The Influence of Temperature on the Performance of Dielectric Soil Moisture Probes

The increased use of new inexpensive and energy efficient soil water probes for continuous and long term monitoring in sensor networks is likely to boost the current understanding of soil moisture dynamics and its variability at catchment scale. We monitored water content with custom-made time domain reflectometry (TDR) probes and commercial capacitance probes (ECH2O-EC5) in packed soil columns in a temperature controlled environment. The temperature effects on capacitance probes and TDR probes were registered and compared. A soil specific calibration of the capacitance probes was initially performed in order to best match the readings in selected soil types. Soil columns with three different soil types (coarse sand, loamy sand, and sandy loam) each at 4 different water contents were equipped with soil water probes, temperature probes and tensiometers. The effects of temperature on the soil water probes were monitored during a 96 hour cycle ranging from 2°C up to 36°C. During the measurement cycle, data on temperature, relative permittivity (dielectric constant), and soil matric potential were continuously measured. The results supported that the permittivity is expected to increase with increasing temperature for fine soils at low water content because of the higher relative amount of bound water. For course soils at high water content, a negative relation between permittivity and temperature was therefore expected, which is in line with effects measured in pure water. The controlling factor considering temperature dependency on permittivity readings with dielectric probes is the competing effect on bound water and on bulk water. Existing calibration models relating permittivity to soil water content and correcting for temperature effects have been used. Recommendations for soil and probe specific calibrations of the capacitance probes are presented. The results of this investigation will be applied to support the installation of a planned soil moisture sensor network at sub-catchment scale.