Article
A Novel Dielectric Tomography System for In-Situ
Tracking Three-Dimensional Soil Water Dynamics
Song Yu
1
, Chao Chen
1
, Qiang Xu
1
, Qiang Cheng
1,
*, Xiaofei Yan
2,
*, Zhou Yu
1
, Yihan Ma
1
and
Haonan Chen
1
1
College of Information and Electrical Engineering, China Agricultural University, Beijing 100083, China;
song_y@cau.edu.cn (S.Y.); chern@cau.edu.cn (C.C.); xuqiang@cau.edu.cn (Q.X.);
yz1157799015@163.com (Z.Y.); myh495836915@outlook.com (Y.M.); chenhaonan9612@163.com (H.C.)
2
School of Technology, Beijing Forestry University, Beijing 100083, China
* Correspondence: chengqiang@cau.edu.cn (Q.C.); yanxf@bjfu.edu.cn (X.Y.)
Received: 22 July 2018; Accepted: 20 August 2018; Published: 31 August 2018
Abstract:
In this study, we developed a novel dielectric tomography system for in-situ tracking
three-dimensional (3D) soil water dynamics. The system was designed to control a single dielectric
tube sensor that automatically lowered in a PVC tube array installed in-situ to determine the water
content of a soil profile, which eliminated probe-to-probe uncertainties and labor costs. Two tests for
evaluating the novel system were conducted (i) to analyze and correct the positional error of the probe
due to out-of-step errors of stepper motors, and (ii) to track and visualize 3D soil water temporal
variations in a soil tank with heterogenetic bulk densities and initial water contents under drip
irrigation.
The results
show that the positioning correcting algorithm combined with starting point
alignment can minimize the positioning error of the probe during the 3D tomography.
The single
drip emitter test illustrated spatial and temporal variations of soil water content due to heterogeneous
soil properties in vertical and horizontal directions around the access tube array. Based on
these data
,
3D distributions of soil water dynamics were visualized. The developed tomography system has
potential application to be extended to the local scale in a greenhouse or the large scale in an
agricultural field. Future research should explore the performance for agricultural crop irrigation
or for modeling and validating soil water flow or hydrological process under either steady state or
non-steady state condition.
Keywords:
dielectric tomography; three-dimension; soil water dynamic; in-situ measurement;
tube array
1. Introduction
Monitoring variation of soil water content (SWC) in three dimensions (3D) can benefit researchers
studying agricultural water management, soil hydrological process, fluid pollutant transport
phenomena, etc., in heterogeneous soils [
1
] where spatial variability of soil properties and field and
meteorological conditions significantly affect the distribution of SWC at different scales [
2
–
8
]. With this
intention, diverse sensor techniques such as time-domain reflectrometry (TDR) [
9
], frequency-domain
impedance (FDI) sensor [
8
], neutron moisture meter (NMM) [
10
], heat pulse (HP) sensor [
11
] and
electrical resistivity tomography (ERT) [
12
] have been developed and widely used to monitor the
temporal variations of soil water content at multiple positions.
In early studies, needle-structure-based sensors (e.g., TDR, FDI, HP, etc.) with multiple probes
were used to automatically monitor 3D soil water dynamics [
9
]. However, the installation of the
multiple probes has to excavate a large pit to insert a series of probes at different depths and sites,
inevitably disturbing the soil structure surrounding the sensors [
10
,
13
,
14
]. Besides, multiple probes
Sensors 2018, 18, 2880; doi:10.3390/s18092880 www.mdpi.com/journal/sensors
