the experiments. This experiment showed that the connectivity of the hillslope preferential flow network is an important factor governing the average subsurface flow velocity.