Field Determination of Soil Moisture in the Root Zone of Deep Vertosols Using EM38 Measurements: Calibration and Application Issues

Abstract

Electromagnetic induction sensors, such as the Geonics EM38, are used widely for monitoring and mapping soil attributes via the apparent electrical conductivity (EC_a) of the soil. The sensor response is the depth-integrated combination of the depth–response function of the EM38 and the local electrical conductivity EC_a(z). In deep Vertosols, assuming that the depth–response function is not perturbed by the soil and that the volumetric moisture content θ(z) dominates EC_a(z), the EM38 should be capable of predicting θ(z). A multi-height EM38 experiment was conducted over deep Vertosols to confirm the validity of the EM38 depth–response function, to test the hypothesis that the EM38 response was an additive combination of its depth–response function and θ(z), and to investigate if on-ground EC_a measurements could estimate average θ within the root zone. A simple model, involving mathematical summation of measured θ(z) from sectioned ‘calibration cores’ and the EM38’s known depth–response function, was found to explain 87 and 83% of the variance in measured EC_a for both horizontal and vertical dipole configurations, respectively. This included all data acquired at multiple sensor heights above the ground. However, a subsequent comparison of on-ground, EM38-derived EC_a and average θ from surface to 0.8 m ($\bar \theta _{{\textrm{0}}{\textrm{.8}}}$) and surface to 1.2 m ($\bar \theta _{{\textrm{1}}{\textrm{.2}}}$) demonstrated that $\bar \theta _{{\textrm{0}}{\textrm{.8}}}$ and $\bar \theta _{{\textrm{1}}{\textrm{.2}}}$ explained only 37 and 46% of the variance in EC_a for vertical dipole configuration measurements, compared to 55 and 56% of the variance for horizontal dipole configuration measurements. This result can be attributed to the small depth-specific changes in the EC_a(z) and θ(z) relationship and the limited proportion of the depth–response function of the EM38 interacting with the soil volumes investigated. Whereas the best calibration over these depth ranges was achieved using a horizontal dipole configuration, further improvements in both dipole orientations might be achieved by calibrating, then deploying, the sensors while they are elevated tens of centimetres above the ground.