3.2 Controls of primary soil moisture patterns
The spatial patterns of soil moisture show the higher Pearson
correlation coefficients with soil-topography properties at each
measurement depth (Table 1). The results generally indicated that
terrain features were larger contributors to the variance in soil
moisture than the soil properties. While most of topographical
attributes (e.g., topography wetness index and slope) had strong
correlations with the derived EOFs, only soil texture among the soil
parameter showed significant correlations. Soil organic matter displayed
lower correlations at the surface soil (0-20 cm). Depth to bedrock,
which is related to both soil thickness and topography, seems to have
had a large influence on soil moisture variability at all depths. This
result may be confirmed by examination of the soil moisture values
within the wet locations which are characterized by the soils with
>1 m depth to bedrock. Elevation, slope, and curvature were
negatively related to soil moisture contents, while upslope contributing
area, depth to bedrock, topographic wetness index, and percent silt and
clay were positively correlated to the soil moisture contents. This
result is likely due to the fact that most soils with the deep soil
profiles are generally limited to lower elevations and concave slope
areas (i.e., valley floor and swales) where soil moisture is normally
the highest. Henninger et al. (1976) reported that soil moisture
increased toward the near-stream zone within a predominantly
agricultural watershed, which was a result of both topographic
convergence and moderately to poorly drained soils within the
near-stream zone. Our regression analysis indicated that soil texture
did not exert a strong influence on the soil moisture spatial
distributions at the catchment scale, particularly at soil depth
intervals between 0.4 and 0.6 m. This finding is due to the relatively
small variations in soil textural properties throughout the measured
locations for the different soil-landform units (Takagi and Lin, 2011).
Famiglietti et al. (1998) reported that under wet conditions, the best
correlations existed with porosity and hydraulic conductivity along a
profile of a 200-m length; whereas under dry conditions, the relative
elevation, aspect and clay content provided the best correlations.
The EOF analysis was repeated for the spatial anomaly data in two
categories (depth/wetness), and the degree to which these factors
affected the soil moisture distribution was calculated (Table 1). At the
Shale Hills, the correlation coefficient values generally increased with
soil depth for elevation, slope, and percent silt and clay content,
whereas the highest values were observed at intermediate depths (0.4 m)
for curvature, depth to bedrock, and topographic wetness index. These
results indicate that soil moisture becomes strongly aligned with
convergent topography and suggests that lateral flow processes may be
important driver of soil moisture redistribution at these depths. The
increased influence of the parameters with depth may relate to the
seasonal changes of soil moisture which undergoes more dramatic changes
near the soil surface. As indicated in Takagi and Lin (2012), the
subsurface soil moisture exhibited weak temporal variability in the
correlation coefficient values that suggested the dampened effects of
climate and hydrological fluxes. Thus, the subsurface soil moisture
distribution in this catchment is a function of both topographic
parameters and soil depth. An observation that was reinforced by the
transient hydrological fluxes such as the presence of the ephemeral
shallow water table that seasonally exist within the valley.