FIELD INVESTIGATION OF MOISTURE DYNAMICS IN WEATHERED BEDROCK

Though runoff through bedrock has been shown to contribute significantly to streamflow in soil mantled landscapes, investigations are typically limited to tracer and water balance analyses due to difficulties accessing hillslope interiors for direct observation. Consequently, moisture storage and transport within unsaturated, fractured bedrock is poorly understood. Within the 17 km² Elder Creek watershed in northern California, we use a variety of techniques to probe the interior of a 4000 m² hillslope to explore how subsurface moisture dynamics are influenced by the development of a weathering front into bedrock. Deep drilling (12 wells 6-35 m deep), geophysical imaging and neutron probe logging reveal a structured weathered bedrock zone. Thin soils mantle a porous saprolite layer which transitions into fractured,weathered rock. An abrupt boundary between weathered rock and dense, very low conductivity fresh bedrock occurs at 4 m at the base of the hillslope and thickens to a depth of 25 m depth near the topographic divide. Six years of monitoring reveal that seasonal rainfall (approximately 1900 mm annually) rapidly penetrates the soil and weathered rock, becomes perched on the fresh bedrock boundary, and flows laterally downslope. The first storms of the season increase rock moisture in the weathered bedrock to depths of up to 6 m and produce a groundwater response at depths of 5 to 19 m. Subsequent rain events cause pulses in soil moisture, but a damped response in the rock moisture of the saprolite. During the dry season, the soil rapidly dries and the rock moisture content declines by approximately 115 mm in the first 6 m. No significant detectable change in moisture content is observed at greater depth within the weathered bedrock despite the rise and fall of the water table through this zone. These observations reveal that the upper portion of the weathered, fractured bedrock gains and loses significant moisture, which in turn may influence seasonal recharge rates to the perched water table, the isotopic signature and solutes in runoff, and water availability to vegetation.