Catchment-Scale Soil Water Dynamics in a Mediterranean-Type Oak Woodland

Water availability is one of the most serious natural resource issues facing California, especially given projections for future climate change. The 3 million ha of oak (Quercus spp.) woodlands play a critical role in California's water supply system, providing runoff primarily from winter rainfall events and hosting two-thirds of the state's drinking water reservoirs. Thus, understanding water storage and streamflow regulation by soils in these watersheds is essential for water resource planning under future climate change scenarios. The primary objectives of this research were to identify the drivers of soil moisture dynamics and the hydrologic budget at the catchment scale and their relationships with streamflow generation. A 33-ha headwater catchment in the Sierra Nevada foothills of northern California was instrumented to monitor climate, subsurface lateral flow, streamflow, and soil moisture within 100 pedons distributed throughout the catchment. A catchment-scale water balance was used to examine factors regulating spatial and temporal soil water distribution within the catchment. Relationships between soil, topographic, and vegetation characteristics and soil water content at each pedon were analyzed using a linear mixed-effects model with four fixed effects: vegetation type or cover, presence or absence of a claypan, compound topographic index, and solar radiation. Streamflow was limited to periods when lower soil horizons were at or near saturation. Following saturation, each rainfall event generated subsurface lateral flow above the clay-rich argillic horizon, resulting in sharp hydrograph responses with little base flow between rainfall events. For the two water years examined, 9.6 and 11.5% of rainfall left the catchment as streamflow. Vegetation (tree vs. grassland), presence or absence of a claypan, aspect, and compound topographic index (index of wetness) were all significantly correlated to soil moisture status and thus streamflow generation at various times throughout the year. Findings indicated that watershed-scale hydrologic models based solely on surface topography will not fully explain dynamic temporal and spatial variability in hydrologic flow paths and streamflow generation in these oak woodland catchments. In particular, watershed-scale knowledge of soil strati graphy (e. g., claypan distribution) was important for understanding catchment hydrology, especially the occurrence of subsurface lateral flow dynamics.