Temperature and Probe‐to‐Probe Variability Effects on the Performance of Capacitance Soil Moisture Sensors in an Oxisol

Abstract

Core Ideas Temperature variability and hysteresis and probe variability affect sensor measurements. Probe‐to‐probe variability is significant but random. Temperature variability is the major source of error in sensor performance. Magnitude of temperature‐induced error is a function of water content and sensor type. Dependent calibration functions could improve sensor measurement performance. Reliable and accurate monitoring of soil water content (θ) across the landscape is indispensable for many water resources applications. Capacitance‐based in situ soil water content measuring devices are extensively used despite their sensitivity to soil properties besides water content, e.g., temperature and organic matter content. The main goals of this study were to: (i) examine the effects of temperature, hysteresis of the temperature response, and probe‐to‐probe variability on the performance of three (5TE, EC‐5, and EC‐TM) single capacitance sensors (SCS) in a Hawaiian Oxisol; and (ii) develop empirical calibration equations to correct for temperature and improve measurement accuracy. The SCS raw output and thermocouple temperature measurements were recorded at 1‐min intervals during heating and cooling cycles between 1 and 45°C. The three SCS and thermocouples were inserted in uniformly packed soils with θ varying from 0 to 0.55 m3 m−3. We used three probes for each SCS, and the entire experiment was replicated with two heating and cooling cycles. Temperature, hysteresis, and the probe‐to‐probe variability effects were highly significant (p < 0.05) for all three SCS. Estimated θ using soil‐specific calibrations at 25°C significantly increased with increasing temperature for all SCS. The 5TE sensor showed increasing temperature sensitivity with increasing water content. However, the EC‐5 and EC‐TM sensors exhibited a bidirectional response to temperature, with the highest sensitivity at ∼0.10 m3 m−3 water content. An empirically derived temperature‐dependent calibration equation substantially reduced the variability (>90% reduction in interquartile range) in measured water content due to changing soil temperature. Applying differing temperature corrections for heating and cooling did not improve the calibration any further.