Pedologic influences on hillslope hydrology: The relationships between soil and hydrologic connectivity in a Californian oak-woodland

Understanding what controls hydrologic connectivity and how it develops has important implications for ecosystem services. It can affect water quality, nutrient and sediment delivery to the stream, carbon and nitrogen cycling, and more. Bedrock topography and soil act in concert as primary physical controls on hydrologic connectivity. However, the important role soil can play is not well understood. 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 Californian northern Sierra foothills. 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 a 210-m hillslope transect was instrumented to collect soil moisture data every five minutes. The instruments were put at multiple locations and depths to capture the soil spatial variability. Once the soil became too dry for monitoring the transect was trenched, characterized and sampled. Texture, bulk density, saturated hydraulic conductivity and soil water retention curves were measured in the lab. Structure, color, redoximorphic features, soil horizon spatial differentiation, saprolite and bedrock characteristics, and coarse fragment percentage were recorded in the field. Prior to excavation an electromagnetic induction (EMI) and ground penetrating radar (GPR) survey in conjunction with the Natural Resource Conservation Service (NRCS) was performed along the hillslope. The goal of the survey was to explore non-invasive techniques to determine spatial variability of hydrologically significant soil horizons and bedrock. The GPR was found not to be reliable at the site. However, the EMI showed potential in showing the discontinuous distribution of the claypan, a horizon characterized by a large and abrupt increase in clay content and very low permeability. The data obtained from the transect excavation was used to create a two-dimensional hillslope model using HYDRUS-2D. Coupled with the soil moisture and local precipitation data the hillslope hydrology was modeled at individual storm event time scale. The field data showed that the hillslope was very complex and comprised of a discontinuous claypan, undulating bedrock topography and highly variable saprolite. The soil moisture data and modeling efforts showed that the surface horizons, which are highly permeable and contain numerous macropores, are the primary hydrologic flowpaths during storm events. The model showed that the presence of claypan decreased effective soil depth, increased antecedent wetness and created a perched water table. The model also showed that the undulating bedrock acted like a dam along the hillslope. The claypan network and undulating bedrock created isolated zones of wetness that only become connected and flow downhill into the stream when a storm caused the disconnected zones to rise in the highly permeable surface horizons.