Development and Performance Evaluation of a Sensor for Monitoring Simultaneously Soil Water Content and Electrical Conductivity

Objective This study aim was to develop a low-cost sensor capable of synchronous and high-precision monitoring soil moisture and salinity, and to systematically compare its performance with several domestic and international sensors, thereby to provide technical support for the optimization of Chinese-made sensors and their application in large-scale ecological monitoring.

Method Utilizing the standing wave ratio method, a sensor (SM926) capable of high-precision, real-time monitoring and transmission of soil water and salt content were developed by independently resolving the real and imaginary components of the complex dielectric constant, combined with a multi-parameter calibration approach. Its performance was systematically evaluated against other sensors.

Result The measurement accuracy of SM926 for soil volumetric water content (θ_v) and apparent electrical conductivity (σ_a) was better than 0.02 cm³ cm⁻³ and 0.3 dS m⁻¹. Under indoor conditions, a comparative accuracy test involving three international sensors (CS655, TEROS 12, Hydra) and three domestic sensors (SMET-3, WHXPH, NH148) showed that SM926 performed optimally overall. Under indoor conditions of varying θ_v, σ_a, and temperature, comparisons between SM926 and the six other sensors revealed SM926 maintained stable and high accuracy (RMSE < 0.035 cm³ cm⁻³ and < 0.55 dS m⁻¹), under varying θ_v (0.1 - 0.4 cm³ cm⁻³), salinity (0.6 - 4 dS m⁻¹, low to medium levels), and temperature (0 to 40℃) conditions. Under the same conditions, the accuracy of SM926 was comparable to that of Hydra; its θ_v measurement accuracy was comparable to or slightly lower than that of TEROS 12 and CS655, but its σ_a measurement accuracy was higher than that of the latter two. Compared with WHXPH and SMET-3, SM926 exhibited better θ_v measurement accuracy and slightly lower temperature sensitivity in σ_a measurement. Moreover, SM926 outperformed NH148 in both θ_v and σ_a measurement accuracy. Under field conditions, the accuracy of SM926 was similar to that of Hydra and superior to that of WHXPH.

Conclusion SM926 provides a reliable solution for the stable monitoring of θ_v and σ_a dynamics under low to medium salinity. For deployment across large and heterogeneous areas, site-specific calibration is recommended to ensure sustained high accuracy.