INTENSIVE MONITORING OF STREAMFLOW AND BEDROCK GROUNDWATER IN A GRANITIC HEADWATER CATCHMENT WITH STEEP TOPOGRAPHY

In spite of the importance of bedrock groundwater on streamflow generation, intensive groundwater monitoring is rare in mountains with steep topography. Consequently, how bedrock groundwater contributes to streamflow hydrographs is in dispute. In the steep Rokko mountain range of central Japan, which consists of granite and has been greatly affected by diastrophic activities, discharge hydrographs are characterized by significant amount of baseflow. In order to elucidate contributions of bedrock groundwater to the hydrograph formation, this study conducted long-term hydrological observations using bedrock wells with depths of 7−78 m drilled at 31 points within a 2.1-ha headwater catchment in the Rokko mountain range. Results indicated a fairly regionalized distribution of bedrock groundwater; that is, upper, middle, and lower aquifers were present. We observed large differences in water level among the aquifers, instead of a gradual and continuous decline in water level. Discharge hydrograph from the catchment was notably characterized by gentle and significant variations in base flow and exhibited triple-peak responses. Flashy first peaks occurred just after rainfall peaks, while the second peaks lagged behind the rainfall peaks by a few days. Broad peaks in the base-flow discharge corresponded to the third peaks, which occurred once or twice in each hydrological year. The triple-peak discharge responses were explained by three types of water pathways: the first peak was caused by the peak in soil mantle groundwater around the outlet of the watershed; the second peak was caused by the first peak in the lower aquifer, which was fed by vertical rainwater infiltration; and the third peak was caused by the second peak in the lower aquifer, resulting from an increased lateral water supply from the middle aquifer. The middle aquifer was recharged by vertical infiltration through weathered bedrock and lateral flow from the upper aquifer. Because of its broad regional expanse and large capacity, the middle aquifer had a dominant effect on formation of the discharge hydrograph. Thus, this study has demonstrated how discharge from the steep headwater catchment is dominated by complex flow systems within bedrock groundwater; the spatial expanse of bedrock aquifers and interaction among aquifers are key factors.