TIME-LAPSE MONITORING OF VARYING SOIL WATER SATURATION AFTER PRECIPITATION USING ELECTROMAGNETIC IMAGING

Soil biogeochemical functioning has been shown to depend on the soil water saturation which vary both spatially and temporally in response to wetting and drying cycles. Understanding such variation is important in capturing hydrological and biogeochemical dynamics in earth system models at large spatial scales. In situ sensors like time-domain reflectometers (TDR) and frequency-domain reflectometers (FDR) provide soil water measurements with high temporal resolution but are expensive, invasive, and time-consuming with limited spatial coverage. Electromagnetic induction (EMI) and electrical resistivity instruments are useful alternatives for delineating the spatiotemporal variation of soil moisture content because changes in soil water potential create electrokinetic gradient. However, previous work that used EMI and electrical resistivity tomography (ERT) to monitor soil moisture content mostly ignored the contribution of surface conduction at the grain-water interface. This study assessed the contributions of surface conductivity in the use of time-lapse electrical conductivity for quantifying changes in soil moisture content using field and laboratory measurements of soil electrical conductivity. This was done by characterizing the soil using 20 undisturbed soil cores taken at 3.5m depth to determine the porosity, permeability, formation factor and surface conductivity. Using EMagPy, inversion of a 5-day time-lapse bulk apparent electrical conductivity (ECa) data was done during and after a drip irrigation experiment simulating rainfall. The preliminary results showed that around the depth of 1m, there was more than 50% increase in the true electrical conductivity of the soil. It is hoped that the effect of the surface conductivity will improve a quantitative estimation of the soil saturation across the survey area. This would be of specific benefit to environmental scientists interested in quantifying spatial variation in soil water content in response to precipitation and evapotranspiration.