Author Contributions
Conceptualization: Y.W. (Yukun Wang), Q.L., X.D., M.D. and X.T.; Software: Y.W. (Yukun Wang), C.X. and X.D.; Formal analysis: Y.W. (Yukun Wang), Y.W. (Yao Wang) and K.L.; Investigation: Y.W. (Yukun Wang), C.X., Y.W. (Yao Wang), J.G., J.Y. and J.Z.; Resources: H.L., M.D., X.T. and Z.L.; Data curation: Y.W. (Yukun Wang), K.Y., X.D. and M.D.; Writing—original draft: Y.W. (Yukun Wang); Writing—review and editing: Q.L., X.D., M.D. and X.T.; Visualization: Y.W. (Yukun Wang) and C.X.; Project administration: M.Z., M.D., X.T. and Z.L. All authors have read and agreed to the published version of the manuscript.
Figure 1.
(a) Location of the study area and (b) overview of the experiment site. The five letters in (b), T0 to T4, represent the control group and four defoliating agent treatment groups, which were 0 mL Xinsaili ha−1 (T0), 1500 mL Xinsaili ha−1 (T1), 1500 mL + 1500 mL (i.e., application of a second spray one week after the first application) Xinsaili ha−1 (T2), 3000 mL Xinsaili ha−1 (T3), and 7500 mL Xinsaili ha−1 (T4).
Figure 1.
(a) Location of the study area and (b) overview of the experiment site. The five letters in (b), T0 to T4, represent the control group and four defoliating agent treatment groups, which were 0 mL Xinsaili ha−1 (T0), 1500 mL Xinsaili ha−1 (T1), 1500 mL + 1500 mL (i.e., application of a second spray one week after the first application) Xinsaili ha−1 (T2), 3000 mL Xinsaili ha−1 (T3), and 7500 mL Xinsaili ha−1 (T4).
Figure 2.
Ground survey of cotton boll opening rate. (a) Image of the cotton group during the harvest season in the year 2021. (b) Diagram of an individual cotton plant in the year 2022.
Figure 2.
Ground survey of cotton boll opening rate. (a) Image of the cotton group during the harvest season in the year 2021. (b) Diagram of an individual cotton plant in the year 2022.
Figure 3.
Remote sensing data collection. (a) An empirical line method was employed for each band (blue, green, red, red edge, and NIR), utilizing three reference reflectance panels (25%, 50%, and 75%). (b) Multispectral data were collected using a DJI P4M.
Figure 3.
Remote sensing data collection. (a) An empirical line method was employed for each band (blue, green, red, red edge, and NIR), utilizing three reference reflectance panels (25%, 50%, and 75%). (b) Multispectral data were collected using a DJI P4M.
Figure 4.
Overall workflow of the study.
Figure 4.
Overall workflow of the study.
Figure 5.
The range of DN values for opening bolls and others in RGB images. (B, G and R) represent the three drone spectral bands mentioned previously: blue (450 nm ± 16 nm), green (560 nm ± 16 nm), and red (650 nm ± 16 nm), respectively. The sub-images represent the range of values for opening bolls and others on September 7th (A), 13th (B), 23rd (C), and 29th (D) and October 2nd (E), 12th (F), 16th (G), and 23rd (H), respectively.
Figure 5.
The range of DN values for opening bolls and others in RGB images. (B, G and R) represent the three drone spectral bands mentioned previously: blue (450 nm ± 16 nm), green (560 nm ± 16 nm), and red (650 nm ± 16 nm), respectively. The sub-images represent the range of values for opening bolls and others on September 7th (A), 13th (B), 23rd (C), and 29th (D) and October 2nd (E), 12th (F), 16th (G), and 23rd (H), respectively.
Figure 6.
The original multispectral images of all time stages. The sub-images represent the original multispectral images on August 26th (A), September 7th (B), 13th (C), 23rd (D), and 29th (E), and October 2nd (F), 12th (G), 16th (H), and 23rd (I), respectively. Image A marks the date when the first boll on the first fruiting branch of the cotton field began to open, signifying that the cotton entered the boll opening period.
Figure 6.
The original multispectral images of all time stages. The sub-images represent the original multispectral images on August 26th (A), September 7th (B), 13th (C), 23rd (D), and 29th (E), and October 2nd (F), 12th (G), 16th (H), and 23rd (I), respectively. Image A marks the date when the first boll on the first fruiting branch of the cotton field began to open, signifying that the cotton entered the boll opening period.
Figure 7.
Changes in the boll opening rate over time. September 7th, 13th, 23rd, and 29th and October 2nd, 12th, 16th, and 23rd represent 134, 140, 150, 156, 159, 169, 173, and 180 days after sowing, respectively.
Figure 7.
Changes in the boll opening rate over time. September 7th, 13th, 23rd, and 29th and October 2nd, 12th, 16th, and 23rd represent 134, 140, 150, 156, 159, 169, 173, and 180 days after sowing, respectively.
Figure 8.
The opening boll extraction results in different time periods in a certain cell. (A–H) represent the opening bolls extraction results on September 7th, 13th, 23rd, and 29th and October 2nd, 12th, 16th, and 23rd, respectively.
Figure 8.
The opening boll extraction results in different time periods in a certain cell. (A–H) represent the opening bolls extraction results on September 7th, 13th, 23rd, and 29th and October 2nd, 12th, 16th, and 23rd, respectively.
Figure 9.
Results of the training set and the validation set based on PLSR. (a) Prediction results of the training set based on PLSR. (b) Prediction results of the validation set based on PLSR.
Figure 9.
Results of the training set and the validation set based on PLSR. (a) Prediction results of the training set based on PLSR. (b) Prediction results of the validation set based on PLSR.
Figure 10.
Results of the training set and the validation set based on RF. (a) Prediction results of the training set based on RF. (b) Prediction results of the validation set based on RF.
Figure 10.
Results of the training set and the validation set based on RF. (a) Prediction results of the training set based on RF. (b) Prediction results of the validation set based on RF.
Table 1.
Some parameters related to the UAV and sensors.
Table 1.
Some parameters related to the UAV and sensors.
Aircraft Parameters | | Camera Parameters | |
---|
Takeoff weight | 1487 g | FOV | 62.7° |
Diagonal distance | 350 mm | Focal length | 5.74 mm |
Maximum flying altitude | 6000 m | Aperture | f/2.2 |
Max ascent speed | 6 m/s | RGB sensor ISO | 200–800 |
Max descent speed | 3 m/s | Monochrome sensor gain | 1–8× |
Max speed | 50 km/h | Max image size | 1600 × 1300 |
Max flight time | 27 min | Photo format | JPEG/TIFF |
Operating temperature | 0 °C to 40 °C | Supported file systems | FAT32(32 GB); exFAT(>32 GB) |
Operating frequency | 5.72 to 5.85 GHz | Operating temperature | 0 °C to 40 °C |
Table 2.
The calculation formula of the spectral index.
Table 2.
The calculation formula of the spectral index.
Vegetation Index | Formula | References |
---|
Normalized Difference Vegetation Index (NDVI) | (NIR − R)/(NIR + R) | [28] |
Normalized Difference Red Edge (NDRE) | (NIR-RE)/(NIR + RE) | [29] |
Visible-band Difference Vegetation Index (VDVI) | (2 × G − R − B)/(2 × G + R + B) | [30] |
Green Normalized Difference Vegetation Index (GNDVI) | (NIR − G)/(NIR + G) | [31] |
Simple Ratio Index (SR) | NIR/R | [32] |
Green Ratio Vegetation Index (GRVI) | NIR/G | [33] |
Red Green Ratio Index (RGRI) | R/G | [34] |
Difference Vegetation Index (DVI) | NIR − R | [35] |
Excess Green Vegetation Index (EXG) | 2 × G − R − B | [36] |
Excess Red Vegetation Index (EXR) | 1.4 × R − G | [37] |
Red Edge Soil-Adjusted Vegetation Index (RESAVI) | 1.5 × (NIR − RE)/(NIR + RE + 0.5) | [38] |
Enhanced Vegetation Index (EVI) | 2.5 × (NIR − R)/(NIR + 6 × R − 7.5 × B + 1) | [39] |
Table 3.
Accuracy evaluation results of opening boll extraction.
Table 3.
Accuracy evaluation results of opening boll extraction.
Date | Overall Accuracy | Kappa Coefficient |
---|
7 September | 99.82% | 0.945 |
13 September | 99.86% | 0.958 |
23 September | 99.16% | 0.966 |
29 September | 99.83% | 0.991 |
2 October | 99.93% | 0.997 |
12 October | 99.98% | 0.999 |
16 October | 99.82% | 0.994 |
23 October | 99.64% | 0.988 |
Table 4.
Correlation between vegetation indices and opening bolls’ area.
Table 4.
Correlation between vegetation indices and opening bolls’ area.
Vegetation Index | r |
---|
DVI | −0.654 *** |
EVI | −0.835 *** |
EXG | −0.815 *** |
EXR | 0.814 *** |
GNDVI | −0.835 *** |
GRVI | −0.838 *** |
NDRE | −0.750 *** |
NDVI | −0.835 *** |
RESAVI | −0.810 *** |
RGRI | 0.777 *** |
SR | −0.838 *** |
VDVI | −0.794 *** |
Table 5.
Results of the training set and the validation set based on the regression equations.
Table 5.
Results of the training set and the validation set based on the regression equations.
Vegetation Index | Model | Training Set | Validation Set |
---|
R2 | rRMSE (%) | R2 | rRMSE (%) |
---|
EVI | y = 88.580x + 2.305 | 0.811 | 22.496 | 0.839 | 20.165 |
GNDVI | y = 53.333x + 24.562 | 0.901 | 16.318 | 0.909 | 15.225 |
GRVI | y = 97.183x + 3.581 | 0.825 | 21.666 | 0.841 | 19.968 |
NDVI | y = 101.316x + 15.372 | 0.912 | 15.387 | 0.929 | 13.414 |
SR | y = 81.510x − 1.543 | 0.724 | 27.222 | 0.74 | 25.421 |
Table 6.
Fitting results of boll opening percentage trained by PLSR with varying numbers of components.
Table 6.
Fitting results of boll opening percentage trained by PLSR with varying numbers of components.
Data | 1 Comps | 2 Comps | 3 Comps | 4 Comps | 5 Comps |
---|
X dimension | 98.56 | 99.24 | 99.84 | 100.98 | 100.00 |
Y dimension | 89.78 | 93.50 | 94.12 | 94.17 | 94.26 |