[1] Recent studies have suggested that bedrock groundwater can exert considerable influence on runoff generation, water chemistry, and the occurrence of landslides in headwater catchments. To clarify water infiltration and redistribution processes between soil and shallow bedrock and their effect on storm and base flow discharge processes in a small headwater catchment underlain by weathered granite, we conducted hydrometric observations using soil and bedrock tensiometers combined with hydrochemical measurements and water budget analyses at three different spatial scales. Results showed that in an unchanneled 0.024-ha headwater catchment, saturated and unsaturated infiltration from soil to bedrock is a dominant hydrological process at the soil-bedrock interface. Annual bedrock infiltration ranged from 35 to 55% of annual precipitation and increased as precipitation increased, suggesting a high level of potential bedrock infiltration, partly explained by the high buffering capacity of the soil layer overlying the bedrock. This physical property of the soil layer was an important factor in controlling the generation of bedrock infiltration and saturated lateral flow over the bedrock. In a 0.086-ha watershed including the unchanneled headwater catchment, exfiltration from the bedrock toward the soil layer composed more than half the annual discharge.

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.

As a first step toward describing water flow processes in bedrock, we determined the hydraulic properties of three trimmed samples of weathered granitic bedrock (referred to as Samples A, B, and C, in order of size) in the laboratory. Silicone rubber was used to fill the space between each sample and the surrounding cylinder wall, ensuring accurate measurement of hydraulic properties of the samples. All samples showed similar saturated hydraulic conductivity values of 1 3 10 24 cm s 21 , with the saturated water flow in all samples obeying Darcy's Law. Unsaturated hydraulic conductivity and water retention functions of Sample A were determined by means of a multistep outflow experiment. Parameters in both functions were optimized by comparing observed and computed cumulative outflow rates. The resulting computed cumulative outflow rates using the optimized parameters showed a good match to the observed cumulative outflow data. Moreover, the derived water retention function agreed closely with the function measured by the pressure plate method. We conclude that the methods proposed in this study are effective for determining the hydraulic properties of weathered bedrock. The bedrock water retention curve exhibited small changes in volumetric water content throughout the measurement range where the pressure head, c, was greater than 2200 cm. The bedrock hydraulic conductivity function showed a small decrease in hydraulic conductivity in the very wet range of c greater than 230 cm, and then declined gradually with decreasing c.

[1] Understanding a discharge hydrograph is one of the leading interests in catchment hydrology. Recent research has provided credible information on the importance of bedrock groundwater on discharge hydrographs from headwater catchments. However, intensive monitoring of bedrock groundwater is rare in mountains with steep topography. Hence, how bedrock groundwater controls discharge from a steep headwater catchment is in dispute. In this study, we conducted long-term hydrological observations using densely located bedrock wells in a headwater catchment underlain by granitic bedrock. The catchment has steep topography affected by diastrophic activities. Results showed a fairly regionalized distribution of bedrock aquifers within a scale of tens of meters, consisting of upper, middle, and lower aquifers, instead of a gradual and continuous decline in water level from ridge to valley bottom. This was presumably attributable to the unique bedrock structure; fault lines developed in the watershed worked to form divides between the bedrock aquifers. Spatial expanse of each aquifer and the interaction among aquifers were key factors to explain gentle and considerable variations in the base flow discharge and triplepeak discharge responses of the observed hydrograph. A simple model was developed to simulate the discharge hydrograph, which computed each of the contributions from the soil mantle groundwater, from the lower aquifer, and from the middle aquifer to the discharge. The modeling results generally succeeded in reproducing the observed hydrograph. Thus, this study demonstrated that understanding regionalized bedrock aquifer distribution is pivotal for explaining discharge hydrograph from headwater catchments that have been affected by diastrophic activities.Citation: Kosugi, K., M. Fujimoto, S. Katsura, H. Kato, Y. Sando, and T. Mizuyama (2011), Localized bedrock aquifer distribution explains discharge from a headwater catchment, Water Resour. Res., 47, W07530,

[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,

Electrical resistivity imaging (ERI) as an effective method to evaluate water flow processes through bedrock in a hillslope in a headwater catchment was validated by invasive hydrometric observations. Distributions of increases and decreases in electrical resistivities ρ relative to a reference ρ profile (Δρ) corresponded well with the increases and decreases in volumetric water content θ (Δθ) calculated from the directly observed pressure head ψ using tensiometers and borehole wells. This demonstrates the applicability of time‐lapse ERI measurement for qualitatively evaluating the spatial and temporal variations in θ (i.e., wetting and drying processes) for not only soil mantles but also for bedrock in a natural hillslope. There was a reasonable correlation (R2 = 0.69 to 0.77) between each average θ and ρ in regions assumed to have different degrees of weathering, indicating the potential of ERI for quantitatively evaluating moisture conditions within an entire natural hillslope, including bedrock, based on field‐scale calibrations with invasive methods. Fluctuations in groundwater tables in boreholes within bedrock along the survey line and discharge from two differently sized catchments including the study slope were both successfully reflected in the temporal variation in mean ρ in the regions located just above and below the groundwater tables. This indicates the potential of ERI for estimating groundwater levels and runoff from a watershed based on temporal ρ monitoring within an entire slope, including the bedrock; such estimations may be more difficult to achieve with invasive methods in many mountain slopes.

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