INVESTIGATION OF HILLSLOPE FLOWPATHS AND STREAMFLOW GENERATION IN A SEMI-ARID HEADWATER CATCHMENT

Hillslope flowpaths influence riparian and stream biogeochemistry and hydrologic conditions. For ephemeral catchments in semi-arid landscapes, the temporal variation in the controls on hillslope flowpaths is of particular importance in forecasting streamflow responses to seasonal water inputs. The timing and quantity of water delivered from the hillslope to the stream channel affect near-stream water table fluctuations and hydrologic processes. In this study, measured near-surface (30 cm depth) and simulated (SHAW model) deep (at the soil-bedrock interface) soil moisture patterns were combined with a distributed water budget analysis to investigate seasonal soil water distributions and hillslope flowpaths and to infer controls on streamflow initiation in a 0.02 km² semi-arid headwater catchment underlain by shallow soils (45 cm average depth) and granitic bedrock.

Spatial mapping of soil moisture patterns identified a subsurface source area near the channel head during the fall wet-up period. The subsurface source area resulted from the convergence of vertical unsaturated flow and laterally distributed bedrock flow and was the source for early season streamflow near the channel head. Early season streamflow downstream from the channel head was lost to the subsurface until hydrologic connection linked shallow (30 cm) upper slope and deep (100 cm) lower slope soil profiles in the central portion of the catchment. Bedrock flow generation in deep soil profiles lagged behind bedrock flow in shallow soil profiles. The lag in bedrock flow delivery from upper slopes to the near-stream environment delayed the timing to peak streamflow and flow through the catchment outlet. Peak streamflow and the timing of streamflow through the catchment outlet was dependent on the growth of the subsurface source area near the channel head and the subsequent hydrologic connection of upper and lower slopes in the central portion of the catchment. This study provides insight into the spatial and temporal variability of ephemeral hillslope flowpaths in shallow soils over granitic bedrock.