Journal of the Japan Society of Erosion Control Engineering, Vol.56,No.1,p.13-19,2003

Effects of bedrock groundwater seepage on runoff generation at a granitic first order stream

Shusuke MIYATA, Taro UCHIDA, Yuko ASANO, Hiroyuki ANDO and Takahisa MIZUYAMA

Abstract

To evaluate the contribution of bedrock groundwater seepage to stormflow generation in a granitic headwater catchment, runoff and water chemistry of springs and streamflow were observed simultaneously at a headwater catchment, Ibi, central Japan. In the catchment covered with a natural closed forest, there were about20springs along the first‐order stream, one from soil‐bedrock interface, and the other from cracks in the bedrock. Hydrologic and hydrochemical observation indicates that bedrock springs at the catchment can be divided into two types; relatively responsive to rainfall, and low SiO2 concentration and EC (“Type A");little runoff fluctuation and relatively high SiO2 concentration and EC (“Type B"). The SiO2 concentration indicated the source of “Type A" spring was shallower than “Type B". Hydrologic observation indicates that the runoff from soil‐bedrock interface is dominant to rapid and large peak runoff that coincided with the rainfall peak (45mm, 110mm). However on the heaviest rain event (287mm), bedrock groundwater seepage is dominant to delayed and large peak discharge. This peak flow was three times greater than the initial peak flow, which was conjunction with the peak of subsurface storm flow.