Analyses of Water Flux Vectors Through Weathered Bedrock in a Granitic Headwater Catchment

Although recent studies in hillslope hydrology have emphasized the importance of bedrock in hydrological and hydrochemical processes and shallow landslides occurrence in headwater catchments, few have analyzed water flow processes in bedrock. We observed the pressure head distribution using intensively installed tensiometers and the bedrock groundwater level in two boreholes in a granitic headwater catchment in central Japan. Moreover, we measured hydraulic properties of variously weathered bedrock core samples taken from the boreholes. By coupling the obtained hydraulic properties with the observed pressure head distribution, subsurface water flux vectors were analyzed based on the Darcy's Law. Water fluxes were larger in the bedrock than in the soils during periods of no rainfall, implying contribution of bedrock groundwater to sustaining baseflow. During rainfall events, very strong fluxes were observed in the soil layers, suggesting contribution of soil layers to storm runoff generation. During heavy storm events, water temporarily flowed from the downslope areas toward the ridge within the deep bedrock (inverse flow). Because the distance from the ground surface to the bedrock groundwater table was generally smaller in the downslope areas than in the upslope areas, it was inferred that the inverse flow was caused by faster and larger water supply to the bedrock groundwater body in the downslope areas than in the upslope areas, leading to the temporarily higher groundwater table in the downslope areas.