Recent studies have emphasized the importance of bedrock in hydrologic processes occurring in headwater catchments. To understand water flow processes through variously weathered bedrock, we measured the saturated hydraulic conductivity, Ks, and water retention characteristics of weakly to highly weathered Tanakami granite and Rokko granite core samples. On the basis of these core‐scale properties, along with the core shape and in situ Ks measurements, we defined two groups of bedrock: CM class (weakly weathered) and CL to DL class (moderately to highly weathered). The CM class bedrock cores had almost no effective porosity (i.e., the amount of porosity that effectively contributes to water flow) and therefore extremely small core‐scale Ks, indicating that the matrix could be regarded as essentially impermeable. The in situ Ks was much larger than the core‐scale values, however, and the core shape showed apparent fractures, suggesting that water did flow preferentially through the fractures. The volumetric water content of the CL– to DL–class bedrock water retention curves changed little in the dry range but changed gradually in the wet range, resulting in a moderate core‐scale Ks of 10−5 to 10−3 cm s−1 The core‐scale Ks values were well explained by the parameters characterizing the water retention curve. The similarity of the in situ Ks to the core‐scale values, and the lack of fractures in the core shape, suggested that water flow could be characterized as matrix flow. The hydraulic properties of weathered granite at other sites followed the trends observed at our sites, implying wide applicability of the findings in this study to various types of weathered granite.

The formation of groundwater in the soil mantle has a dominant effect on rainwater discharge and shallow landslide occurrence in headwater catchments. Here, we report two completely different groundwater responses within a single well excavated into the soil mantle. One was an ephemeral‐type response that is well described by physical hydrology models based on a geographic information system (GIS). The other was a semi‐perennial‐type response, rarely reported in previous studies, which cannot be explained by the existing physical models. The semi‐perennial groundwater caused considerably high antecedent groundwater tables between storms, leading to an increased peak in the groundwater level during later heavy storm events and a likely increase in the risk of shallow landslides. Moreover, peaks in the semi‐perennial groundwater lagged considerably behind rainstorm events, which probably affected base flow discharge by forming a delayed peak. Geochemical and geothermal observations indicated that the source of the semi‐perennial groundwater was deep bedrock groundwater, demonstrating the considerable effects of bedrock groundwater on surface hydrological processes.

[1] We investigated processes of soil mantle groundwater generation in a granitic headwater catchment in central Japan. Two types of groundwater were observed: ephemeral-type groundwater (EG), which developed in response to rainfall events and disappeared rapidly after the events ceased, and semiperennial-type groundwater (SPG), which remained formed for more than several months. The groundwater level, chemistry, and temperature within the soil and bedrock layers indicated that the source of EG was rain or soil water, whereas the source of SPG was deep bedrock groundwater. The generation processes of soil mantle groundwater varied both spatially and temporally under the influence of the underlying bedrock. Whereas only EG was generated in upslope areas, bedrock groundwater continuously seeped into the soil layers in downslope areas to generate SPG. In middle-slope areas, an increase in the bedrock groundwater level generated SPG in soil layers, but the SPG disappeared when the bedrock groundwater level fell. Our results indicate that bedrock is important in controlling soil mantle groundwater generation and water flow processes in headwater catchments and that direct measurements of bedrock conditions are vital for clarifying the roles of bedrock in these processes.Citation: Katsura, S., K. Kosugi, T. Mizutani, S. Okunaka, and T. Mizuyama (2008), Effects of bedrock groundwater on spatial and temporal variations in soil mantle groundwater in a steep granitic headwater catchment, Water Resour. Res., 44, W09430,

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