Saturated hydraulic conductivity and water retention curve of variably weathered tuff breccia bedrock in a headwater catchment

Abstract

Although recent studies have emphasized the importance of water flow through bedrock to hydrological processes in headwater catchments, information on the hydraulic properties of bedrock, required for physically characterizing water flow processes, is lacking. In this study, the saturated hydraulic conductivity, Ks, and water retention curve of tuff breccia bedrock core were determined in samples collected from boreholes in a headwater catchment in northern Japan. These core-scale properties, together with core shape and in situ Ks values, were used to define two groups of bedrock: CM- and CL-class (weakly to moderately weathered) and D-class (highly weathered). Little change in the volumetric water content θ of CM- and CL-class bedrock was observed in the dry range at a pressure head ψ < –200 cm, but slight changes occurred in the wet range (ψ > –200 cm), resulting in low core-scale Ks values of 10–8–10–7 cm s−1. Core shape and in situ Ks values much larger than the core-scale values suggested that in situ water flow is restricted to fractures, with little contribution of the matrix to total flow. In D-class bedrock, on the other hand, the change in θ with ψ was somewhat larger than in CM- and CL-class bedrock, especially in the wet range, resulting in moderate core-scale Ks values of 10–5 cm s−1. These results indicated that flow in D-class bedrock is better represented by matrix flow than by preferential flow through fractures. The hydraulic properties presented herein provide a basis for analyzing subsurface water flow, including through bedrock layers. Those analyses will provide a better understanding of water flow processes in headwater catchments.