Accuracy calibration and evaluation of capacitance-based soil moisture sensors for a variety of soil properties

Accurate measurement of soil moisture ( θ ) is key to hydrology and agriculture research. Soil moisture sensor technology is the predominant method for measuring θ , and such measurements are used as a standard for evaluating results from remote sensing and data assimilation . Therefore, improving the θ measurement accuracy of soil moisture sensors is of great significance. This study used the capacitance-based soil moisture sensor (5TM, Decagon Devices, Inc.) as an example to illustrate the necessity of calibration. The 5TM soil moisture sensor calculates θ by measuring the dielectric constant ( ε ) of the soil medium, and ε is affected by soil properties (texture, salinity , soil organic matter , etc.). Consequently, a common calibration model (CCM) was developed to calibrate θ from the 5TM sensor by incorporating the soil properties using soil samples collected from 17 sites in 13 provinces in China at 4 different depths. First, the θ change experiments were conducted in the laboratory for each soil sample. Second, a linear calibration model (LCM) was applied to calibrate the 5TM measured soil moisture ( θ 5 TM ) based on “true” soil moisture ( θ true ) assessed through the gravimetric method. The results indicated (1) high correlation coefficient ( R = 0.95 ) was found for θ 5 TM and θ true , but with a high root mean square error ( RMSE ) of 0.051 m 3 m − 3 , and a more significant underestimation with increasing θ ; (2) LCM calibration results ( θ LCM ) showed a higher R (0.99) and a lower RMSE (0.017 m 3 m − 3 ). Finally, the CCM was established through relating the LCM coefficients ( a LCM and b LCM ) and soil properties based on multiple regression, with RMSE of 0.126 m 3 m − 3 and 0.023 m 3 m − 3 for a CCM and b CCM respectively. The CCM calibrated result ( θ CCM ) showed an RMSE of 0.02 m 3 m − 3 and R of 0.98. CCM can almost replace LCM in terms of similar accuracy. In this study, a CCM for soil moisture sensors is proposed, which provides a new approach for soil moisture sensor calibration . • A soil moisture sensor calibration model based on soil properties is proposed. • The accuracy of soil moisture senor is improved. • Soil properties-based common calibration model is suitable for different soil properties and soil types.