Calibrating electromagnetic induction conductivities with time-domain reflectometry measurements

Giovanna Dragonetti,Nicola Lamaddalena,Ali Ajeel,Gian Piero Deidda,Giuseppe Rodriguez,Alessandro Comegna,Antonio Coppola,Giulio Vignoli

doi:10.5194/hess-22-1509-2018

Abstract

Abstract. This paper deals with the issue of monitoring the spatial distribution of bulk electrical conductivity, σb, in the soil root zone by using electromagnetic induction (EMI) sensors under different water and salinity conditions. To deduce the actual distribution of depth-specific σb from EMI apparent electrical conductivity (ECa) measurements, we inverted the data by using a regularized 1-D inversion procedure designed to manage nonlinear multiple EMI-depth responses. The inversion technique is based on the coupling of the damped Gauss–Newton method with truncated generalized singular value decomposition (TGSVD). The ill-posedness of the EMI data inversion is addressed by using a sharp stabilizer term in the objective function. This specific stabilizer promotes the reconstruction of blocky targets, thereby contributing to enhance the spatial resolution of the EMI results in the presence of sharp boundaries (otherwise smeared out after the application of more standard Occam-like regularization strategies searching for smooth solutions). Time-domain reflectometry (TDR) data are used as ground-truth data for calibration of the inversion results. An experimental field was divided into four transects 30 m long and 2.8 m wide, cultivated with green bean, and irrigated with water at two different salinity levels and using two different irrigation volumes. Clearly, this induces different salinity and water contents within the soil profiles. For each transect, 26 regularly spaced monitoring soundings (1 m apart) were selected for the collection of (i) Geonics EM-38 and (ii) Tektronix reflectometer data. Despite the original discrepancies in the EMI and TDR data, we found a significant correlation of the means and standard deviations of the two data series; in particular, after a low-pass spatial filtering of the TDR data. Based on these findings, this paper introduces a novel methodology to calibrate EMI-based electrical conductivities via TDR direct measurements. This calibration strategy consists of a linear mapping of the original inversion results into a new conductivity spatial distribution with the coefficients of the transformation uniquely based on the statistics of the two original measurement datasets (EMI and TDR conductivities).

Highlights

Soil water content and salinity vary in space both vertically and horizontally
We focus on the use of Time-domain reflectometry (TDR) data to absolutely calibrate the conductivities obtained by inverting the electromagnetic induction (EMI) measurements
The EMI result seems quite insensitive to TDR variability

Summary

Introduction

Soil water content and salinity vary in space both vertically and horizontally. Their distribution depends on management practices and on the complex nonlinear processes of soil water flow and solute transport, resulting in variable storages of solutes and water (Coppola et al, 2015). Monitoring the actual distribution of water and salts in the soil profile explored by roots is crucial for managing irrigation with saline water, while still maintaining an acceptable crop yield. For water and salt monitoring over large areas, there are non-. Dragonetti et al.: Calibrating EMI electrical conductivities with TDR measurements invasive techniques based on electromagnetic sensors which allow the bulk electrical conductivity of soils, σb, to be determined (Sheets and Hendrickx, 1995; Corwin and Lesch, 2005; Robinson et al, 2012; Doolittle and Brevik, 2014; Von Hebel et al, 2014, among many others)

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Conclusion