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

Electromagnetic induction (EMI) measurements are widely used for soil mapping, 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 (theta). The major aim of this study was to further investigate the potential of repeated EMI measurements to map theta, with particular focus on the temporal variability of the spatial patterns of ECa and theta. To this end, we compared repeated EMI measurements with high-resolution theta data from a wireless soil moisture and soil temperature monitoring network for an extensively managed hillslope area for which soil properties and theta dynamics are known. For the investigated site, (i) ECa showed small temporal variations whereas theta varied from very dry to almost saturation, (ii) temporal changes of the spatial pattern of ECa differed from those of the spatial pattern of theta, and (iii) the ECa-theta relationship varied with time. Results suggest that (i) depending upon site characteristics, stable soil properties can be the major control of ECa measured with EMI, and (ii) for soils with low clay content, the influence of theta on ECa may be confounded by changes of the electrical conductivity of the soil solution. Further, this study discusses the complex interplay between factors controlling ECa and theta, and the use of EMI-based ECa data with respect to hydrological applications.