Empirical Temperature Calibration of Capacitance Probes to Measure Soil Water

Dielectric sensors have been widely used for nondestructive determination of volumetric soil water content (θ, m 3 m ―3 ). Since the output of such sensors is affected by soil temperature (T, °C), the calibration for the effect is indispensable for accurate determination of θ. The objectives of this paper were (i) evaluation of the temperature effects on outputs of the commercial capacitance probes called ECH 2 O probes for various types of soils, and (ii) to include temperature in empirical calibration equations. Laboratory experiments were performed to obtain probe outputs at various T (5- 35°C) and θ (air-dry- near-saturation), using four soils and four probe models with different oscillation frequencies (5 and 70 MHz). The results showed that the outputs linearly responded to T at constant θ for all tested soil-probe combinations. The slope values of the linear responses to T depended on θ. The curves of the output-θ functions at a reference temperature (25°C) varied among the soils and probe models. A calibration equation describing the probe output as a function of θ and T was derived for each soil-probe combination by combining the output―θ function at the reference temperature and the slope―θ function. The derived calibration equations substantially reduced the temperature effects on the probe outputs for all soil-probe combinations. We also briefly considered the theoretical background of temperature effects on the probe outputs based on the results from the experiments and the properties of the soils tested. To demonstrate the importance of temperature calibration, the derived calibration equations were applied to two field observations from arid reasons.