Temperature-Corrected Calibration of GS3 and TEROS-12 Soil Water Content Sensors.

The continuous monitoring of soil water content is commonly carried out using low-frequency capacitance sensors that require a site-specific calibration to relate sensor readings to apparent dielectric bulk permittivity ( K b ) and soil water content ( θ ). In fine-textured soils, the conversion of K b to θ is still challenging due to temperature effects on the bound water fraction associated with clay mineral surfaces, which is disregarded in factory calibrations. Here, a multi-point calibration approach accounts for temperature effects on two soils with medium to high clay content. A calibration strategy was developed using repacked soil samples in which the K b - θ relationship was determined for temperature ( T ) steps from 10 to 40 °C. This approach was tested using the GS3 and TEROS-12 sensors (METER Group, Inc. Pullman, WA, USA; formerly Decagon Devices). K b is influenced by T in both soils with contrasting T - K b relationships. The measured data were fitted using a linear function θ = a Kb + b with temperature-dependent coefficients a and b . The slope, a ( T ), and intercept, b ( T ), of the loam soil were different from the ones of the clay soil. The consideration of a temperature correction resulted in low RMSE values, ranging from 0.007 to 0.033 cm 3 cm -3 , which were lower than the RMSE values obtained from factory calibration (0.046 to 0.11 cm 3 cm -3 ). However, each experiment was replicated only twice using two different sensors. Sensor-to-sensor variability effects were thus ignored in this study and will be systematically investigated in a future study. Finally, the applicability of the proposed calibration method was tested at two experimental sites. The spatial-average θ from a network of GS3 sensors based on the new calibration fairly agreed with the independent area-wide θ from the Cosmic Ray Neutron Sensor (CRNS). This study provided a temperature-corrected calibration to increase the accuracy of commercial sensors, especially under dry conditions, at two experimental sites.