Soil Stratigraphic Influences On Hill Slope Hydrology in an Oak-Woodland Catchment.

A hill slope study was conducted in an oak-woodland watershed in the California Sierra Foothills to explore how soil stratigraphy and spatial distribution of clay pans influence hill slope hydrology, hydrologic connectivity and stream flow.  The watershed has approximately 100 characterized pedons instrumented with soil moisture probes at multiple depths.   The stream outlet is instrumented with a Parshall flume and v-notch weir to measure high and low flows, respectively.  An instrumented trench captures subsurface lateral flow from different horizons.  A hill slope transect in the watershed was instrumented with a tensiometer network, which measured soil water potential.  Tensiometers were installed at 15 cm, 30 cm and two deeper depths which varied depending on the soil stratigraphy.  The hill slope transect (~200 m) was chosen based on three criteria: 1) The hill slope had a defined top and bottom; 2) the hill slope contributing area was relatively low; and 3) zones of clay pan and no clay pan existed along the transect.  Five of the previously characterized and instrumented pedons which fell along the transect were used to incorporate soil volumetric water content data with the soil water potential data collected by the tensiometers.  The pedon descriptions were also used to create a cross-section of the hill slope in order to estimate clay pan extent, depth and biomantle thickness (upper bioturbated zone of the soil).  Soil moisture and temperature probes were also installed at 15 cm and 30 cm depths at each tensiometer site to complement the upper two tensiometers.  Preliminary results suggest disconnected perched water tables that become connected during large rain events.  Future plans involve modeling subsurface vertical and lateral flow with HYDRUS-2D and coupling the results with stream flow and subsurface lateral flow from the hill slope trench.