Location: Soil and Water Management Research
Title: Soil permittivity response to bulk electrical conductivity for selected soil water sensors Authors
Submitted to: Vadose Zone Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 16, 2013
Publication Date: March 22, 2013
Citation: Schwartz, R.C., Casanova, J.J., Pelletier, M.G., Evett, S.R., Baumhardt, R.L. 2013. Soil permittivity response to bulk electrical conductivity for selected soil water sensors. Vadose Zone Journal. 12(2): doi10.2136/vzj2012.0133. Interpretive Summary: Variations in bulk electrical conductivity can reduce the accuracy of soil water content measurements using electromagnetic sensors. These inaccuracies arise because of the dependency of the measured permittivity on both soil water content and bulk EC. We evaluated the dependence of measured permittivity on bulk EC in contrasting soils using time-domain reflectometry (TDR), a digital time-domain transmission (TDT) sensor, and a capacitance sensor in the laboratory. Permittivity measured using TDR was sensitive to bulk EC in the clay loam soil but not in the fine sand. The capacitance sensor exhibitted a nonlinear response of permittivity to bulk EC in both media. In contrast, permittivity measured using the TDT probe exhibited little or no sensitivity to bulk EC. Consideration of bulk EC variation is required to accurately estimate soil water content in fine-textured soils using TDR and both soils using the capacitance sensor.
Technical Abstract: Bulk electrical conductivity can dominate the low frequency dielectric loss spectrum in soils, masking changes in the real permittivity and causing errors in estimated water content. We examined the dependence of measured apparent permittivity (Ka) on bulk electrical conductivity in contrasting soils using time-domain reflectometry (TDR), a digital time-domain transmission (TDT) sensor, and a capacitance sensor (5TE) during near saturated solute displacement experiments. Sensors were installed in 0.2 m i.d. columns packed with fine sand or a clay loam soil. Displacement experiments were completed by first equilibrating columns with 1 mM CaCl2 (0.25 dS m**1), introducing a step pulse of about 23 mM CaCl2 (about 4.7 dS m**1) and, after equilibration, displacing the resident solution with 1 mM CaCl2. Using TDR, measured Ka increased bulk electrical conductivity, however the slope of this response averaged 3.47 m dS**1 for clay loam compared with 0.19 m dS**1 for sand. The large response in the clay loam was attributed to bound water relaxation losses that narrowed the effective bandwidth from 821 to an estimated 164 MHz. In contrast, the effective frequency in sand averaged 515 MHz. Permittivity measured using the TDT probe exhibited little or no sensitivity to bulk electrical conductivity (< 0.32 m dS**1) in both media. Measured Ka using the capacitance probe declined with increasing bulk electrical conductivity up to 1 to 1.8 dS m**1 and then increased thereafter with net negative responses for sand (change in permittivity with respect to change in bulk EC = -3.1 m dS**1) and net positive responses for the clay loam (change in permittivity with respect to change in bulk EC= 2.9 m dS**1). Consideration of the Ka-bulk electrical conductivity response is required for accurate soil water content estimation (+/- 0.03 m3 m**3) in the presence of solution EC variations using TDR in fine-textured soils or the 5TE sensor in all media. Large differences in the sampling volumes between 5TE-measured bulk EC and permittivity also confounded the Ka-bulk electrical conductivity response in the presence of a concentration gradient.