Quantifying field-scale soil moisture using electrical resistivity imaging

Electrical resistivity imaging (ERI) and time domain reflectometry (TDR) were used to simultaneously measure resistivity and soil moisture at an experimental field site with the objective of quantifying soil moisture in 2D unsaturated zone profiles (>40 m long and 9 m deep). With a time-series data set and a modified form of Archie’s Law, we used bulk conductivity derived from ERI models of soil resistivity, pore water conductivity derived using an exchangeable cation proxy (Ca + Mg), and percent clay content of soils, to convert 2D ERI profiles into 2D soil moisture profiles. We optimized the model using 1D access-tube time domain reflectometry (TDR) derived soil moisture measurements. Results show that this approach can be used to quantify soil moisture in heterogeneous soils, and thus provides a useful method for measuring 2D soil moisture at a variety of field sites and scales.