Calibration of Temperature Effect on Dielectric Probes Using Time Series Field Data

Dielectric sensors have been widely used for nondestructive determination of volumetric soil water content (θ). Since the output of such sensors is influenced by soil temperature ( T ), the calibration for the dependence is indispensable for accurate determination of θ. The objectives of this study were (i) to apply an existing empirical temperature calibration method for capacitance probes to other types of commercial dielectric probes and (ii) to propose a new calibration method using time series of field data that requires no laboratory experiments for temperature calibration. Laboratory experiments were performed to obtain probe outputs at various T (5– 35°C) and θ (air dry–near saturation), using two soils and seven probes. From the results, a calibration equation describing the probe output as a function of θ and T was derived for every soil–probe combination and confirmed that this method is applicable not only to capacitance probes but also to impedance and TDR probes. Time series of field data (θ and T ) had been previously monitored with ECH 2 O EC-5 and EC-20 capacitance probes in the Loess Plateau, China. Hourly outputs from the probes and T at the 5-cm depth were analyzed, and calibration equations were derived using the responses of the probe outputs to daily fluctuation of T . The derived calibration equations from the field data were in good agreement with those from laboratory experiment for both probes. The equation was applied to field data and validly removed the effect of T on the probe output.