Repeated Electromagnetic Induction Measurements for Mapping Soil Moisture at the Field Scale: Comparison with Data from a Wireless Soil Moisture Monitoring Network

Abstract:

Electromagnetic induction (EMI) measurements offer great potential for field-scale mapping of various soil properties and states such as texture, organic carbon content or soil moisture (θ). Limitations to the use of EMI for estimating any of these properties exist, due to the ambiguous relationship between the measured apparent electrical conductivity (ECa) and the soil properties of interest. To further investigate the potential of EMI for field-scale mapping of θ, we conducted repeated EMI surveys during different hydrological states on a hillslope site where soil properties and θ dynamics were known in detail from a wireless soil moisture monitoring network. Repeated EMI measurements offered the potential to reveal the limits of applicability of the method. For the investigated site we found that i) ECa showed small temporal variations, whereas the range of soil moisture was very large; ii) temporal changes in spatial patterns of ECa differed from temporal changes in spatial patterns of soil moisture; and iii) the ECa-θ relationship varied with time, independent of both the moisture state (dry, intermediate or wet) and the hydrological regime (drying, stable or wetting). This suggests that, at the investigated site, θ has little influence on ECa. Because ECa and θ are predominantly controlled by the same factors and their relative importance changes over time, the ambiguous ECa-θ relationship changes over time, limiting the use of EMI for estimating θ.