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.