Miniaturized, Field-Deployable, Continuous Soil Water Potential Sensor

Abstract

Soil water potential is a significant factor in determining the dynamics of water in the soil. However, few soil water potential sensors are available to conduct long-term, continuous measurements with full automation, due to cavitation formation inside the sensors while interacting with the soil. This paper presents a miniature, field-deployable soil water potential sensor capable of real-time measurement with a wide dynamic range from 0 to −800 kPa, a minimum detectable change of water potential of ~40 Pa, and a high sensitivity of $\sim 0.248~\mu \text{A}$ /kPa. The sensor consists of a shallow water reservoir sandwiched between a nanoporous ceramic plate and a thin silicon diaphragm with thermal oxide. The nanoscale pores of the ceramic plate allow for the increase of air entry tension, while the smooth and hydrophilic interior surfaces of the water reservoir help to minimize the trapping of air bubbles in the reservoir. When the sensor is embedded in unsaturated soils, the pre-filled water in the reservoir tends to leave the reservoir through the nanopores of the ceramic plate, until an equilibrium in water potential is achieved between the reservoir and the soil. The loss of water leads to bending of the silicon-based diaphragm toward the reservoir. The displacement of the diaphragm is quantified by a miniature optical displacement detector assembled with the sensor, which corresponds to the soil water potential. The presented soil sensor has been validated through both greenhouse and field experiments to monitor dynamic changes in soil water potential in real-time over multiple days.