An improved model and field calibration technique for measuring liquid water content in unfrozen and frozen soils with dielectric probes

Abstract

Dielectric soil moisture probes can be used to obtain frequently logged field observations of volumetric water content, but because this is an indirect method, it is challenging to ensure that you have a suitable calibration relationship and parameters. Challenges are associated with (a) limitations in the probes' ability to accurately measure the soil bulk dielectric constant and (b) limitations in obtaining reliable direct observations to calibrate the probe against, due to the spatial heterogeneity in the field. Furthermore, in soils that freeze, we do not have a robust approach to account for the effect of ice on the instrument. In this study, we propose a calibration relationship for all dielectric probes that is physically based, parsimonious, accounts for errors in the bulk dielectric constant, and quantifies the uncertainty in the liquid water content caused by the presence of ice. We show that our relationship has a better performance and more realistic parameter values than existing approaches. To calibrate a dielectric probe in the field, we show that it is necessary to account for spatial heterogeneity in samples taken for calibration. There is value in taking simultaneous measurements of the bulk dielectric constant (using the same dielectric probe as is being calibrated) and gravimetric samples for water content at a number of points in space. Taking too few samples is likely to be misinformative-we show that it is better to not calibrate the probe at all than to calibrate it with too few data points.

Recent Advances in Dielectric Properties-Based Soil Water Content Measurements

“…To evaluate the uncertainty limit of a dielectric probe to detect the liquid water content in variably frozen soils, the dielectric mixing model has been extended to frozen circumstances [102]. To describe the characteristic curves of soil freezing, this tiny uncertainty limit is helpful, but due to the regional diversity of soil parameters, the field calibration of dielectric devices presents challenges [71,110].…”

“…Depending on the application, one may choose to field-calibrate. Amankwah et al [102] further plotted the mixing models with the most sensitive parameter, α, which influences the intercept of the relationship for the absolute water content, as seen in Figure 6. Nevertheless, ε s and n are insensitive to variations in water content.…”

“…By simulating an uncharged silica surface, Kargas et al [37] discovered that surfaces can impede the ability of water molecules to rotate up to 10 nm away. The orientation of the dipole is changed by the structural arrangement of the confined water, not by electrostatic forces caused by the hydration of ions or negatively charged surfaces [102]. There is still disagreement about whether the drop in the dielectric profile is a reflection of the intrinsic or ionic characteristics of the water.…”