Seepage area and rate of bedrock groundwater discharge at a granitic unchanneled hillslope

[ 1] Recent studies have demonstrated the importance of water movement through the bedrock in the rainfall-runoff process on steep hillslopes. However, quantitative information on this process is still limited. The objective of this study was to address the following questions: ( 1) How large is the area where bedrock groundwater seeps into the soil layer, and ( 2) what is the rate of water flow out of the bedrock? To address these questions, detailed hydrological, hydrochemical, and thermal measurements were conducted at a forested steep unchanneled granitic concave slope in the Tanakami Mountains, central Japan. The relationship between the amplitude of annual soil temperature variation and the measurement depth showed that in a normal low-flow period, the seepage area ranged between 14 and 21 m 2 and the ratio of this area to that of the whole catchment was about 2.0%. In a drought period the seepage area ranged between 3.5 and 5.5 m(2), and the ratio to the whole catchment was around 0.5%. The variation in the area of seepage was controlled both by the short-term precipitation pattern during the preceding several weeks and by the long-term pattern over several preceding months. A two-component geochemical hydrograph separation indicated that the ratio of bedrock groundwater to streamflow was about 0.82 for the normal low-flow periods and 0.90 for the drought period. The rate of flow out of the bedrock into the soil layer ranged from 0.5 to 3.3 m(3) d(-1). That is, although the seepage area was small (0.5-2.0% of the catchment), the contribution of bedrock groundwater was considerable (50-95% of streamflow).