To understand runoff processes in steep mountains, it is important to clarify the groundwater dynamics in the bedrock layer. Recent studies have found that bedrock groundwater is a dominant source of runoff from headwater catchments. To increase our understanding of this factor, our research group examined the spatial variability of baseflow characteristics based on hydrological and hydrochemical observations of a second-order catchment (2.3 ha) and seven nested zero-order catchments (0.1-0.5 ha) included in the second-order catchment underlain by granite bedrock. We also investigated bedrock groundwater using densely nested bedrock wells within each catchment. We discovered a catchment with a higher baseflow rate and SiO2 concentration relative to the other catchments. The SiO2 concentrations of the bedrock groundwater were variable and affected by the depth of the flowpath in the bedrock layer. The distribution of the bedrock groundwater table indicated that bedrock groundwater flowed to a neighboring catchment across the catchment boundary. The differences in baseflow among the zero-order catchments arose from the mixture of relatively shallow and deep bedrock groundwater. This study demonstrated that the bedrock groundwater flow across the surface divide produces spatial variability in baseflow, even in a small headwater catchment within homogeneous granite geology.
