Dielectric properties are crucial in understanding the behavior of water within soil, particularly the soil water content (SWC), as they measure a material’s ability to store an electric charge and are influenced by water and other minerals in the soil. However, a comprehensive review paper is needed that synthesizes the latest developments in this field, identifies the key challenges and limitations, and outlines future research directions. In addition, various factors, such as soil salinity, temperature, texture, probing space, installation gap, density, clay content, sampling volume, and environmental factors, influence the measurement of the dielectric permittivity of the soil. Therefore, this review aims to address the research gap by critically analyzing the current state-of-the-art dielectric properties-based methods for SWC measurements. The motivation for this review is the increasing importance of precise SWC data for various applications such as agriculture, environmental monitoring, and hydrological studies. We examine time domain reflectometry (TDR), frequency domain reflectometry (FDR), ground-penetrating radar (GPR), remote sensing (RS), and capacitance, which are accurate and cost-effective, enabling real-time water resource management and soil health understanding through measuring the travel time of electromagnetic waves in soil and the reflection coefficient of these waves. SWC can be estimated using various approaches, such as TDR, FDR, GPR, and microwave-based techniques. These methods are made possible by increasing the dielectric permittivity and loss factor with SWC. The available dielectric properties are further synthesized based on mathematical models relating apparent permittivity to water content, providing an updated understanding of their development, applications, and monitoring. It also analyzes recent mathematical calibration models, applications, algorithms, challenges, and trends in dielectric permittivity methods for estimating SWC. By consolidating recent advances and highlighting the remaining challenges, this review article aims to guide researchers and practitioners toward more effective strategies for SWC measurement
No | Soil Type | Texture (in Percent) | Wilting Point (cm3/cm3) |
---|
No | Soil Sample (pH) |
ɛr fr = 1.88 GHz |
ɛr fr = 2.45 GHz | Transition Water Content (cm3/cm3) |
Real Part of the Complex Dielectric Permittivity | Imaginary Part of Dielectric Permittivity | ||
---|---|---|---|---|---|---|---|---|
ɛ | r | fr = 5.35 GHz | Sand | Silt | Clay | |||
1 | Harlingen clay | 2.0 | 37.0 | 61.0 | 0.358 | 0.31 | 0.30 | 0 |
2 | Yuma sand | 100.0 | 0 | 0 | 0.004 | 0.17 | 0.50 | 0 |
3 | Eufaula fine sand | 90.0 | 7.0 | 3.0 | 0.024 | 0.16 | 0.50 | 0 |
4 | Dougherty fine sand | 82.0 | 14.0 | 4.0 | 0.34 | 0.17 | 0.50 | 0 |
5 | Minco very fine sand | 70.0 | 22.0 | 8.0 | 0.051 | 0.17 | 0.50 | 0 |
6 | Chinkasha loam | 58.0 | 28.0 | 14.0 | 0.098 | 0.22 | 0.40 | 8 |
7 | Open street silt | 22.0 | 70.0 | 8.0 | 0.092 | 0.23 | 0.50 | 8 |
Experiment | Objectives/Aim | Findings | References | ||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
Soil’s specific features and calibration | Focused on the FDR sensors on their factory calibration |
|
[9] | ||||||||
Calibration procedure for electromagnetic SWC sensors | To demonstrate the recent and effective calibration methods for low-cost EM sensors |
|
[53] | ||||||||
Laterite’s dielectric characteristics and constant model | To examine the mechanical and physical aspects of in situ laterite dielectric properties |
|
[47] | ||||||||
Measurement and modelling of the dielectric permittivity of soil | To suggest, locate, and demonstrate fresh approaches to determining the dielectric permittivity during freezing |
|
[38] | ||||||||
Dielectric analysis models for measurement of SWC | To presents a normalization-based calibration model. |
|
[19] | ||||||||
Saturated prediction model using TDR | To suggest the level of soil’s saturation with different control criterion for compaction quality |
|
[90] | ||||||||
Calibration of the dielectric permittivity model for agricultural soils | To investigates using three pre-established dielectric permittivity models |
|
8 | Zanies loam | 48.0 | 36.0 | 16.0 | 0.114 | 0.22 | 0.40 | 8 |
9 | Collinville loam | 45.0 | 39.0 | 16.0 | 0.115 | 0.23 | 0.40 | 8 | |||
10 | Kirkland silt loam | 26.0 | 56.0 | 18.0 | 0.137 | 0.20 | 0.40 | 8 | |||
11 | Vernon clay loam | 16.0 | 56.0 | 22.0 | 0.192 | 0.28 | 0.45 | 26 | |||
12 | Tabler silt loam | 22.0 | 56.0 | 22.0 | 0.159 | 0.19 | 0.40 | 8 | |||
13 | Long lake clay | 6.0 | 54.0 | 40.0 | 0.255 | 0.26 | 0.40 | 26 | |||
14 | Sand | 86.0 | 7.0 | 7.0 | 0.046 | 0.20 | 0.40 | 0 | |||
15 | Miller clay | 3.0 | 35.0 | 62.0 | 0.361 | 0.33 | 0.30 | 20 |
1 | 4.7 | 3.99 | 3.90 | 3.84 |
2 | 4.9 | 3.62 | 3.43 | 3.32 |
3 | 5.0 | 3.79 | 3.53 | 3.27 |
4 | 5.2 | 3.83 | 3.75 | 3.52 |
5 | 5.8 | 3.45 | 3.76 | 3.68 |
6 | 6.1 | 3.64 | 3.47 | 3.14 |
7 | 6.3 | 3.48 | 3.55 | 3.21 |
8 | 7.0 | 3.32 | 3.72 | 3.56 |
9 | 7.4 | 3.78 | 3.80 | 3.30 |
| |||
[ | |||
43 | |||
] | |||
Dielectric models for estimating SWC | Examining the link between soil dielectric permittivity, volumetric water content, and dielectric permittivity |
|
[10] |
Mixing models describing dielectric dispersion | To study the dielectric response in the frequency domain of clay minerals and clayey soils |
|
[91] |
Modeling and measurement of soil dielectric properties | To investigate the dielectric properties at room temperature |
|
[40] |
Dielectric damping and configuration effects on TDR | To investigate the impacts of phase configuration and bound water in four high-surface-area soils |
|
[92] |
Evaluation of the thermal conductivity model | Classification into physical, mixing, normalized, linear, and non-linear regression |
|
[93] |
Application of TDR in porous media | To study TDR applications and analyzing waveforms for electrical conductivity and permittivity |
|
[94] |
Using TDR probes, field observations of topsoil moisture | To assesses the effectiveness of a novel inverse method to predict water content profiles |
|
[95] |
Logarithmic TDR calibration formulas: a physical interpretation | To give an empirical estimate of the solid percentage permittivity in volcanic soils |
|
[96] |
TDR field calibration for determining SWC | Examining the dielectric permittivity and gravimetric water content in damaged peatlands |
|
[97] |
Temperature-dependent measurement error in TDR determinations of SWC | To compared soil temperature fluctuations in Ka measurement errors with those estimated using a dielectric mixing model |
|
[98] |
Calculating effective approaches for the dielectric permittivity of moist soil | To calculate the effective dielectric permittivity of multiphase soil |
|
[57] |
SWC estimation | To determine the precise dielectric permittivity by calibrating the wave velocity |
|
[99] |
A novel soil water sensor that adjusts soil temperature and water content | To adjust and monitor SWC reflectometers for various soil types |
|
[36] |
Soil water remote sensing | To enhance retrieval algorithms and transfer empirical observation at different resolutions |
|
[79] |
Dielectric study to quantify the water content of soil | To examine SWC using a new dielectric analysis model |
|
[100] |
Effective field calibration method and model for determining liquid water content | To calculate the amount of uncertainty in the liquid water content |
|
[101] |
Soil water measurement by dielectric method | To investigate the dielectric method of measuring SWC and identify sensor values that are differentially influenced by complex dielectric permeability |
|
[15] |
Measurement of SWC with dielectric dispersion frequency | To investigate the possibility of measuring variations in theta using the soil dielectric spectrum |
|
[102] |
Soil water content retrieval from multispectral remote sensing | To measure SWC with machine learning algorithms and remote sensing |
|
[81] |
Dielectric properties calibration, methods and devices for measuring soil water content | To investigate talc, glass beads, and their combinations at various levels of salinity and water content. |
|
[70] |
Distributed fiber-optic sensing for long-range Monitoring |
DiTeSt is a laser-based distributed sensing system that utilizes optical scattering within the sensing fiber. |
|
[103] |
Detecting SWC with GPR | Evaluation of the latest advancements in GPR applications in SWC measurement |
|
[8] |
GPR outside the ground for soil water content determination | To examine the connection between SWC and surface characteristics. |
|
[81] |
SWC estimation from remote sensing | To examine recent developments and applications related to SWC estimate from remote sensing |
|
[104] |
Temperature and electrical conductivity effects on an inexpensive SWC | To use a two-sensor array to measure the electrical conductivity sensor used in agricultural fields |
|
[105] |
Soil water retention curves from water content measurements | To develop a new method to estimate soil water retention curves. |
|
[6] |
TDR to quantify the SWC and bulk density | Implementation and testing of a novel software for TDR-waveform analysis to measures SWC |
|
[25] |
Monopole antenna-based spectroscopy technique for measuring SWC | To suggest a new approach to measuring soil water that uses frequency scanning |
|
[4] |
Determining SWC and bulk density | To determine the TDR calibration slope and effects of electromagnetic waveform on soil salinity |
|
[106] |