FIELD SCALE SOIL MOISTURE MEASUREMENTS USING TDR-CALIBRATED ERT DATA

Electrical Resistivity Tomography (ERT) is frequently used to remotely characterize subsurface features in many settings, including the vadose zone. Differential ERT (DERT) has been used to qualitatively model changes in soil moisture at the field scale (10s of meters) by monitoring changes in resistivity assumed to be the result of changes in soil moisture. DERT has also been used in conjunction with other measurements to quantitatively measure moisture content and changes at cm to meter scale in lab or small-scale field experiments. In order for DERT to be useful for quantitative field-scale vadose-zone modeling, ERT measurements must first be converted into a representative model of soil moisture.

We present a method for quantitatively modeling soil moisture and time-series changes in soil moisture in 2-D heterogeneous soil profiles by linking calibrated access-tube Time Domain Reflectometry (TDR) soil moisture measurements, soil properties, and ERT data. We use a modified form of Archie's law to convert 72m long by 10m deep 2-D DERT profiles into time-series soil moisture profiles. Parameters included in the equation we use are: bulk modeled resistivity values, a proxy for pore water resistivity, and clay content. Preliminary results indicate that this soil moisture model can predict moisture values with one standard deviation in soil moisture of ± 6%. The ability to convert field scale ERT measurements into reasonable estimates of soil moisture is a valuable tool for modeling soil moisture distribution in 2-D soil profiles.