Hydropedology and Hydrologic Connectivity of an Oak-Woodland Hillslope in the Northern Sierra Foothills of California.

Understanding hydrologic controls that influence hydrologic connectivity and how it develops has implications for water resource sustainability, water quality and other ecosystem services.  A hillslope study was conducted to explore the dynamics between soil and hydrologic connectivity.  The hillslope was in a zero-order watershed with a flashy ephemeral stream.  It was located in an oak-woodland in the Northern Sierra Foothills of California.  The research objectives were to 1) identify and characterize hydrologically significant soil properties; 2) explore how soil stratigraphy and morphology influence hydrologic connectivity; and 3) examine potential causes for connection and disconnection of hydrologic flowpaths during and between rain storm events.  During the 2012 wet season soil hydrology data was collected on a 210 m hillslope transect.  Once the soil became too dry the hillslope was trenched, characterized, sampled and mapped.  The data obtained from the transect excavation was used to create a two-dimensional hillslope model using HYDRUS 2D.  The hillslope hydrology was modeled prior to, during and following a streamflow event.  The field data showed that the hillslope stratigraphy was complex and comprised of a discontinuous network of claypan, undulating bedrock topography and highly variable weathered bedrock.  The primary hydrologic flowpath feeding peak streamflows was found to be rapid subsurface lateral flow in the permeable surface horizon.   The presence of a claypan decreased effective soil depth, increased antecedent wetness and created a perched water table.  The undulating bedrock created disconnected perched water tables along the hillslope.  The isolated zones of wetness only became connected when a storm event saturated the entire subsurface and moved the water table into the permeable surface horizons.  Further investigation on the hydrologic role of the varied weathered bedrock would greatly improve our understanding of hillslope and catchment scale hydrology in the Northern Sierra Foothills.