Temporal Stability of Soil Moisture Spatial Pattern and Subsurface Preferential Flow Pathways in the Shale Hills Catchment

Abstract

Hydropedologic approaches utilize a strategy of “map first, then design” and “direction first, then velocity” in enhancing the understanding of complex landscape processes. This is illustrated in this study by examples dealing with (i) the mapping of soils and landforms in monitoring and interpreting soil moisture dynamics and (ii) the identification of flow pathways in determining landscape water fluxes. Year‐round monitoring at 77 sites in the Shale Hills Catchment in central Pennsylvania revealed a temporal stability of soil moisture spatial pattern as governed by soil types and landforms, and suggested the significance of subsurface preferential flow in rapid channeling of precipitation to stream discharge. The five soil series identified in the catchment had the following decreasing trend of moisture storage within the upper 1.1‐m solum: Ernest > Blairton ≥ Rushtown ≥ Berks > Weikert. The four landform units showed a decreasing trend of soil moisture storage: Valley > Swale > Hillslope > Hilltop. The 77 monitoring sites exhibited considerable ranking stability throughout the monitoring year at multiple depths, with the subsurface's moisture ranking stability being slightly stronger than that at the surface. A slope‐intercept analysis of linear regression further described the four conditions of temporal stability as related to soil moisture and hydrologic dynamics. Because of more extensively distributed deeper soils and hydrologically active swales, plus favorable subsurface lateral flow pathways and slightly higher cumulative rainfall, the south‐facing slope in this V‐shaped catchment was hydrologically more active than the north‐facing slope in terms of draining more water at a faster rate to the stream. Approximately two‐thirds of the soil horizons measured in the catchment had lateral saturated hydraulic conductivity (Ksat) values 1.5 to 142.5 higher than vertical values. Because of a moderate slope (up to 25–48%), horizontally dipping shale bedrock (11.5–17.1°), and shallow tree rooting systems (branching laterally), subsurface lateral flow was prominent in this humid forested catchment.