The role of subsurface runoff through bedrock on storm flow generation

Abstract The influence of bedrock subsurface flow on storm runoff generation was investigated 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 watersheds underlain by granite and Mesozoic shale were studied. Two of these watersheds were instrumented to monitor stream discharge and the other four are springs occurring at the bedrock exposure. The other study area is in northern Kyoto prefecture, western Japan (Oe region). Here, spring distribution and seasonal flow were monitored in two steep basins underlain by serpentinite rocks. Distinctly different runoff responses were observed: the granite watershed exhibited a large and rapid runoff peak that coincided with the rainfall peak (type 1); and the shale and serpentinite basins exhibited small initial runoff peaks followed by a maximum peak discharge five to ten times greater with a convex recession limb (type 2). Runoff response from bedrock springs had characteristics similar to type 1 hydrographs; however, discharge 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 the Oe region, especially in dry periods. This decline in discharge with elevation suggests that a deep subsurface flow system through bedrock fissures contributes to the storm water flow in serpentinate basins. When comparing runoff response and spring discharge, slow or double peak runoff response may be a good indicator of the influence of bedrock subsurface outflow on storm runoff generation in steep mountainous regions. Copyright © 2001 John Wiley & Sons, Ltd.