An Autonomous Sensor to Monitor In Situ Soil Moisture, Salinity, and Temperature Profiles for Wireless Networks

The in situ sensor we develop is aimed at monitoring continuously essential variables of the soil such as its moisture, salinity, and temperature at different horizons or depths up to 60 cm. Its capability permits to follow accurately water movements in soil resulting from such events as rain infiltration, evapotranspiration or intrusions of underground saltwater (Fig. 1). It is intended for industrial-scale duplication and deployment (series of several tens) (Chavanne and Frangi 2024). The instrument principle relies on soil permittivity to determine water content and salinity like many others. However, its unique measurement technique presents an output linear to the complex permittivity with a large resolution, not possible with other techniques. Conversions are based on physical laws to reach high accuracy and made instrument bias negligible. Hence are deduced soil real permittivity (for water molecules) and conductivity (for ions). Conversion of permittivity into soil water content should be based on established relations like Archie’s law or even Topp’s correlation, that are independent of the instrument. Soil temperature gradients are measured owing to thermocouples offering also unique precision and resolution without the need of a calibration. The device profits from recent advances in Information and Communication Technology, namely LoRaWAN, for autonomous operations and communication. Data are transferred over several km and made available in real time on Internet. Consumption is minimized to permit operations over a year without intervention at a sampling frequency as low as 5 min. Both rare but intense events and long term trends are captured. The technology allows equally to manage networks of several tens of sensors such as those in a catchment for mapping spatial variability. Sensor features a compact design with a water proof box of 12 x 12 cm2 area housing on ground all electronics and power supply, and connected to low-cost modular electrodes inserted into soil. Installation labor and invasiveness are minimized. Electronic simplification and integration on one board, as well as standardized components and procedures of assembly reduce costs and labors. Tight tolerances for key characteristics and a on-board reference limit time spent on quality control for each device of a series while maintaining a low sensor-to-sensor variability. Prototypes are operating presently at different observatories, some in mountains, to test their robustness and to identify further improvements before industrial duplication. They also start to provide data.