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ORIGINAL RESEARCH

Temperature Effects on Soil Dielectric Properties Measured at 50 MHz

^a USDA-ARS-NWRC, 800 Park Blvd., Boise, ID 93712
^b 3310 Bren School of Environmental Science and Management, Univ. of California, Santa Barbara, CA 93106-5131. Mention of manufactures is for the convenience of the reader only and implies no endorsement on the part of the authors or the USDA
^* Corresponding author (mseyfrie{at}nwrc.ars.usda.gov).

Received 30 December 2006.

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.

Abbreviations: TDR, time domain reflectometry