Calibrating and using an EM31 electromagnetic induction meter to estimate and map soil salinity in the jarrah and karri forests of south-western Australia

Abstract

Knowledge of the spatial distribution of soil salinity in areas of jarrah and karri forest where groundwater levels may be close to the surface is important when planning land use activities that may reduce leaf area and thus increase the risk of a rise in stream salinity. Hence this study aimed, firstly, to test whether a Geonics EM31 induction meter could be used to accurately estimate soil salinity in jarrah and karri forest soils. Secondly, to determine an appropriate method of surveying the stream zone with an EM31 to adequately represent the spatial variation in soil salinity. A comparison of EM31 meter estimates of soil apparent electrical conductivity (ECa) with average soil salinity profiles (EC 1:5) to 6 m depth across a broad range of sites resulted in good correlations in spring ( R 2 = 0.66, n = 164, p < 0.001) and autumn ( R 2 = 0.64, n = 156, p < 0.001). ECa measured in spring, when soil moisture was at its highest, was 25% higher than in autumn, when soil moisture was at its lowest. EM31 surveys along second-order stream zones showed that the highest ECa levels generally occurred within 10 m of the stream centre. However, ECa was highly variable along the length of the streams and the highest levels did not necessarily occur at the lowest part of the catchment. A suitable method for conducting surveys of stream zones is to carry an EM31, while walking along a traverse, taking a reading approximately every 2 m.