Field Determination of Soil Moisture in the Root Zone by Multi-height EM38 Measurements

Electromagnetic induction sensors, such as EM38, are used widely for monitoring and mapping soil attributes. These sensors create and measure electric currents, induced at depth in response to an external time-varying (primary) magnetic field. The strength of the induced current is determined by the apparent electrical conductivity (ECa) of the soil, often driven by the moisture profile in the soil matrix. In this project the predicted electrical conductivity, calculated by combining the depth-response function of the EM38 unit with volumetric moisture content (VMC) measurements taken at various depths, were compared with the actual measured ECa by the EM38. The EM38 was calibrated with VMC in two different methods: a pit method and single core method at two depths of 0-1.2 m and 0-0.8 m. A strong linear relationship (R2 = 99%) was observed at each site between EM38 response (both vertical and horizontal mode) at different heights and predicted electrical conductivity. The pit method calibration outperformed the core method. The horizontal dipole measurements consistently provided superior results, explaining 99% of the variation to VMC in the pit method at 0-1.2 m depth.