A new universal dielectric mixing model and field calibration technique for measuring liquid water content in unfrozen and frozen soils with dielectric probes
Dielectric moisture probes used for continuous measurement of soil moisture content face two challenges: a) the limitations of the probe to accurately measure the soil bulk dielectric constant; and b) difficulty obtaining reliable and representative direct observations to calibrate the probes. Furthermore, the uncertainty associated with dielectric instruments in frozen soils due to the growth of ice in the soil pore is poorly understood. In this study, we propose a universal dielectric mixing model for calibrating dielectric moisture probes. This model is parsimonious and accounts for errors in the measured bulk dielectric permittivity due to salinity, and clay. Our new model performed better and provided more physically meaningful parameters than manufacturer’s supplied calibration. The model was extended to frozen soils to show that dielectric instruments have small uncertainty in measuring the liquid moisture content, proving that dielectric instruments can approximately measure the liquid water content in frozen soils. The study highlighted the relevance of accounting for spatial heterogeneity when calibrating dielectric moisture probes in the field by taking simultaneous bulk dielectric permittivity measurements (using the same dielectric probe as is being calibrated). The results of the study also showed that taking few samples can be misinformative and can lead to bigger errors in the resulting moisture content. We show that it is better not to calibrate the probe than to calibrate it with too few data points.