Linking soil stratigraphy and catchment hydrology in a semi-arid oak woodland in the Sierra Nevada Foothills, California

Watersheds within oak woodland plant communities play a major role in California's water supply. Soils in Mediterranean climates store a large portion of the precipitation when vegetation is dormant, resulting in unique soil moisture dynamics that regulate hydrologic flowpaths and runoff. This study examined factors regulating water flows in a typical oak woodland watershed with a particular focus on soil stratigraphy at the watershed scale. A 33-ha headwater catchment in the Sierra Nevada Foothills was equipped with a hydrologic monitoring network consisting of a meteorological station, a perched water collection system to collect subsurface lateral flow, and a V-notch weir and flume to record streamflow from the catchment. One hundred soil pedons were equipped with soil moisture sensors at four depths to quantify soil water status on a continuous basis over three years. Five soil moisture states were identified based on rainfall, streamflow, subsurface lateral flow, and soil water storage. Soil water in each soil moisture state and at different depths was controlled by different factors, including vegetation (oak vs grasslands), soil characteristics (presence or absence of claypan), solar radiation, and topography. At the initiation of the rainy season in the fall, canopy interception controled the availability of water at the soil surface. During the wet-up state the wetting front advances downward in the profile, depending on the rate of water input in excess of evapotranspiration (ET). Streamflow generation occured once a threshold (¡"195 mm of rainfall) for the spatial organization of soil moisture was reached. Once the soil was at or near saturation, subsurface lateral flow above the claypan (a clay-rich horizon with very low saturated conductivity) became the primary hydrologic flowpath. Geostatistical analysis of soil moisture and tensiometric, and piezometric data demonstrate the importance of the claypan in providing hydrological connectivity, which strongly controls water movement and distribution at the catchment scale. During the spring dry-down state, oak and annual grasses consumed soil water by ET and streamflow ceased. Throughout the dry summer, ET by oak continued to deplete soil water in the deeper soil horizons, while in contrast, the subsurface horizons remained moist. In terms of water quality, potential transport of contaminants is highest once soils reach saturation at the watershed scale and rapid subsurface lateral flow through the soil surface horizons limits soil filtration and retention capacity of the soil. Practically, grazing management of oak woodlands should consider soil moisture status and hydrologic flowpaths in order to implement best management practices most effectively. Furthermore, this study demonstrates that integrated watershed-scale knowledge of soil stratigraphy (e.g., claypan distribution) is important for understanding catchment hydrology in semi-arid oak woodland catchments.