Temporal Controls of Soil Moisture Spatial Organization In the Shale Hills Catchment.

Modeling hydrological processes often requires the identification of dominant controls on soil moisture organization under different climatic conditions at various soil depths.  In this study, we utilized a four-year database consisting of soil moisture measurements at 106 locations from the surface down to 1.1-m depth within the Shale Hills Catchment.  Our objectives were to 1) compare the spatial organization of soil moisture within different soil-landform units and its temporal persistence at different depths under varying catchment wetness conditions and 2) investigate correlation strength between soil moisture content and 11 soil-terrain attributes and the temporal change of such correlation.  Our results showed that the catchment’s near-surface (< 0.3-m) soil moisture organization exhibited clear seasonal trends: during the winter through early summer, areas of high soil moisture were concentrated within convergent landforms; while during the summer through early fall, soil moisture was more uniformly distributed throughout the catchment.  This indicates that under dry conditions soil moisture removal (primarily through evapotranspiration) had a significant influence on the organization of near-surface soil moisture, while topography was an important control on soil moisture organization under wet conditions.  Subsurface (> 0.3-m down to 1.1-m) soil moisture organization, however, exhibited increasing temporal persistence with increasing depth, and subsoil moisture above the catchment-wide average was concentrated within convergent landforms under both wet and dry conditions.  Deep soil profiles in convergent landforms provide a store of soil moisture that persisted throughout year, indicating that soil moisture organization within the subsurface is influenced by both surface topography and soil depth.  Topographic wetness index (TWI), slope, depth to bedrock, and percent clay and rock fragment were significant (p < 0.05) factors influencing soil moisture on at least 80% of 91 measurement days analyzed for all soil depths.  Intermediate depths (0.3 to 0.7-m) exhibited the highest coefficient of determination (R²) in linear regression for TWI and upslope contributing area, suggesting that lateral subsurface flow may be an important driver of soil moisture dynamics at these depths in this catchment.   Mean R² values for slope and depth to bedrock increased with increasing soil depth, confirming the importance of deep soil moisture storage on subsoil moisture organization.    We conclude that the controls on this catchment’s soil moisture spatial organization at the near-surface (< 0.3-m) fluctuates seasonally between evapotranspiration and topography; that at intermediate depths (0.3- to 0.7-m) the soil moisture organization is controlled significantly by lateral subsurface flow; and that the organization at deeper depths (> 0.7-m) becomes more temporally persistent and is primarily a function of both topography and soil depth.