Distinguishing Capillary Fringe Reflection in a GPR Profile for Precise Water Table Depth Estimation in a Boreal Podzolic Soil Field

Chameera Illawathure,Vanessa Kavanagh,Mumtaz Cheema,Lakshman Galagedara

doi:10.3390/w12061670

Chameera Illawathure, Vanessa Kavanagh + Show 2 more

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https://doi.org/10.3390/w12061670

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Abstract

Relative permittivity and soil moisture are highly correlated; therefore, the top boundary of saturated soil gives strong reflections in ground-penetrating radar (GPR) profiles. Conventionally in shallow groundwater systems, the first dominant reflection comes from the capillary fringe, followed by the actual water table. The objective of this study was to calibrate and validate a site-specific relationship between GPR-estimated depth to the capillary fringe (DCF) and measured water table depth (WTDm). Common midpoint (CMP) GPR surveys were carried out in order to estimate the average radar velocity, and common offset (CO) surveys were carried out to map the water table variability in the 2017 and 2018 growing seasons. Also, GPR sampling volume geometry with radar velocities in different soil layers was considered to support the CMP estimations. The regression model (R2 = 0.9778) between DCF and WTDm, developed for the site in 2017, was validated using data from 2018. A regression analysis between DCF and WTDm for the two growing seasons suggested an average capillary height of 0.741 m (R2 = 0.911, n = 16), which is compatible with the existing literature under similar soil conditions. The described method should be further developed over several growing seasons to encompass wider water table variability.

Highlights

Knowledge of the water table depth (WTD) is essential for understanding many environmental scenarios, including water management in agriculture
The ground penetrating radar (GPR) velocity is independent of the frequency and dependent only on the relative permittivity and the magnetic permeability [67]
The results from this study indicate that 250 MHz is suitable to examine shallow

Summary

Introduction

Knowledge of the water table depth (WTD) is essential for understanding many environmental scenarios, including water management in agriculture. The capillary fringe, sometimes called the transition zone, in the vadose zone mediates space, water, and nutrients for plants and soil organisms [2,3]. Both WTD and the depth to the top of the capillary fringe (DCF ) are subject to seasonal fluctuations, and can affect agricultural water management practices throughout the growing season. Sixteen 250 MHz GPR CO surveys (42 m in length, antenna separation = 0.38 m, sampling interval = 0.05 m, temporal sampling interval = 200 ps) and 16 CMP surveys (near the borehole) were performed in 2017 and 2018.

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