Dryout of a Steep Hillslope During Periods of Extended Seasonal Drought

Studies investigating the mechanics of hillslope hydrology typically explore processes that are prominent as precipitation is transformed to runoff. In seasonally water-stressed environments such as arid Mediterranean climates, re-distribution of subsurface moisture during extended periods of seasonal drought is also a significant, though often ignored, component of the hydrologic system. During this period, ground moisture sustains vegetation for upwards of 6 months without replenishment, and researchers have suggested that weathered bedrock is a critical source of water during the summer drought. We have monitored a 4000 m2 hillslope in the Angelo Coast Range Reserve along the Eel River in Northern California to explore hydrologic processes. Since September 2007, devices for tracking saturation changes in soil and rock (e.g., TDR probes, Electrical Resistance Probes, Neutron logging), and climate attributes have been installed throughout the catchment. In addition, 7 deep wells up to nearly 30 m depth were drilled across the hillslope and water levels have been continuously monitored since then. Drilling revealed a thin (< 0.3 m) soil mantle overlying a fractured mudstone weathering profile that extends up to 20 m near the ridgetop and tapers downslope towards a bedrock lined channel named Elder Creek. Baseflow sustains Elder Creek year round. Vegetation at the site is dominated by Pseudostuga menziesii (Douglas Fir), Arbutus menziesii (Madrone), Quercus wislizeni (Interior live oak), Umbellularia californica (California bay), Notholithocarpus densiflorus (Tan-bark oak) and Sequoia sempervirens (Redwood) trees. Average rainfall is about 2 m with nearly all of it occurring between November and April. For approximately six months during the warm summer, there is no recordable precipitation. We have observed that during the long precipitation-free period, there is significant reduction in the amount of water retained in the shallow soil/weathered rock profile, and in the location of the underlying water table. In both, the rate of decline is largest at the start of the summer, which then gradually decreases over the next few months. In the unsaturated rock zone, extending ~4-25m below the ground surface, there are small, though measurable, reductions in moisture content to depths less than 10 m. In the deeper unsaturated profile, there are no measurable changes in saturation.