Evidence of subsurface preferential flow using soil hydrologic monitoring in the Shale Hills catchment

Characterization of preferential flow at multiple spatial and temporal scales is fundamental to the understanding of complex subsurface heterogeneity and catchment hydrology. Evidence of subsurface preferential flow and the conditions under which it occurs were investigated in the Shale Hills catchment, a humid forested region in central Pennsylvania, USA. Seven monitoring sites, plus five replicates, were established along a concave hillslope, a convex hillslope and a valley floor to monitor in situ the hydrology in various soil horizons and their interfaces at half-minute intervals. Using the indicator of a lower horizon that responded to a rainstorm earlier than an upper horizon within the same soil profile, we investigated the subsurface preferential flow processes and their dynamics in each of the five soil series mapped in the catchment. Threshold behaviour, hydrophobicity impact, influence of soil thickness and topography were observed in the spatial and temporal variation of the subsurface preferential flow, which was initiated more readily under the conditions of more intense rain, drier initial soil, shallower soil, and steeper slope. Whereas preferential flow seemed common in this catchment, its frequency during the 15 storm events from 23 September 2006 to 1 January 2007 ranged from 0 to 73.3% for the 68 soil horizons monitored at the 12 stations, with an overall average frequency of 7.5% (i.e. similar to 5 horizons per storm event). This preferential flow was more frequent during the drier period than that during the wetter one. Variation was observed within the same soil series, even for those profiles adjacent to one another. This was due to the differences in hillslope position, slope gradient and orientation, the underlying bedrock fracture and orientation, or some combinations. Whereas different soil series help differentiate the processes and dynamics involved in the subsurface preferential flow, a combined consideration of soil types and landscape features is important to ensure proper use of the soil data for hydrological applications.