Soil moisture is an important component of the hydrologic cycle. The spatial and temporal variability of soil moisture influences environmental processes at a wide range of spatial scales. This study utilized a four-year database consisting of soil moisture measurements at 106 locations from the surface down to 1.1-m depth within a forested catchment in central Pennsylvania to address the following objectives: 1) characterizing the relationship between soil moisture spatial variability and catchment-wide wetness to assess the uncertainty in estimating spatially-averaged soil moisture; 2) examining temporal changes in vertical soil moisture profile along a topographic gradient to identify processes that influence the above stated relationship; 3) compare the spatial organization of soil moisture at different measurement depths; 4) examine the correlation between soil moisture at different measurement depths and soil terrain indices. Our results showed that spatial variability (defined as the standard deviation) increased exponentially with increasing catchment-wide spatially-averaged soil moisture (R2 = 0.863, p < 0.05). This relationship led to the widening of confidence intervals as spatially-averaged soil moisture increased at all depths (increased CI of 0.040, 0.038, 0.033, 0.037, 0.031, 0.029-m3/m3 between lowest and highest spatially-averaged soil moisture for 10, 20, 40, 60, 80 and 100-cm depths, respectively). Similarly, the number of samples required for obtaining 95% confidence interval in the estimates of spatially-averaged soil moisture increased as spatially-averaged soil moisture increased at all depths (number of samples increased by 23, 25, 24, 27, 19, and 16 between lowest and highest spatially-averaged soil moisture for 10, 20, 40, 60, 80 and 100-cm depths, respectively). Our analysis of temporal changes in the vertical soil moisture profile indicated that during the winter through early summer, the emergence of a shallow water table in the valley and spatially and temporally limited concentrated lateral flow along hillslopes, increased catchment-wide soil moisture spatial variability during wet periods. Conversely, during periods of low soil moisture, an increase in hydrological processes operating across the watershed, particularly evapotranspiration, acted to decrease the catchment-wide soil moisture variability. The results of this analysis have implications for optimizing soil moisture monitoring in forested watersheds, particularly watersheds which exhibit strong seasonal fluctuations in water table height.
At the Shale Hills catchment, surface (<20-cm) soil moisture organization exhibited 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, areas of high soil moisture were more randomly distributed throughout the catchment. This indicates that seasonal removal of soil moisture, primarily through evapotranspiration, had a significant influence on near-surface soil moisture organization under dry conditions, while topography was an important control on soil moisture organization under wet conditions. Sub-surface (>20-cm) soil moisture organization exhibited temporal persistence, with soil moisture above the catchment-wide average concentrated within convergent landforms under both wet and dry conditions. Deep soil profiles in convergent landforms provide a store of soil moisture that persisted even under dry conditions, indicating that soil moisture organization within the subsurface is a function of both topographic controls and soil depth. The results of linear regression between soil moisture and soil-terrain attributes indicate that terrain features generally outperformed soil textural properties in explaining the variability of soil moisture within the forested catchment. Slope, depth to bedrock, topographic wetness index (TWI) and rock fragments were significant (α = 0.05) on at least 85% of the measurement days for all depth intervals measured. Intermediate depths (40 and 60-cm) exhibited the highest mean R2 values for TWI and upslope contributing area, while mean R2 values for slope and depth to bedrock increased with increased soil depth. No seasonal trend is present in the R2 values for sub-surface soil moisture (60, 80 and 100-cm) and depth to bedrock and topographic wetness index, with depth to bedrock exhibiting high R2 values throughout the four-year period. These results support our conclusion that both topography and soil depth are important in controlling soil moisture organization at depth, while seasonal fluctuations in evapotranspiration and topography are important controls on soil moisture organization at the near-surface.
Takagi, K. (2009): Static and Dynamic Controls of Soil Moisture Variability in the Shale Hills Catchment. Master of Science, Soil Science, The Pennsylvania State University, p. 68.
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