Performance assessment of factory and field calibrations for electromagnetic sensors in a loam soil

Accurate continuous measurements of temperature (T), apparent electrical conductivity (ECa), apparent dielectric permittivity (εra), and volumetric water content (θv) are extremely valuable to irrigation management and other agronomic decisions. The performance of eight electromagnetic (EM) sensors (TDR315, CS655, HydraProbe2, 5TE, EC5, CS616, Field Connect, AquaCheck), were analyzed through a field study in a loam soil. T, ECa, and εra were compared in reference to overall average among all sensors, and θv in reference to a neutron moisture meter (NMM). The reported T and ECa difference among the sensors were within 1 °C and 1 dS m−1, respectively, at 0.15 and 0.76 m depths. Among the single-sensor probes, the range of depth-combined (0.15 and 0.76 m) RMSD for factory calibration varied from 0.039 m3 m−3 (5TE) to 0.157 m3 m−3 (CS616). In comparison to single-sensor probes, RMSD of Field Connect at combined depths (0.30 and 0.51 m) was moderate (0.083 m3 m−3), and RMSD of AquaCheck at combined depths (0.30 and 0.61 m) was high (0.163 m3 m−3). Regression calibrations improved θv accuracy substantially beyond factory calibrations, as RMSD of the evaluated sensors except Field Connect was below 0.025 m3 m−3 using regression calibrations. The betterment in θv accuracy gained by using offset calibrations was smaller and less consistent than the improvements gained by using regression calibrations. The lower and upper bounds of the 95% confidence interval for mean RMSD of most sensors were below 0.02 and 0.04 m3 m−3, respectively, when using depth-specific offset calibrations. The relative success of offset calibrations for certain sensors in this study is encouraging and may signal new opportunities. Because much of the uncertainty in sensor-reported θv for the sensors under evaluation was systematic, future work should aim to develop universal calibrations or facilitate site-specific calibrations.