Bedrock Groundwater Catchment Area Unveils Rainfall-Runoff Processes in Headwater Basins

Bedrock groundwater (BGW) has a contribution area that differs from that of the surface watershed. To date, there has been no analysis using BGW observations to determine to what extent the BGW catchment area influences surface runoff characteristics. This study determined the BGW catchment area from the water table, using densely nested boring wells (61 in bedrock and three in the soil layer) drilled in the granitic basin (2.3 ha). Streamflow runoff was also measured concurrently across the entire watershed (F0) and the subwatersheds (F1 to F6). The BGW catchment area of F3 accounted for 191% of the watershed area derived from surface topography, reducing the area of adjacent watersheds through cross-watershed flow. The annual precipitation input to the BGW catchment area and baseflow volume was highly correlated, indicating that the BGW catchment area reflected the actual catchment effect in the bedrock layer more accurately than the surface watershed area. The degree of bedrock weathering showed that the BGW table was distributed along the boundary of the C-L-C-M classes. The estimated actual permeability of the C-M class bedrock based on the deep percolation rate suggests that the depth near the C-L-C-M boundary is the hydrological basement surface; this defines the BGW catchment area. This study demonstrates that the BGW catchment area concept provides an accurate prediction of streamflow runoff in mountainous headwater basins that are heavily governed by hydrological processes in bedrock.