Spatial variability of soil water content and soil electrical conductivity across scales derived from Electromagnetic Induction and Time Domain Reflectometry

Quick, reliable and accurate estimates of soil water content (SWC) at intermediate (slope) to larger scale (catchment) are important for understanding hydrological processes and may be provided by electromagnetic induction (EMI). EMI measures the apparent electrical conductivity of the subsurface (ECapp) which represents a depth weighted average value of the bulk soil electrical conductivity (ECb). The relation between ECb and SWC has generally been investigated in soil cores or using local measurements of SWC and ECb. Studies that investigated the relation between ECapp measured with EMI and SWC in considerably larger and internally more heterogeneous support volumes are far scarcer and cover a limited range of environments with a limited range of factors contributing to ECapp. This study developed a new calibration method to obtain quantitative estimates of SWC using EMI measured ECapp data in a sub-tropical region in Southern Brazil at sites with different soil properties. SWC and ECb were measured in soil pits with Time Domain Reflectometry (TDR) probes. Collocated ECapp was simultaneously measured with EMI using different coil separations and orientations to measure over increasing sensing volume. EMI measured ECapp data were first calibrated against calculated ECapp, which were derived from ECb profiles inserted in an exact EMI forward model. A depth averaged SWC (SWCavg) was calculated and different calibrations that relate ECapp to SWCavg were evaluated. ECapp measurements of the deeper sensing coil configurations could predict best the variability of SWCavg using a non-linear relation. Spatio-temporal variations of pore water electrical conductivity (ECw) were found to be an important cofounding factor. Temporal variations of ECw and the small temporal variability of SWCavg prevented the prediction of temporal variability of SWCavg using ECapp measurements. Overall, the combination of both calibration steps resulted in the description of 83% of the spatial variability of SWCavg from ECapp measurements.