Investigation of non-unique relationship between soil electrical conductivity and water content due to drying-wetting rate using TDR

The characterization of the relationship between volumetric soil water content (θ) and bulk electrical conductivity (EC) (or electrical resistivity) is of high interest in fields of engineering, fundamental petrophysical, hydrological model parameterization, and quantitative hydrogeophysical interpretations. Based on the relationship, for instance, the broad-area image using electrical resistivity tomography (ERT) can be transformed into θ tomograms as initial/boundary conditions of the slope stability analysis. However, the relationship is affected by the hysteresis, leading the inconsistency in drying-wetting circles. Therefore, this study focused on the factor of soil drying-wetting rate using the proposed time domain reflectometry (TDR) method and related sensors for such characterization. A modified pressure plate apparatus enhanced by TDR was first employed with a relatively low rate in which EC and θ were measured simultaneously, without evident non-unique relationship. A TDR penetrometer was then designed for acquiring the temporal variations of θ and EC in the field. On the basis of certain assumptions, observations showed an apparent dependency of the slope of the θ–EC rating curve on rates, but no hysteresis pattern. Finally, a laboratory controlled fast wetting–drying cell was used to increase the response time, proving that both the hysteresis and slopes of the θ–EC rating curves were obviously affected by the soil drying-wetting rate, as a significant finding to the related topics.