Soil Water Content Estimation Using High-Frequency Ground Penetrating Radar

Ligang Zhou,Xiangdong Wang,Dongsheng Yu,Zhaoyan Wang

doi:10.3390/w11051036

Abstract

The rapid high-precision and nondestructive determination of shallow soil water content (SWC) is of vital importance to precision agriculture and water resource management. However, the low-frequency ground penetrating radar (GPR) technology currently in use is insufficient for precisely determining the shallow SWC. Therefore, it is essential to develop and use a high-precision detection technology to determine SWC. In this paper, a laboratory study was conducted to evaluate the use of a high-frequency GPR antenna to determine the SWC of loamy sand, clay, and silty loam. We collected soil samples (0–20 cm) of six soil types of loamy sand, clay, and silty loam and used a high-frequency (2-GHz) GPR antenna to determine the SWC. In addition, we obtained GPR data and images as well as characteristic parameters of the electromagnetic spectrum and analyzed the quantitative relationship with SWC. The GPR reflection two-way travel times and the known depths of reflectors were used to calculate the average soil dielectric permittivities above the reflectors and establish a spatial relationship between the soil dielectric permittivity ( ε ) and SWC ( θ ), which was used to estimate the depth-averaged SWC. The results show that the SWC, which affects the attenuation of wave energy and the wave velocity of the GPR signal, is a dominant factor affecting the soil dielectric permittivity. In addition, the conductivity, magnetic soil, soil texture, soil organic matter, and soil temperature have substantial effects on the soil dielectric permittivity, which consequentially affects the prediction of SWC. The correlation coefficients R2 of the “ θ ~ ε ” cubic curve models, which were used to fit the relationships between the soil dielectric permittivity ( ε ) and SWC ( θ ), were greater than 0.89, and the root-mean-square errors were less than 2.9%, which demonstrate that high-frequency GPR technology can be applied to determine shallow SWC under variable hydrological conditions.

Highlights

The shallow soil water content (SWC) is the water in the upper 20 cm of soil
Taking the red sandstone soil as an example, the x-axis represents the distance traveled by the ground penetrating radar (GPR) shield antenna, and the y-axis represents the two-way travel time (TWTT) of the radar wave (Figure 5)
The overall characteristics are similar, a slight but visible difference between each plot can be noted; the TWTT increases with increasing SWC at the same location (Figure 5); the returning signal from the bottom is delayed at a higher SWC because of the slower propagation of electromagnetic waves in the medium

Summary

Introduction

The shallow soil water content (SWC) is the water in the upper 20 cm of soil. Compared with the total amount of water on the global scale, this thin layer of soil water may appear to be insignificant; it is of fundamental importance to many hydrological, biological, and biogeochemical processes. The rapid, high-precision, and nondestructive determination of shallow SWC is of vital importance in developing precision agriculture and preventing slope soil erosion [1]. The SWC controls plant growth and crop quality [3]. Crop quality can decrease due to the adverse effects of waterlogged plant roots. Agronomists and farmers need information about shallow SWC variability at both spatial and temporal scales in cultivated regions to manage irrigation practices effectively [5,6]. Variations in soil texture, topography, geology, tillage, vegetation cover, and irrigation practices result in large spatial and temporal variabilities in shallow SWC [2]. Rapid, continuous and reliable estimation of shallow SWC is necessary to achieve effective soil water management at the field scale. We briefly describe these methods for measuring shallow SWC

Objectives

Methods

Results

Discussion

Conclusion