Temperature Effects on Soil Dielectric Properties Measured at 50 MHz

Abstract

In recent years a number of soil water monitoring instruments have been developed and made commercially available. These instruments generally respond to the complex soil dielectric permittivity and operate at frequencies between 10 and 150 MHz. Although there is some evidence that these instruments are sensitive to temperature change in certain soils, little empirical data exists describing the degree of this sensitivity. We quantified temperature effects on both the real and imaginary components of the complex permittivity for 19 soils collected around the United States using the Hydra Probe soil water sensor, which operates at 50 MHz. We found that the real component response ranged from positive to negative such that the effect of a 40°C temperature change resulted in a maximum apparent water content change of ± 0.028 m3 m−3 among soils. The effect of temperature on the imaginary component was as much as six times greater than on the real component, changing about 2% °C−1, which is similar to that observed for electrical conductivity. The high imaginary component sensitivity to temperature is probably responsible for the high temperature sensitivity noted for commercial soil water sensors because they generally respond to a composite of both components. In addition, there was a strong linear correlation (R2 = 0.81) between the effect of temperature on the calculated soil water content and the magnitude of the imaginary component. While this relationship suggests the possibility of calculating temperature effects on Hydra Probe–calculated soil water content in the field, it applies only to saturated soil conditions at present.

Measurement of Soil Water Content with Dielectric Dispersion Frequency

“…Seyfried and Grant (2007) reported that temperature responses were both positive and negative for different soils. Figure 4 shows the permittivity traces at soil temperatures of 10 to 50°C for the Ida soil at a constant water content of 0.30 m 3 /m 3 .…”

A sensor array system for profiling moisture in unsaturated rock and soil

“…Temperature effects on TDR measurements is well documented, with dielectric permeability of bulk water increasing with temperature (e.g. Seyfried and Grant, 2007; Freidman and Robinson, 2002; Or and Wraith, 1999). Because the dielectric properties of solids and air are significantly less sensitive to temperature changes than water, the mixture of solid, air and water exhibit a temperature dependence similar to (but smaller than) that of free water (Pepin et al , 1995).…”

“…Various models have been developed to quantify the change in dielectric in response to temperature changes (e.g. Or and Wraith, 1999; Seyfried and Grant, 2007). As with the dielectric‐temperature relationship, the effects of temperature on electrical resistance has also been extensively documented.…”

Integration of hydrogeophysics and remote sensing with coupled hydrological models

“…Finally, we measured the apparent soil electrical conductivity of discrete, homogeneous soil horizons (EC s ) to check the consistency with the surface geophysics EC a , since the latter corresponds to a bulk measurement of the soil horizons EC s above the depth of penetration of the geophysical instrument. We measured the real ( r ) and imaginary (ε r ) dimensionless components of the soil complex dielectric permittivity with a portable Stevens Hydra Probe (Seyfried and Murdock, 2004;Seyfried and Grant, 2007) on the AB and C horizons at 10 vertical soil profiles (locations PD1 to PD10, Figure 2.2). EC s (S/m) is related to ε r as following:…”

“…A correction for temperature effect was considered as suggested by Seyfried and Grant (2007) in order to obtain standardized values of EC s at 25°C. The areas with a ratio of ε r /ε r >2 were identified as affected by soil salinity (Seyfried et al, 2005).…”