Interactions between bedrock groundwater and surface-hydrological processes in headwater catchments with different topographies

Abstract:

Recent hydrological studies have revealed that large amounts of rainwater infiltrate into bedrock. This study investigates interactions between bedrock groundwater and surface-hydrological processes in headwater catchments, and evaluates effects of surface topographies on their rain water storages and discharge hydrographs.

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, water storage observations were conducted by using bedrock wells with depths of 7−78 m drilled at 31 points within a 2.1-ha headwater catchment. On the other hand, discharge hydrographs from the gentle Tanakami mountain range of central Japan, which also consists of granite, produce large storm peaks and sparse baseflow discharges. In headwater regions of the Tanakami mountain range, we established a 2.3-ha catchment, in which water storage observations were conducted by using bedrock wells with depths of 3−42 m drilled at 61 points.

Results indicated a regionalized distribution of bedrock groundwater in the Rokko catchment; 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. Broad peaks in the base-flow discharge corresponded to peaks of water storage of the middle aquifer. Because of its broad regional expanse and large capacity, the middle aquifer had a dominant effect on formation of the discharge hydrograph. In the Tanakami catchment, bedrock groundwater aquifers were separated by small ridges and hollows within the catchment, and their spatial expansions were much narrower than those in the Rokko catchment. As a result, the bedrock water storage exhibited more peaky responses, causing rapid increases and declines in discharge hydrographs. Thus, this study has demonstrated how discharge from headwater catchments are dominated by water storage within bedrock.