Assessing soil salinity dynamics using time-lapse electromagnetic conductivity imaging

Maria Catarina Paz,Mohammad Farzamian,Fernando Monteiro Santos,Nádia Luísa Castanheira,Ana Marta Paz,Maria Conceição Gonçalves

doi:10.5194/soil-6-499-2020

Maria Catarina Paz, Mohammad Farzamian + Show 4 more

Open Access

https://doi.org/10.5194/soil-6-499-2020

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Abstract

Abstract. Lezíria Grande de Vila Franca de Xira, located in Portugal, is an important agricultural system where soil faces the risk of salinization due to climate change, as the level and salinity of groundwater are likely to increase as a result of the rise of the sea water level and consequently of the estuary. These changes can also affect the salinity of the irrigation water which is collected upstream of the estuary. Soil salinity can be assessed over large areas by the following rationale: (1) use of electromagnetic induction (EMI) to measure the soil apparent electrical conductivity (ECa, mS m−1); (2) inversion of ECa to obtain electromagnetic conductivity imaging (EMCI) which provides the spatial distribution of the soil electrical conductivity (σ, mS m−1); (3) calibration process consisting of a regression between σ and the electrical conductivity of the saturated soil paste extract (ECe, dS m−1), used as a proxy for soil salinity; and (4) conversion of EMCI into salinity cross sections using the obtained calibration equation. In this study, EMI surveys and soil sampling were carried out between May 2017 and October 2018 at four locations with different salinity levels across the study area of Lezíria de Vila Franca. A previously developed regional calibration was used for predicting ECe from EMCI. Using time-lapse EMCI data, this study aims (1) to evaluate the ability of the regional calibration to predict soil salinity and (2) to perform a preliminary qualitative analysis of soil salinity dynamics in the study area. The validation analysis showed that ECe was predicted with a root mean square error (RMSE) of 3.14 dS m−1 in a range of 52.35 dS m−1, slightly overestimated (−1.23 dS m−1), with a strong Lin's concordance correlation coefficient (CCC) of 0.94 and high linearity between measured and predicted data (R2=0.88). It was also observed that the prediction ability of the regional calibration is more influenced by spatial variability of data than temporal variability of data. Soil salinity cross sections were generated for each date and location of data collection, revealing qualitative salinity fluctuations related to the input of salts and water either through irrigation, precipitation, or level and salinity of groundwater. Time-lapse EMCI is developing into a valid methodology for evaluating the risk of soil salinization, so it can further support the evaluation and adoption of proper agricultural management strategies, especially in irrigated areas, where continuous monitoring of soil salinity dynamics is required.

Highlights

Lezíria Grande de Vila Franca de Xira is an important agricultural system of alluvial origin located by the estuary of the Tejo River, north-east of Lisbon, Portugal (Fig. 1), where soil faces risk of salinization due to the marine origin of part of the sediments, tidal influence of the estuary, irrigation practices, and projected evolution of future climate with increasing temperature and decreasing precipitation
Electromagnetic induction (EMI) measures the apparent electrical conductivity of the soil (ECa, mS m−1), which is a function of soil properties such as salinity, texture, cation exchange capacity, water content, and temperature
Paz et al.: Assessing soil salinity dynamics other hand, the statistical indicators discriminated by date of measurement, shown in Table 2, reveal that the prediction ability does not vary significantly when comparing the statistical indicators of the three dates

Summary

Introduction

Lezíria Grande de Vila Franca de Xira (hereafter called Lezíria de Vila Franca) is an important agricultural system of alluvial origin located by the estuary of the Tejo River, north-east of Lisbon, Portugal (Fig. 1), where soil faces risk of salinization due to the marine origin of part of the sediments, tidal influence of the estuary, irrigation practices, and projected evolution of future climate with increasing temperature and decreasing precipitation. EMCI can be converted to a cross section of the soil properties which show strong correlation with σ This methodology has been applied in Lezíria de Vila Franca to study soil salinity risk (Farzamian et al, 2019; Paz et al, 2019b) and salinity and sodicity risk (Paz et al, 2019a). In this later study, the authors performed a principal component analysis of the soil properties in the study area and found that the water content was correlated with σ but with a relatively lower influence when compared to the properties related to salinity and sodicity (ECe, sodium adsorption ratio, and exchangeable sodium percentage)

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