Critical zone evolution at the hillslope scale
Wednesday, 17 December 2014: 10:35 AM
The critical zone, which extends from the top of the canopy to the elevation of unweathered bedrock, hosts a variety of processes that control the evolution of the topographic surface. Though studies of hillslope evolution tend to focus on erosion and soil production mechanisms near the surface, evidence suggests that weathering and fluid transport processes that occur at depths beyond the soil influence hillslope form. Despite this, little is known about what controls the boundary between weathered and fresh bedrock (Zb) under hillslopes and few direct observations of Zb and moisture dynamics within weathered rock exist. This study presents observations from a heavily monitored 4000 m2 experimental hillslope within the Eel River Critical Zone Observatory in northern California. Drilling data, groundwater monitoring, and geophysical imaging reveal a weathered bedrock zone that thickens towards the divide and is consistent with a theory for predicting Zb under hillslopes that links channel incision to the drainage of fresh bedrock within the hillslope. Direct observations of rock moisture indicate that moisture dynamics within weathered rock regulate streamflow and transpiration. Water transport in the weathered bedrock occurs via two mechanisms: a diffuse wetting front and rapid transport along fractures. This water recharges a seasonally perched water table that provides baseflow. Rock moisture storage increases upslope and remains seasonally elevated longer into the dry season of this Mediterranean climate suggesting that Zb and the spatial distribution of weathered rock controls the spatial and temporal moisture dyanmics in hillslopes. Data from vegetative water use monitoring efforts suggest that rock moisture influences forest ecosystem dynamics and thus physical weathering, erosion mechanisms, climate (via transpiration), and baseflow. Our observations within the interior of this hillslope highlight the consequences of critical zone evolution on the mechanisms that influence hillslope form.