Widespread interest in soil water content (θ, m3 m−3) information for both management and research has led to the development of a variety of soil water content sensors. In most cases, critical issues related to sensor calibration and accuracy have received little independent study. We investigated the performance of the Hydra Probe soil water sensor with the following objectives: (i) quantify the inter-sensor variability, (ii) evaluate the applicability of data from two commonly used calibration methods, and (iii) develop and test two multi-soil calibration equations, one general, “default” calibration equation and a second calibration that incorporates the effects of soil properties. The largest deviation in the real component of the relative dielectric permittivity

\(\left({\epsilon}_{r}^{{^\prime}}\right)\)
determined with the Hydra Probe using 30 sensors in ethanol corresponded to a water content deviation of about 0.012 m3m−3, indicating that a single calibration could be generally applied. In layered (wet and dry) media, εr′ determined with the Hydra Probe was different from that in uniform media with the same water content. In uniform media, θ was a linear function of √εr′. We used this functional relationship to describe individual soil calibrations and the multi-soil calibrations. Individual soil calibrations varied independently of clay content but were correlated with dielectric loss. When applied to the 19-soil test data set, the general calibration outperformed manufacturer-supplied calibrations. The average θ difference, evaluated between
\(\mathrm{{\epsilon}}_{\mathrm{r}}^{{^\prime}}\ =\ 4\)
and
\(\mathrm{{\epsilon}}_{\mathrm{r}}^{{^\prime}}\ =\ 36\)
⁠, was 0.019 m3m−3 for the general equation and 0.013 m3m−3 for the loss-corrected equation.

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