Runoff generation mechanisms in high-relief mountainous watersheds with different underlying geology

In order to examine the runoff generation processes in an extremely steep terrain underlain by relatively unweathered granite and shale, four small (0.78–7.1 ha) watersheds were monitored – two in shale and two in granite. The two shale watersheds exhibited distinctly different runoff responses to identical storm events, whereas the two granite watersheds exhibited similar responses in which the maximum runoff peak coincided with the rainfall peak. The majority of storm water runoff events in the shale watersheds and more than half of those in the granite watersheds was “old water”. Based on soil water percolated into the bedrock in the shale watersheds even during storm events, whereas subsurface storm flow parallel to the slope was observed in the granite watersheds. These data and observation suggest that the subsurface storm flow was dominant even in these extremely steep mountains. Furthermore, subsurface stormflow through the soil mantle was dominant in the granite watersheds, whereas bedrock flow was dominant in the shale watersheds.