A Pixel-Scale Measurement Method of Soil Moisture Using Ground-Penetrating Radar
Abstract
:1. Introduction
2. Study Area
3. Methodology
- According to the experiment design, GPR and the gravimetric method were both used to measure the soil moisture of four 30 × 30 m2 plots.
- Based on the measuring results, a random combination method was applied to analyze the influence of a different number and sampling design of GPR survey lines on the measurement accuracy at pixel scale. Then, the necessary number and appropriate sampling design were obtained, resulting in a pixel-scale measurement method of soil moisture by GPR.
- The random combination method and the statistical sampling were used to determine the necessary sampling sizes of point measurements by gravimetric method under different accuracy requirements, respectively. Additionally, the pixel-scale measurement method by GPR and the sampling method by point measurements were compared.
- The soil moisture by remote sensing in the study area was retrieved by Landsat 8 data, obtaining the soil moisture of Plots A–D. The pixel-scale measurement method by GPR and the sampling method by point measurements were used to validate the remote sensing soil moisture in four plots, and the validation effects were analyzed, respectively.
3.1. GPR Theory
3.2. Experiment Design
3.3. Statistical Sampling
3.4. Random Combination Method
- Select m samples randomly from n samples (m = 1, 2, 3,…, n), and repeat times to cover all the combinations each time.
- Calculate the mean of m samples obtained by each selection, and obtain mean values in total.
- Calculate the relative error between the mean and the mean of all n measured samples, and analyze the confidence level within 5%, 6%, 7%, 8%, 9%, and 10%.
- Plot the relationship between the confidence level and m to determine the necessary sampling size corresponding to a given confidence level (95% or 90%) when the relative error ranges from 5% to 10%, respectively.
3.5. Remote Sensing Soil Moisture by Landsat 8
4. Results
4.1. The Necessary Number of GPR Survey Lines for Pixel-Scale Soil Moisture
4.2. The Sampling Design of GPR Survey Lines for Pixel-Scale Soil Moisture
4.3. Pixel-Scale Measurement Method by GPR and Sampling Method by Point Measurements
4.4. Ground Validation Comparison of Remote Sensing Soil Moisture
5. Conclusions
Author Contributions
Funding
Data Availability Statement
Acknowledgments
Conflicts of Interest
Abbreviations
GPR | Ground-penetrating radar |
FO | Fixed offset |
ET | Evapotranspiration |
TDR | Time domain reflectometry |
LULC | Land use and land cover |
CMP | Common-midpoint |
WARR | Wide angle reflection and refraction |
NDVI | Normalized difference vegetation index |
LST | Land surface temperature |
VSWI | Vegetation supply water index |
FDR | Frequency domain reflectometry |
ESTARFM | Enhanced spatial and temporal adaptive reflectance fusion model |
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RMSE (m3/m3) | |||||
---|---|---|---|---|---|
Shape Ⅰ | Shape Ⅱ | Shape Ⅲ | Shape Ⅳ | Shape Ⅴ | |
Plot A | 0.0083 | 0.0086 | 0.0066 | 0.0066 | 0.0067 |
Plot B | 0.0076 | 0.0079 | 0.0063 | 0.0063 | 0.0065 |
Plot C | 0.0078 | 0.0072 | 0.0059 | 0.007 | 0.0069 |
Plot D | 0.0077 | 0.0138 | 0.0119 | 0.0087 | 0.0097 |
Average | 0.0079 | 0.0094 | 0.0077 | 0.0072 | 0.0075 |
Confidence Level | Relative Error | Number of Sampling Points | |||||||
---|---|---|---|---|---|---|---|---|---|
Plot A | Plot B | Plot C | Plot D | ||||||
S | R | S | R | S | R | S | R | ||
90% | 5% | 16 | 16 | 11 | 10 | 15 | 16 | 15 | 15 |
10% | 8 | 8 | 5 | 4 | 8 | 8 | 7 | 7 | |
95% | 5% | 18 | 18 | 13 | 12 | 17 | 18 | 17 | 17 |
10% | 10 | 10 | 6 | 5 | 10 | 9 | 10 | 9 |
Plot | Soil Moisture Measurements (m3/m3) | ||||||
---|---|---|---|---|---|---|---|
Gravimetric Measurements (25 Samples/Plot) | Number of Survey Lines | ||||||
1 | 2 | 4 | 8 | 10 | 12 | ||
A | 0.069 | 0.064 | 0.071 | 0.071 | 0.070 | 0.070 | 0.071 |
B | 0.058 | 0.055 | 0.060 | 0.063 | 0.060 | 0.061 | 0.061 |
C | 0.066 | 0.061 | 0.070 | 0.069 | 0.068 | 0.067 | 0.068 |
D | 0.071 | 0.060 | 0.066 | 0.066 | 0.065 | 0.066 | 0.065 |
60 × 60 m2 | 0.064 (by 25 samples) 0.066 (by 81 samples) | 0.062 | 0.066 | 0.068 | 0.066 | 0.065 | 0.066 |
Remote Sensing Soil Moisture (m3/m3) | Validation Results by Point Measurements (Relative Error) | ||||||
---|---|---|---|---|---|---|---|
1 Point | 5 Points | 10 Points | 15 Points | 20 Points | 25 Points | ||
Plot A | 0.037 | 9.93–58.21% | 19.61–54.36% | 29.31–51.49% | 34.55–51.49% | 38.61–46.40% | 46.38% |
Plot B | 0.047 | 3.90–37.10% | 2.62–29.08% | 6.74–26.16% | 10.91–23.69% | 14.27–20.78% | 18.97% |
Plot C | 0.043 | 7.40–49.62% | 8.78–46.85% | 18.40–42.50% | 23.22–39.20% | 27.33–36.40% | 34.85% |
Plot D | 0.046 | 11.20.–51.83% | 11.65–46.29% | 20.54–42.58% | 24.39–36.70% | 28.43–36.70% | 35.21% |
1 point | 5 points | 10 points | 15 points | 20 points | 25 points | ||
RMSE of four plots (m3/m3) | 0.0036– 0.0438 | 0.0059– 0.0365 | 0.0110–0.0316 | 0.0142– 0.0284 | 0.0173– 0.0250 | 0.0240 |
Remote Sensing Soil Moisture (m3/m3) | Validation Results by GPR Survey Lines (Relative Error) | ||||
---|---|---|---|---|---|
2 Lines | 4 Lines | 8 Lines | 12 Lines | ||
Plot A | 0.037 | 47.60% | 47.89% | 47.14% | 47.89% |
Plot B | 0.047 | 21.77% | 25.40% | 21.67% | 22.95% |
Plot C | 0.043 | 38.71% | 37.68% | 36.76% | 36.76% |
Plot D | 0.046 | 30.17% | 30.30% | 29.23% | 29.23% |
2 lines | 4 lines | 8 lines | 12 lines | ||
RMSE of four plots (m3/m3) | 0.0247 | 0.0249 | 0.0237 | 0.0242 |
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Song, W.; Lu, Y.; Wang, Y.; Lu, J.; Shi, H. A Pixel-Scale Measurement Method of Soil Moisture Using Ground-Penetrating Radar. Water 2023, 15, 1318. https://doi.org/10.3390/w15071318
Song W, Lu Y, Wang Y, Lu J, Shi H. A Pixel-Scale Measurement Method of Soil Moisture Using Ground-Penetrating Radar. Water. 2023; 15(7):1318. https://doi.org/10.3390/w15071318
Chicago/Turabian StyleSong, Wenlong, Yizhu Lu, Yu Wang, Jingxuan Lu, and Haixian Shi. 2023. "A Pixel-Scale Measurement Method of Soil Moisture Using Ground-Penetrating Radar" Water 15, no. 7: 1318. https://doi.org/10.3390/w15071318