Electrical Resistivity Sounding to Study Water Content Distribution in Heterogeneous Soils.

Electrical resistivity (ER) sounding is increasingly being used as non-invasive technique to reveal and map soil heterogeneity. The objective of this work was to assess ER sounding applicability to study soil water distribution in spatially heterogeneous soils. The 30x30-m study plot was located at the ARS Beltsville OPE3 site in Maryland. Soil was presented by six textural classes in the range from sandy loam to silty clay loam.  The relationship between ER and soil water content was studied in disturbed 80-cm³ soil samples taken at 10 depths with 20 cm increments at 12 locations. The ER was measured at field water content in samples uniformly packed into 4-electrode cells. Soil water contents (WC) and bulk densities (BD) were in the ranges from 0.096 to 0.403 cm³ cm^-3 and from 1.14 to 1.86 g cm^-3 respectively. Generally ER in soil samples decreased as WC increased, however experimental data did not fall onto a single curve. A regression tree algorithm was used to derive parameters affecting ER-WC relationship. They were BD, clay and silt content in the topsoil, and BD and sand content in the subsoil. The power equation with parameters calculated as linear regression of defined soil properties fitted experimental data adequately. The vertical electrical sounding (LandMapper ERM-02) was used to infer spatial distribution of soil resistivity along a 24-m transect for different dates when soil was dry and when it was relatively uniformly wetted with long low-intensity rainfall. The ER values measured with Wenner-Shlumberger array (50-electrodes spaced 50-cm apart) were processed using the RES2DINV software and corrected for field temperature. The 2-D WC distributions were obtained from the corrected ER values using the power equation with soil properties measured at different depths. Water content images revealed high spatial variability in soil water content with distinct vertical fingers representing dry zones.