Spatiotemporal Patterns of Soil Matric Potential in the Shale Hills Critical Zone Observatory

Abstract:

While spatiotemporal characteristics of soil moisture content have been frequently studied, that of soil matric potential (ψ) remains sparse. The objects of this study was to investigate the spatial variability and temporal stability of ψ at multiple depths and their relationships with soil moisture content (θ) across a forested catchment under the influence of soil type, terrain, and season. In addition, we also attempted to identify representative locations across the catchment where overall-averaged values of ψ may be found. We analyzed a 5.5-yr database consisting of 62 sites from the surface down to 1.0 m depth that were distributed throughout the 7.9-ha Shale Hills Critical Zone Observatory. The results clearly showed a downward parabolic trend in spatial variability of ψ with decreasing spatial mean value of ψ across all depths. The catchment’s overall spatial variability of ψ generally increased with soil depth and was relatively high during summer and fall due to varying responses of sites within different landforms and soil types to wetting and drying. Percent of sand and silt were significant factors (p<0.05) influencing ψ at surface soil layers; while ψ at subsurface layers (40 to 100 cm) were highly correlated with elevation, suggesting convergent topography and subsurface lateral flow as important drivers of soil water dynamics in this catchment. Temporal stability of the spatial pattern in ψ was generally higher in surface soil layers than that in the subsurface due to more frequent preferential flow and more heterogeneous tree root water demand in the subsurface. In addition, the temporal stability of ψ spatial pattern was also weaker in the spring as compared to other seasons, and wet areas tended to have less evident temporal stability at each depth, all due to more pronounced preferential flow occurrence under wet conditions. Finally, at least one of the representative sites of the catchment-averaged ψ at each depth was found within a south-facing concave hillslope at the middle elevation. The results from this study have implications for upscaling soil water from point-based observations to the hillslope and catchment scales.