Application of Electrical Resistivity Imaging for MeasuringWater Content Distribution on Hillslopes

Abstract

Electrical resistivity imaging (ERI) as a method for effectively evaluating soil water content distribution on natural hillslopes was validated by combining ERI technique with the invasive measurement of volumetric water content (θ) using a combined penetrometermoisture probe (CPMP) on a hillslope in a head-water catchment underlain by weathered granite porphyry. There was a reasonable correlation (R2= 0.54) between θ and electrical resistivity (ρ). The correlation between (θ and ρ measured on two natural hillslopes in a head-water catchment underlain by weathered granite in our previous studies was also analyzed, and there was some reasonable correlation (R2= 0.33 to 0.53) between θ and ρ within each slope, indicating the potential of ERI for quantitatively evaluating moisture conditions within soil layers of natural hillslopes based on field-scale calibrations with invasive methods. These θ-ρ datasets were roughly consistent with a common fitted functionalmodel (Archie’s equation) (R2= 0.37), indicating the possibility of quantitatively evaluating θ of soil layer on natural hillslopes using ERI without directly measuring θ using any invasive method, although results still showed the importance of combining invasive methods with ERI and obtaining sitespecified θ-ρ correlation models for providing a more accurate spatial distribution of θ within the soil mantle. Inconsistencies between θ and ρ within datasets may be significantly attributable to not only limitations on spatial resolution of ERI technique related to the issue of representative volumes of the technique and inversion analysis to obtain ρ profiles but also the assumption that soil properties and pore-water resistivity of the entire slope are homogeneous. Using a CPMP as invasive method, detecting heterogeneous θ distribution more accurately than ERI technique, together with ERI is one of the most reasonable ways of effectively quantifying soil water content distribution on natural hillslopes.