A COMPARISON OF DISCHARGE AND BEDROCK GROUNDWATER HYDROGRAPHS BETWEEN CATCHMENTS UNDERLAIN BY SEDIMENTARY AND GRANITIC ROCKS

Hydrological processes in headwater catchments are closely related to the topography. Studies in some basins in Japan have suggested that bedrock features determine the trends in runoff hydrographs. This suggests that rainfall infiltrations into the bedrock cannot be neglected in headwater hydrological processes. Some previous studies have compared river runoff in catchments underlain by sedimentary rock and granitic rock. However, catchment conditions, such as watershed area and topography, varied, which made it difficult to determine the effects of bedrock characteristics on runoff generation. Moreover, direct measurements of bedrock groundwater have not been adequately conducted. In this study, we evaluated the differences in runoff response to rainfall events between sedimentary and granitic rock catchments, which were similar in area and topography.

We studied the Shigaraki-catchment and the Fudoji catchment, which are underlain by sedimentary rock and granitic rock, respectively. The area of the sedimentary rock catchment was 0.896 ha, and that of the granitic rock catchment was 1.073 ha. In the sedimentary rock catchment, we excavated 12 boreholes to depths of 10 to 30 m. In the granitic rock catchment, we excavated 15 boreholes to depth of 10 to 36 m.

The discharge hydrograph from the sedimentary rock catchment had sharper peaks and more immediate declines than that from the granitic rock catchment. In both catchments, groundwater responses to rainfall events were relatively small in topographic hollows. In ridge areas, groundwater responses were larger in the sedimentary rock catchment than in the granitic rock catchment, which were comparable to the trends in the runoff hydrographs. The contours of the groundwater table suggested that ground water flows down to in a different direction to that indicated by the surface topography. In the granitic rock catchment, the contour is further apart than that in the sedimentary rock catchment. We concluded that, in the granitic rock catchment, the groundwater table was deep and unsaturated flow in thick weathered rock produced minimal responses in the groundwater hydrographs. In the sedimentary rock catchment, the groundwater table was shallow and its hydrograph responded sharply to rainfall, because of rapid water flow in rock fractures.