An in-situ methodology to separate the contribution of soil water content and salinity to EMI-based soil electrical conductivity

Abstract. Salt accumulation in the root zone limits agricultural productivity and can eventually lead to land abandonment. Therefore, monitoring the spatial distribution of soil water content and solution salinity is crucial for effective land and irrigation management. However, assessing soil water content and salinity at the field scale is often challenging due to the heterogeneity of soil properties.

Electromagnetic induction (EMI) offers a fast, non-invasive, in situ geophysical method to map spatial variability in soil. EMI instruments measure the apparent soil electrical conductivity (EC_a), which reflects the integrated contribution of the bulk electrical conductivity (σ_b) of different soil layers. By inverting the measured EC_a, it is possible to obtain the distribution of the σ_b along the soil profile, which provides indirect information on soil salinity. However, in saline soils, σ_b is influenced by both water content (θ) and soil solution electrical conductivity (σ_w) (the salinity), making it difficult to independently quantify these two variables through a single, straightforward procedure.

The objective of this study is to separate the respective contributions of θ and σ_w to σ_b, as obtained from the EMI inversion. To achieve this, EC_a was measured using a CMD-MiniExplorer instrument in two maize plots irrigated with saline and non-saline water, respectively, in an agricultural field in southern Italy. The dataset was then inverted in order to obtain the σ_b distribution. By employing a site-specific calibrated Rhoades linear model and assuming homogeneity between the two plots, the spatial distribution of θ and σ_w in the saline plot was successfully estimated. To validate the results, independent measurements of soil water content by Time Domain Reflectometry (TDR) and direct measurement of soil solution electrical conductivity, σ_w, were performed.

The proposed procedure enables the estimation of θ and σ_w with high accuracy along the soil profile, except in the soil surface, where EMI reliability is limited. These findings demonstrate that the integration of EMI with a site-specific θ - σ_b - σ_w model is a reliable and efficient in-situ approach for mapping soil salinity and water content at field scale, offering valuable insights for optimizing agricultural irrigation management in systems using saline water.