Repeated electromagnetic induction measurements for mapping soil moisture at the field scale: validation with data from a wireless soil moisture monitoring network

Abstract:

Among hydrogeophysical techniques, electromagnetic induction (EMI) measurements are widely used for environmental studies, as they allow fast and relatively low-cost surveys of soil apparent electrical conductivity (ECa). Although the use of non-invasive EMI for imaging spatial soil properties is very attractive, the dependence of ECa on several factors challenges any interpretation with respect to individual soil properties or states such as soil moisture (θ).

The major aim of this study was to further investigate the potential of repeated EMI measurements to map θ, with particular focus on the temporal variability of the spatial patterns of ECa and θ. To this end, we compared time-lapse EMI measurements with high-resolution θ data from a wireless soil moisture and soil temperature monitoring network for an extensively managed hillslope area (the Schäfertal hillslope site) for which soil properties and θ dynamics are known. Soil ECa were acquired following careful measurement and processing procedures which are presented in this contribution.

For the investigated site, i) ECa showed small temporal variations whereas θ varied from very dry to almost saturation; ii) temporal changes of the spatial pattern of ECa differed from those of the spatial pattern of θ; and iii) the ECa-θ relationship varied with time, regardless the moisture state. We discussed the results with respect to the physical processes that control the spatio-temporal dynamics of θ and to the factors affecting ECa. Results show that, for the Schäfertal hillslope site and generally for low conductive soils, i) stable soil patterns (i.e. textural heterogeneity) are the major control of ECa measured with EMI, and ii) the influence of θ on ECa may be confounded by changes of the electrical conductivity of the soil solution, which may become the dominant signal.

These findings are important for future applications of time-lapse EMI measurements to hydrological applications. Further, this study stimulates a thorough discussion about the complex interplay between factors controlling ECa and θ, and the use of EMI-based ECa data with respect to environmental studies which aim to estimate soil properties or states from the measured ECa.