Evaluation of a low-cost soil water content sensor for wireless network applications

SummaryWireless sensor networks are a promising new in situ measurement technology for monitoring soil water content changes with a high spatial and temporal resolution for large areas. However, to realise sensor networks at the small basin scale (e.g. 500 sensors for an area of 25 ha), the costs for a single sensor have to be minimised. Furthermore, the sensor technique should be robust and operate with a low energy consumption to achieve a long operation time of the network. This paper evaluates a low-cost soil water content sensor (ECH2O probe model EC-5, Decagon Devices Inc., Pullman, WA) using laboratory as well as field experiments. The field experiment features a comparison of water content measurements of a forest soil at 5 cm depth using TDR and EC-5 sensors. The laboratory experiment is based on a standardized sensor characterisation methodology, which uses liquid standards with a known dielectric permittivity. The results of the laboratory experiment showed that the EC-5 sensor has good output voltage sensitivity below a permittivity of 40, but is less sensitive when permittivity is higher. The experiments also revealed a distinct dependence of the sensor reading on the applied supply voltage. Therefore, a function was obtained that allows the permittivity to be determined from the sensor reading and the supply voltage. Due to the higher frequency of the EC-5 sensor, conductivity effects were less pronounced compared to the older EC-20 sensor (also Decagon Devices Inc.). However, the EC-5 sensor reading was significantly influenced by temperature changes. The field experiment showed distinct differences between TDR and EC-5 measurements that could be explained to a large degree with the correction functions derived from the laboratory measurements. Remaining errors are possibly due to soil variability and discrepancies between measurement volume and installation depth. Overall, we conclude that the EC-5 sensor is suitable for wireless network applications. However, the results of this paper also suggest that temperature and electric conductivity effects on the sensor reading have to be compensated using appropriate correction functions.