Field and laboratory evaluation of the CS655 soil water content sensor

Authors: CALDWELL, T. - University Of Texas; BONGIOVANNI, T. - University Of Texas; Cosh, Michael; HALLEY, C. - University Of Texas; YOUNG, M. - University Of Texas

Submitted to: Vadose Zone Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/14/2018
Publication Date: 5/3/2018
Citation: Caldwell, T., Bongiovanni, T., Cosh, M.H., Halley, C., Young, M. 2018. Field and laboratory evaluation of the CS655 soil water content sensor. Vadose Zone Journal. 17:170214. https://doi.org/10.2136/vzj2017.12.0214.
DOI: https://doi.org/10.2136/vzj2017.12.0214

Interpretive Summary: The CS655 soil water content sensor is a relatively new, low-frequency electromagnetic sensor that determines relative permittivity (Ka) which can be used to estimate soil water content for a soil volume around two 12 cm probe rods. This sensor’s generalized calibration had a root mean square error from greater than 0.05 m3/m3 to less than 0.03 m3/m3 by various methods of soil specific calibration. This error is a critical threshold for field estimation, as most hydrologic applications need an error of less than 0.04 m3/m3. This calibration study is of importance to experimenters and network developers as it demonstrates the advantages of different calibration techniques with a new sensor.

Technical Abstract: Soil moisture sensors infer volumetric soil water content (SWC) from other properties of the bulk porous media. The CS655 water content reflectometer is a relatively new, low-frequency electromagnetic sensor that determines relative permittivity (Ka) using the two-way travel period and voltage attenuation of the applied signal along two 12-cm rods. This measured attenuation is linearly related to bulk electrical conductivity (EC). Along with an onboard thermistor, the CS655 allows for a more robust correction of propagation time and Ka, which its predecessors, the CS615 and CS616, lacked. However with new sensors, it is necessary to quantify their practical accuracy in the field. Here, we present an overview of the CS655 sensor and an evaluation under both laboratory and field conditions, using five surface soils (0-10 cm depth) in the laboratory and gravimetric samples collected in the field. Overall, a site-specific calibration using a two-term linearization of the SWC-Ka function reduced the root mean square error (RMSE) of the factory-derived SWC of 0.073 and 0.043 m3 m-3 during batch and infiltration experiments, respectively, to 0.025 and 0.028 m3 m-3. Results further indicate that a soil-specific calibration additionally reduced the RMSE to <0.02 m3 m-3. Field evaluation across the Texas Soil Observation Network found calibration reduced the variance across the network but did not affect the arithmetic mean or the RMSE against gravimetric sampling, which remained ~0.05 m3 m-3 regardless of the SWC–Ka–EC function applied. At the regional scale, a global calibration is sufficient.