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SOIL PHYSICS

Temperature-Dependent Scaled Frequency: Improved Accuracy of Multisensor Capacitance Probes

^a Natural Resources & Environmental Management Dep., Univ. of Hawaii-Manoa, 1910 East West Rd., Honolulu, HI 96822
^b Molecular Bioscience and Bioengineering Dep., Univ. of Hawaii at Manoa, 1955 East West Rd., Honolulu, HI 96822

^* Corresponding author (AFares{at}Hawaii.edu).

The response of multisensor capacitance probes (MCPs) to water content depends on multiple soil properties including temperature. The goal of this study was to evaluate a new temperature-dependent scaled frequency algorithm to correct for the temperature effect on the performance of MCPs. Plastic columns with MCPs and thermocouples in the middle were filled with air, deionized water, or quartz sand at different water contents (0.0, 0.02, 0.04, 0.06, 0.08, 0.12 and 0.38 m³ m^–3) and placed in a water bath with temperature varying between 5 and 45°C. Scaled frequency (SF) readings in saturated sand (0.38 m³ m^–3) were negatively correlated with temperature. There were positive correlations observed, however, between isothermal SF readings and media temperature for air and unsaturated quartz sand. Temperature effects in the unsaturated sand decreased with increasing water content; observed SF for quartz sand at 0.0 and 0.12 m³ m^–3 water contents increased 58 and 4%, respectively, when temperature increased from 5 to 45°C. A hysteretic temperature effect was observed in all tested media. A new temperature-dependent SF calibration methodology that we developed mitigated the increase in apparent water content caused by the use of the isothermal SF calibration equation. Our experimental data indicate that quartz sand temperature effects on apparent volumetric water content measured with MCPs can be mitigated using this new temperature-dependent SF methodology.

Abbreviations: MCP, multisensor capacitance probe • SF, scaled frequency

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