Spatial and Temporal Monitoring of Water Content in Weathered Granitic Bedrock Using Electrical Resistivity Imaging

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 rho relative to a reference rho profile (Delta rho) corresponded well with the increases and decreases in volumetric water content theta (Delta theta) calculated from the directly observed pressure head psi using tensiometers and borehole wells. This demonstrates the applicability of time-lapse ERI measurement for qualitatively evaluating the spatial and temporal variations in theta (i.e., wetting and drying processes) for not only soil mantles but also for bedrock in a natural hillslope. There was a reasonable correlation (R-2 = 0.69 to 0.77) between each average theta and rho in regions assumed to have different degrees of weathering, indicating the potential of ERI for quantitatively evaluating moisture conditions within an en tire 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 rho 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 rho monitoring within an en tire slope, including the bedrock; such estimations may be more difficult to achieve with invasive methods in many mountain slopes.