Author Contributions
Conceptualization, Y.T.; methodology, Y.Z.; software, M.C.; validation, C.S.; formal analysis, M.C.; investigation, Y.T.; resources, Y.Z.; data curation, Y.T.; writing—original draft preparation, Y.T.; writing—review and editing, C.S.; visualization, Y.Z.; supervision, C.S.; project administration, C.S.; funding acquisition, M.C. All authors have read and agreed to the published version of the manuscript.
Figure 1.
Orthophoto image of the test field taken with a drone.
Figure 1.
Orthophoto image of the test field taken with a drone.
Figure 2.
The flowchart of methods.
Figure 2.
The flowchart of methods.
Figure 3.
Correlation analysis between the characteristic parameters at the jointing stage and CGIavg. Note: the solid orange line indicates the significance level at p > 0.01; the red dotted line indicates the significance level at p > 0.05.
Figure 3.
Correlation analysis between the characteristic parameters at the jointing stage and CGIavg. Note: the solid orange line indicates the significance level at p > 0.01; the red dotted line indicates the significance level at p > 0.05.
Figure 4.
Correlation analysis between characteristic parameters at the booting stage and CGIavg. Note: the solid orange line indicates the significance level at p > 0.01; the red dotted line indicates the significance level at p > 0.05.
Figure 4.
Correlation analysis between characteristic parameters at the booting stage and CGIavg. Note: the solid orange line indicates the significance level at p > 0.01; the red dotted line indicates the significance level at p > 0.05.
Figure 5.
Correlation analysis between characteristic parameters at the flowering stage and CGIavg. Note: the solid orange line indicates the significance level at p > 0.01; the red dotted line indicates the significance level at p > 0.05.
Figure 5.
Correlation analysis between characteristic parameters at the flowering stage and CGIavg. Note: the solid orange line indicates the significance level at p > 0.01; the red dotted line indicates the significance level at p > 0.05.
Figure 6.
The process of filtering the characteristic parameters of each period using the SPA algorithm. Note: The black curves represent the variables selected at elongation stage, the red curve represents the variables selected booting stage, the blue curves represent the variables selected for flowering stage.
Figure 6.
The process of filtering the characteristic parameters of each period using the SPA algorithm. Note: The black curves represent the variables selected at elongation stage, the red curve represents the variables selected booting stage, the blue curves represent the variables selected for flowering stage.
Figure 7.
Correlation analysis between characteristic parameters at the jointing stage and comprehensive index CGIcv.
Figure 7.
Correlation analysis between characteristic parameters at the jointing stage and comprehensive index CGIcv.
Figure 8.
Correlation analysis between characteristic parameters t the booting stage and comprehensive index CGIcv.
Figure 8.
Correlation analysis between characteristic parameters t the booting stage and comprehensive index CGIcv.
Figure 9.
Correlation analysis between characteristic parameters at the flowering period and comprehensive index CGIcv.
Figure 9.
Correlation analysis between characteristic parameters at the flowering period and comprehensive index CGIcv.
Figure 10.
The process of filtering the characteristic parameters of each period using the SPA algorithm. Note: The black curves represent the variables selected at elongation stage, the red curve represents the variables selected booting stage, the blue curves represent the variables selected for flowering stage.
Figure 10.
The process of filtering the characteristic parameters of each period using the SPA algorithm. Note: The black curves represent the variables selected at elongation stage, the red curve represents the variables selected booting stage, the blue curves represent the variables selected for flowering stage.
Figure 11.
Scatterplot with a 1:1 reference line of CGIavg measured and predicted values for the SVR model validation set at the jointing stage.
Figure 11.
Scatterplot with a 1:1 reference line of CGIavg measured and predicted values for the SVR model validation set at the jointing stage.
Figure 12.
Scatterplot with a 1:1 reference line of CGIavg measured and predicted values in the booting stage GPR model validation set.
Figure 12.
Scatterplot with a 1:1 reference line of CGIavg measured and predicted values in the booting stage GPR model validation set.
Figure 13.
Scatterplot with a 1:1 reference line of CGIavg measured and predicted values for the SVR model validation set at the flowering stage.
Figure 13.
Scatterplot with a 1:1 reference line of CGIavg measured and predicted values for the SVR model validation set at the flowering stage.
Figure 14.
Scatterplot with a 1:1 reference line of the measured and predicted values of the MSR model validation set CGIcv at the jointing stage.
Figure 14.
Scatterplot with a 1:1 reference line of the measured and predicted values of the MSR model validation set CGIcv at the jointing stage.
Figure 15.
Scatterplot with a 1:1 reference line graph of the measured and predicted values of the GPR model validation set CGIcv at the booting stage.
Figure 15.
Scatterplot with a 1:1 reference line graph of the measured and predicted values of the GPR model validation set CGIcv at the booting stage.
Figure 16.
Scatterplot with a 1:1 reference line of the measured and predicted values of the SVR model validation set CGIcv at the flowering stage.
Figure 16.
Scatterplot with a 1:1 reference line of the measured and predicted values of the SVR model validation set CGIcv at the flowering stage.
Table 1.
Selected color indices and their calculation methods.
Table 1.
Selected color indices and their calculation methods.
Parameters | Equation | References |
---|
Visible atmospherically resistant index (VARI) | (g − r)/(g + r − b) | [9] |
Excess green vegetation index (ExG) | 2 × g − r − b | [10] |
Modified GRVI (MGRVI) | (g2 − r2)/(g2 + r2) | [10] |
Neighborhood deprivation index (NDI) | (r − g)/(r + g + 0.01) | [10] |
Excess green minus excess red index (ExGR) | 3 × g − 2.4 × r − b | [10] |
Red–green–blue vegetation index (RGBVI) | (g2 − b × r)/(g2 + b × r) | [11] |
Red–green ratio index (RGRI) | R/G | [11] |
Normalized green–blue difference index (NGBDI) | (G − B)/(G + B) | [11] |
Normalized pigment chlorophyll index (NPCI) | (R − B)/(R + B) | [12] |
Vegetative index (VEG) | G/(R0.667 × B0.33) | [12] |
Table 2.
Selected vegetation indices and their calculation methods.
Table 2.
Selected vegetation indices and their calculation methods.
Parameters | Equation | References |
---|
Normalized difference vegetation index (NDVI) | (R800 − R680)/(R800 + R680) | [13] |
Radar vegetation index (RVI) | R800/R680 | [13] |
Difference vegetation index (DVI) | R800 − R680 | [13] |
Optimized soil-adjusted vegetation index (OSAVI) | (1 + 0.16) × (R800 − R670)/(R800 + R670 + 0.16) | [14] |
Renormalized difference vegetation index (RDVI) | (R800 − R680)/(R800 + R680) | [14] |
Vogelmann index (VOGI) | R740/R720 | [14] |
Enhanced vegetation index (EVI) | 2.5 × (R860 − R645)/(1 + R860 + 6 × R645 − 7.5 × R470) | [14] |
Green normalized difference vegetation index (GNDVI) | (R750 − R550)/(R750 + R550) | [15] |
Modified simple ratio (MSR) | (R750/R705 − 1)/((R750/R705) + 1) | [15] |
Modified soil adjusted vegetation index (MSAVI) | [2 × R750 + 1 − ((2 × R750 + 1)2 − 8 × (R750 − R705))0.5]/2 | [15] |
Plant senescence/reflectance index (PSRI) | (R680 − R500)/R750 | [16] |
Transformed chlorophyll absorption and reflectance index (TCARI) | 3 × [(R700 − R670) − 0.2 × (R700 − R550) × (R700/R670)] | [16] |
Table 3.
Correlation analysis of agronomic parameters and comprehensive index CGIavg in each growth stage.
Table 3.
Correlation analysis of agronomic parameters and comprehensive index CGIavg in each growth stage.
Growth Stage | AGB | SPAD | LNC |
---|
Jointing Stage | 0.910 ** | 0.782 ** | 0.727 ** |
Booting Stage | 0.900 ** | 0.854 ** | 0.893 ** |
Flowering Stage | 0.884 ** | 0.851 ** | 0.771 ** |
Table 4.
The formula constructed for CGIcv, the comprehensive growth index, at each growth stage.
Table 4.
The formula constructed for CGIcv, the comprehensive growth index, at each growth stage.
Growth Stage | Comprehensive Growth Indicator CGIcv |
---|
Jointing Stage | CGIcv = 0.28 × IAGB + 0.37 × ISPAD + 0.35 × ILNC |
Booting Stage | CGIcv = 0.31 × IAGB + 0.34 × ISPAD + 0.35 × ILNC |
Flowering Stage | CGIcv = 0.29 × IAGB + 0.33 × ISPAD + 0.38 × ILNC |
Table 5.
Correlation analysis of agronomic parameters and comprehensive index CGIcv at each growth stage.
Table 5.
Correlation analysis of agronomic parameters and comprehensive index CGIcv at each growth stage.
Growth Stage | AGB | SPAD | LNC |
---|
Jointing Stage | 0.897 ** | 0.802 ** | 0.719 ** |
Booting Stage | 0.891 ** | 0.816 ** | 0.807 ** |
Flowering Stage | 0.862 ** | 0.844 ** | 0.799 ** |
Table 6.
CGIavg estimation model at each growth stage.
Table 6.
CGIavg estimation model at each growth stage.
Growth Stage | Model | Modeling Set | Validation Set |
---|
R2 | RMSE | NRMSE (%) | R2 | RMSE | NRMSE |
---|
Jointing Stage | MSR | 0.83 | 0.087 | 11.45 | 0.77 | 0.095 | 14.16 |
GPR | 0.76 | 0.098 | 12.91 | 0.69 | 0.105 | 15.56 |
SVR | 0.68 | 0.107 | 14.09 | 0.71 | 0.102 | 15.21 |
Booting Stage | MSR | 0.69 | 0.103 | 13.56 | 0.62 | 0.113 | 13.94 |
GPR | 0.78 | 0.092 | 12.11 | 0.71 | 0.098 | 12.23 |
SVR | 0.73 | 0.096 | 12.64 | 0.67 | 0.106 | 13.08 |
Flowering Stage | MSR | 0.76 | 0.095 | 11.59 | 0.69 | 0.104 | 12.93 |
GPR | 0.72 | 0.101 | 12.32 | 0.67 | 0.107 | 13.31 |
SVR | 0.81 | 0.082 | 10.02 | 0.78 | 0.087 | 10.81 |
Table 7.
CGIcv estimation model for each growth stage.
Table 7.
CGIcv estimation model for each growth stage.
Growth Stage | Model | Modeling Set | Validation Set |
---|
R2 | RMSE | NRMSE | R2 | RMSE | NRMSE |
---|
Jointing Stage | MSR | 0.77 | 0.075 | 10.12 | 0.73 | 0.084 | 14.21 |
GPR | 0.71 | 0.087 | 11.85 | 0.74 | 0.079 | 13.67 |
SVR | 0.66 | 0.094 | 12.74 | 0.63 | 0.098 | 16.96 |
Booting Stage | MSR | 0.65 | 0.107 | 14.05 | 0.51 | 0.128 | 15.77 |
GPR | 0.80 | 0.087 | 11.42 | 0.74 | 0.092 | 12.31 |
SVR | 0.78 | 0.090 | 11.85 | 0.72 | 0.096 | 12.61 |
Flowering Stage | MSR | 0.75 | 0.087 | 12.08 | 0.69 | 0.102 | 14.16 |
GPR | 0.71 | 0.092 | 12.78 | 0.66 | 0.113 | 15.69 |
SVR | 0.84 | 0.077 | 10.69 | 0.78 | 0.085 | 11.82 |