Investigation of temperature effects and performance evaluation of a newly developed capacitance probe

Abstract

Capacitance probes are soil water monitoring device that have the advantages of high accuracy, low cost and ease of automation of data acquisition. This study was carried out to investigate the effect of temperature and to evaluate soil water measurement capability of a newly developed capacitance probes (HydraSCOUT). Undisturbed soil core samples were de-aired and saturated in a vacuum chamber at a suction pressure of −70 kPa. At saturation, the probes were inserted into the soil core, thereafter it was left to drain. The results shows that the response of the sensors of the capacitance probes at constant water content to temperature can be classified into 3 different stages: Constant stage (between 15 °C and 38 °C), with no temperature effect on the readings; Falling stage (between 3 °C and ≤15 °C), where the readings declined with temperature; Erratic stage (2 °C to ≤3 °C), where the readings had no pattern. Volumetric water contents were plotted against the sensors readings to develop calibration equations. Results indicated that calibration functions developed for the soils were linear (Clay loam) and polynomial (Sandy loam). Volumetric water content was well predicted with the soil specific equation for clay loam (RMSE of 0.001 m3 m−3 and D-index of 0.99) and Sandy loam (RMSE of 0.001 m3 m−3 and D-index of 0.99) soil textural classes. It was recommended that the soil specific laboratory calibration equations developed during this study should be used especially, for the Sandy loam soil type, in a situation where many probes are in use.