The role of subsurface runoff through bedrock on storm flow generation

The influence of bedrock subsurface flow on storm runoff generation was inv estigated in Japan in two regions in Japan underlain by three bedrock types . One region, with approximately 2500 m of relief, is located in the Japan Alps, central Japan (In a region), where six small forest-covered watershed s underlain by granite and Mesozoic shale were studied. Two of these waters heds were instrumented to monitor stream discharge and the other four are s prings occurring at the bedrock exposure. The other study area is in northe rn Kyoto prefecture, western Japan (Oe region). Here, spring distribution a nd seasonal flow were monitored in two steep basins underlain by serpentini te rocks. Distinctly different runoff responses were observed: the granite watershed exhibited a large and rapid runoff peak that coincided with the r ainfall peak (type 1); and the shale and serpentinite basins exhibited smal l initial runoff peaks followed by a maximum peak discharge five to ten tim es greater with a convex recession limb (type 2). Runoff response from bedr ock springs had characteristics similar to type 1 hydrographs; however, dis charge increased only when the antecedent precipitation index was large and the runoff peak was delayed between 10 h and 1 week after rain events. The specific discharge of the springs decreased with increase in altitude in t he Oe region, especially in dry periods. This decline in discharge with ele vation suggests that a deep subsurface flow system through bedrock fissures contributes to the storm water how in serpentinate basins. When comparing runoff response and spring discharge, slow or double peak runoff response m ay be a good indicator of the influence of bedrock subsurface outflow on st orm runoff generation in steep mountainous regions. Copyright (C) 2001 John Wiley & Sons, Ltd.