Hydraulic Properties of Variously Weathered Granitic Bedrock in Headwater Catchments

Abstract

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.

Estimation of Bedrock Infiltration on a Weathered Granitic Mountain Covered by Japanese Cypress Forest using Water-Budget and Eddy Covariance Methods

“…Katsura et al [2009] reported that the hydraulic conductivity of weathered granite bedrock (10 -5 -10 -3 cm s -1 ) is higher than that of weakly weathered bedrock (10 -8 cm s -1 ) in deeper layers. Based on intensive observations of the SiO 2 concentrations of groundwater and streamwater, Katsuyama et al [2005] showed that groundwater flow within a permeable weathered granite bedrock contributed to streams year-round in one small headwater catchment.…”

Combined geophysical methods for detecting soil thickness distribution on a weathered granitic hillslope

“…However, it should be noted that CPT results are significantly affected by the presence of core stones. Moreover, Katsura et al (2009) demonstrated that the saturated hydraulic conductivity, K s , of the D L -class bedrock (10 − 4 cm s − 1 ) was dramatically lower than that of the soil (10 − 1 cm s − 1 ). These findings were based on core samples obtained from boreholes drilled on line C (Fig.…”

“…1c), were 12, 20, and 15 m, respectively; all were drilled by Katsura et al (2009) for geotechnical investigations and groundwater-level observations.…”

“…1c), drilled by Katsura et al (2009), together with the results of CPT conducted at the corresponding points by Kosugi et al (2006). These borehole logs were described by Katsura et al (2009) based on the system of classifying granite cores by weathering degree (Japanese Society of Soil Mechanics and Foundation Engineering, 1985). Table 3 summarizes the characteristics of each weathering class (A to D).…”

“…R D with M = 5 kg, N =50 and . 1c), as determined by Katsura et al (2009). The profiles include stratigraphic weathering profiles, estimated based on borehole logs, and profiles of penetration resistance (N c ), measured at the corresponding points by Kosugi et al (2006).…”