Evaluation of soil water content and bulk electrical conductivity across the US Climate Reference Network using two electromagnetic sensors

Soil bulk electrical conductivity (BEC) was evaluated alongside soil volumetric water content (VWC) and soil temperature measurements using the HydraProbe (model HydraProbe, Stevens Water Monitoring Systems, Inc.) (hereafter called HP) with accuracy range of BEC <= 0.3 S m-1, and the time domain reflectometry (TDR)-315L Probe (model TDR-315L, Acclima, Inc.) (hereafter called AP) suitable for BEC up to 0.6 S m-1, at 23 stations of the U.S. Climate Reference Network. Previous evaluations revealed inconsistent performance of both sensors in some clay soils using manufacturer-recommended calibrations in converting dielectric permittivity measurements to VWC. Here, we found that hourly values of BEC reached 0.6 S m-1 in high clay content soils and exceeded 2 S m-1 in high saline soils, and these high values of BEC were associated with poor performance and failures of both HP and AP sensors. Large values of BEC occurred in predominantly saturated soils where VWC values reached about 0.5 m3 m-3 for saline soils and about 0.7 m3 m-3 for clay soils, while low magnitudes of BEC were associated with low soil water content and seldomly saturated soils. Low hourly BEC values of less than 0.1 S m-1 were observed in wide variety of soil types, where sensor performance was typically excellent. The most influential factor on BEC was high soil water content conditions. Although dielectric permittivity measurements in estimating the soil water content were sensitive to BEC as some high clay content and high salinity soils increased BEC, the impact of large BEC on dielectric permittivity measurements was smaller in the well-drained top soil layers than in deep soil layers that remained near saturation. Soil temperature had only a small impact on BEC. With high clay content and high salinity, the specific area of clay minerals was also associated with the magnitude of BEC. Estimated soil bulk electrical conductivity (BEC) and soil volumetric water content (VWC) at soil depths of 5, 10, 20, 50, and 100 cm were evaluated across 23 US Climate Reference Network stations. The estimation of BEC and VWC was based on the use of the 50 MHz HydraProbe and the Acclima 1 GHz time-domain reflectometry (TDR)-315L. The evaluation suggested that the clay type, specific surface area, clay content, soil salinity, and soil temperature combined to affect how soil BEC increased and decreased with soil water content.