Temporal Dynamics of Soil Moisture Spatial Variability in the Shale Hills Critical Zone Observatory

This study utilized a 4-yr database consisting of catchment-wide volumetric soil moisture measurements at 78 locations from the surface down to 1.1 m depth within the 7.9-ha Shale Hills Critical Zone Observatory in central Pennsylvania. The objectives were to: (i) assess the uncertainty in estimating catchment-wide mean soil moisture content by characterizing the relationship between soil moisture spatial variability and spatially averaged moisture content and (ii) examine the relationship between temporal changes in soil moisture spatial variability and seasonal and event-based hydrologic fluxes. Our results showed that soil moisture spatial variability increased exponentially with increasing catchment-wide wetness across all measurement depths and various soil-landform units. This relationship led to the widening of confidence intervals and an increase in the number of sites needed to obtain 95% confidence in spatially averaged soil moisture as the catchment wets up. This was due to the well-drained and steep-sloped hillslope soils in the catchment that confined saturated areas to the swales and the valley floor. Temporal changes in soil moisture spatial variability indicated a seasonal increase in the catchment-wide soil moisture variability that coincided with the emergence of a shallow water table in the valley during the winter and spring months. Event-based increases in soil moisture spatial variability associated with large antecedent precipitation were also observed (but with limited resolution because of weekly to biweekly measurements), which indicated the importance of subsurface lateral flow in influencing the catchment-wide soil moisture spatial variability. Because ephemeral shallow water tables and subsurface lateral flow are common in many temperate forests, our results are useful for guiding soil moisture monitoring campaigns in these catchments and have implications for interpreting and scaling remote sensing footprints of soil moisture in these landscapes during different times of a year.