Figure 1.
Land cover map of the study area using MODIS MCD12Q1 product in 2020.
Figure 1.
Land cover map of the study area using MODIS MCD12Q1 product in 2020.
Figure 2.
Flowchart for MERSI-II LAI estimation.
Figure 2.
Flowchart for MERSI-II LAI estimation.
Figure 3.
Time-series curves of VIs and LAI for grasses/cereal crops.
Figure 3.
Time-series curves of VIs and LAI for grasses/cereal crops.
Figure 4.
Time-series curves of VIs and LAI for broadleaf crops.
Figure 4.
Time-series curves of VIs and LAI for broadleaf crops.
Figure 5.
Time-series curves of VIs and LAI for savannahs.
Figure 5.
Time-series curves of VIs and LAI for savannahs.
Figure 6.
Time-series curves of VIs and LAI for deciduous broadleaf forests.
Figure 6.
Time-series curves of VIs and LAI for deciduous broadleaf forests.
Figure 7.
VI–LAI density scatter plot of grasses/cereal crops: (a) DVI–LAI; (b) EVI–LAI; (c) MSR-LAI; (d) NDVI–LAI; (e) OSAVI–LAI; (f) RDVI–LAI; (g) RVI–LAI; (h) SAVI–LAI.
Figure 7.
VI–LAI density scatter plot of grasses/cereal crops: (a) DVI–LAI; (b) EVI–LAI; (c) MSR-LAI; (d) NDVI–LAI; (e) OSAVI–LAI; (f) RDVI–LAI; (g) RVI–LAI; (h) SAVI–LAI.
Figure 8.
Optimal VIs for MERSI-II LAI estimation across different biomes and growth stages. Biomes 1–8 correspond to grasses/cereal crops, shrubs, broadleaf crops, savanna, EBF, DBF, ENF, and ENF, respectively. The seven colors represent seven different input parameters, respectively.
Figure 8.
Optimal VIs for MERSI-II LAI estimation across different biomes and growth stages. Biomes 1–8 correspond to grasses/cereal crops, shrubs, broadleaf crops, savanna, EBF, DBF, ENF, and ENF, respectively. The seven colors represent seven different input parameters, respectively.
Figure 9.
Estimation results of different methods.
Figure 9.
Estimation results of different methods.
Figure 10.
Comparison of spatial distributions of LAI differences between MERSI-II, Aqua MODIS and GLASS in mainland China in 2020: (a) MERSI-II LAI minus MODIS LAI; (b) STD of the LAI differences between MERSI-II and MODIS; (c) MERSI-II LAI minus GLASS LAI; (d) STD of the LAI differences between MERSI-II and GLASS. No-data pixels in white color are observations contaminated by cloud, shadow, aerosol, etc. Gray pixels are non-vegetation areas.
Figure 10.
Comparison of spatial distributions of LAI differences between MERSI-II, Aqua MODIS and GLASS in mainland China in 2020: (a) MERSI-II LAI minus MODIS LAI; (b) STD of the LAI differences between MERSI-II and MODIS; (c) MERSI-II LAI minus GLASS LAI; (d) STD of the LAI differences between MERSI-II and GLASS. No-data pixels in white color are observations contaminated by cloud, shadow, aerosol, etc. Gray pixels are non-vegetation areas.
Figure 11.
Bar chart for the proportion of different categories of LAI differences under each biome: (a) MERSI-II LAI minus MODIS LAI; (b) MERSI-II LAI minus GLASS LAI. Biomes 1–8 are grasses/cereal crops, shrubs, broadleaf crops, savanna, EBF, DBF, ENF, and ENF.
Figure 11.
Bar chart for the proportion of different categories of LAI differences under each biome: (a) MERSI-II LAI minus MODIS LAI; (b) MERSI-II LAI minus GLASS LAI. Biomes 1–8 are grasses/cereal crops, shrubs, broadleaf crops, savanna, EBF, DBF, ENF, and ENF.
Figure 12.
Bar chart for the proportion of different biomes under each category of LAI difference: (a) MERSI-II LAI minus MODIS LAI; (b) MERSI-II LAI minus GLASS LAI. Biomes 1–8 are grasses/cereal crops, shrubs, broadleaf crops, savanna, EBF, DBF, ENF and ENF.
Figure 12.
Bar chart for the proportion of different biomes under each category of LAI difference: (a) MERSI-II LAI minus MODIS LAI; (b) MERSI-II LAI minus GLASS LAI. Biomes 1–8 are grasses/cereal crops, shrubs, broadleaf crops, savanna, EBF, DBF, ENF and ENF.
Figure 13.
Time series of MERSI, GLASS and MODIS (2020). The red circle indicates large fluctuations in MODIS LAI.
Figure 13.
Time series of MERSI, GLASS and MODIS (2020). The red circle indicates large fluctuations in MODIS LAI.
Figure 14.
Comparison between ground-truth data and FY-3D MERSI-II LAI. Biome 1 is grasses/cereal crops, Biome 2 is shrubs, Biome 4 is savanna, Biome 5 is evergreen broadleaf forests, and Biome 8 is deciduous needleleaf forests.
Figure 14.
Comparison between ground-truth data and FY-3D MERSI-II LAI. Biome 1 is grasses/cereal crops, Biome 2 is shrubs, Biome 4 is savanna, Biome 5 is evergreen broadleaf forests, and Biome 8 is deciduous needleleaf forests.
Table 1.
Information related to GBOV ground stations.
Table 1.
Information related to GBOV ground stations.
ID | Lat (°) | Lon (°) | Dominant Vegetation | Start Date–End Date |
---|
1 | 44.064 | −71.287 | savanna | 03.18–10.12 |
2 | 39.060 | −78.072 | savanna | 02.27–12.11 |
3 | 40.816 | −104.746 | grasses/cereal crops | 03.12–10.06 |
4 | 32.542 | −87.804 | savanna | 04.02–09.09 |
5 | 28.125 | −81.436 | evergreen broadleaf forests | 01.26–11.25 |
6 | 17.970 | −66.869 | shrubs | 01.13–12.14 |
7 | 31.195 | −84.469 | evergreen broadleaf forests | 02.09–11.23 |
8 | 32.591 | −106.843 | shrubs | 03.05–10.15 |
9 | 39.110 | −96.613 | evergreen needleleaf forests | 03.02–10.12 |
10 | 18.025 | −67.073 | shrubs | 01.13–12.30 |
11 | 40.177 | −112.455 | grasses/cereal crops | 03.31–10.09 |
12 | 29.691 | −81.997 | evergreen broadleaf forests | 01.01–11.16 |
Table 2.
VIs for VI–LAI relationship analysis, where L = 0.5,
= 6.0, and
= 7.5 [
20,
22].
Table 2.
VIs for VI–LAI relationship analysis, where L = 0.5,
= 6.0, and
= 7.5 [
20,
22].
Index | Formula | Reference |
---|
RVI | | [25] |
DVI | | [26] |
NDVI | | [19] |
RDVI | | [24] |
MSR | | [23] |
EVI | | [22] |
SAVI | | [20] |
OSAVI | | [21] |
Table 3.
Regressions of the LAI on vegetation indices; bold indicates the highest correlation.
Table 3.
Regressions of the LAI on vegetation indices; bold indicates the highest correlation.
| DVI | EVI | MSR | NDVI | OSAVI | RDVI | RVI | SAVI |
---|
R2 | RMSE | R2 | RMSE | R2 | RMSE | R2 | RMSE | R2 | RMSE | R2 | RMSE | R2 | RMSE | R2 | RMSE |
---|
grasses/cereal crops | Lin | 0.70 | 0.04 | 0.76 | 0.07 | 0.77 | 0.18 | 0.70 | 0.10 | 0.72 | 0.03 | 0.72 | 0.06 | 0.78 | 0.64 | 0.70 | 0.06 |
Log | 0.82 | 0.03 | 0.88 | 0.05 | 0.84 | 0.15 | 0.87 | 0.06 | 0.84 | 0.02 | 0.86 | 0.04 | 0.82 | 0.58 | 0.84 | 0.04 |
shrubs | Lin | 0.77 | 0.03 | 0.80 | 0.06 | 0.81 | 0.18 | 0.75 | 0.10 | 0.78 | 0.02 | 0.78 | 0.05 | 0.83 | 0.61 | 0.78 | 0.05 |
Log | 0.87 | 0.03 | 0.92 | 0.04 | 0.92 | 0.12 | 0.92 | 0.06 | 0.91 | 0.02 | 0.92 | 0.03 | 0.90 | 0.47 | 0.91 | 0.03 |
broadleaf crops | Lin | 0.67 | 0.05 | 0.70 | 0.09 | 0.73 | 0.22 | 0.67 | 0.10 | 0.69 | 0.03 | 0.67 | 0.07 | 0.72 | 0.83 | 0.68 | 0.07 |
Log | 0.81 | 0.04 | 0.87 | 0.06 | 0.81 | 0.18 | 0.85 | 0.07 | 0.85 | 0.02 | 0.85 | 0.05 | 0.75 | 0.79 | 0.84 | 0.05 |
savanna | Lin | 0.49 | 0.06 | 0.54 | 0.09 | 0.50 | 0.31 | 0.46 | 0.12 | 0.49 | 0.04 | 0.51 | 0.08 | 0.49 | 1.28 | 0.50 | 0.07 |
Log | 0.63 | 0.05 | 0.70 | 0.07 | 0.60 | 0.27 | 0.65 | 0.09 | 0.65 | 0.03 | 0.68 | 0.06 | 0.56 | 1.19 | 0.66 | 0.06 |
EBF | Lin | 0.30 | 0.06 | 0.35 | 0.08 | 0.16 | 0.36 | 0.14 | 0.10 | 0.29 | 0.04 | 0.29 | 0.07 | 0.17 | 1.63 | 0.32 | 0.07 |
Log | 0.30 | 0.06 | 0.35 | 0.08 | 0.17 | 0.36 | 0.15 | 0.10 | 0.29 | 0.04 | 0.29 | 0.07 | 0.17 | 1.63 | 0.32 | 0.07 |
DBF | Lin | 0.78 | 0.05 | 0.81 | 0.08 | 0.75 | 0.33 | 0.68 | 0.12 | 0.78 | 0.03 | 0.77 | 0.07 | 0.71 | 1.54 | 0.77 | 0.07 |
Log | 0.83 | 0.05 | 0.87 | 0.07 | 0.81 | 0.29 | 0.82 | 0.09 | 0.85 | 0.03 | 0.86 | 0.06 | 0.75 | 1.45 | 0.84 | 0.06 |
ENF | Lin | 0.38 | 0.05 | 0.45 | 0.08 | 0.23 | 0.34 | 0.22 | 0.12 | 0.37 | 0.04 | 0.36 | 0.07 | 0.27 | 1.40 | 0.36 | 0.07 |
Log | 0.39 | 0.05 | 0.46 | 0.08 | 0.25 | 0.34 | 0.27 | 0.12 | 0.40 | 0.04 | 0.39 | 0.07 | 0.28 | 1.39 | 0.38 | 0.07 |
DNF | Lin | 0.76 | 0.04 | 0.79 | 0.06 | 0.78 | 0.26 | 0.70 | 0.13 | 0.76 | 0.03 | 0.77 | 0.06 | 0.78 | 1.00 | 0.78 | 0.05 |
Log | 0.84 | 0.03 | 0.86 | 0.05 | 0.86 | 0.21 | 0.86 | 0.09 | 0.86 | 0.02 | 0.89 | 0.04 | 0.82 | 0.91 | 0.87 | 0.04 |
Table 4.
The accuracy of the test set for BPNN, RF, and SVM models.
Table 4.
The accuracy of the test set for BPNN, RF, and SVM models.
DOY | BPNN | RF | SVM |
---|
R2 | RMSE | R2 | RMSE | R2 | RMSE |
---|
113 | 0.7265 | 0.7834 | 0.7246 | 0.7889 | 0.6939 | 0.8768 |
193 | 0.7504 | 1.0775 | 0.7503 | 1.0780 | 0.7367 | 1.1366 |
289 | 0.8391 | 0.5933 | 0.8352 | 0.6077 | 0.8280 | 0.6341 |
33 | 0.8105 | 0.3605 | 0.8063 | 0.3685 | 0.7869 | 0.4054 |
Table 5.
Eleven distinct combinations of four vegetation indices: EVI, NDVI, RDVI, and MSR.
Table 5.
Eleven distinct combinations of four vegetation indices: EVI, NDVI, RDVI, and MSR.
Number | Combination of Vegetation Indices |
---|
two VIs | EVI–NDVI | EVI–RDVI | EVI–MSR | NDVI–RDVI | NDVI–MSR | RDVI–MSR |
three VIs | EVI–NDVI–RDVI | EVI–NDVI–MSR | EVI–RDVI–MSR | NDVI–RDVI–MSR |
four VIs | EVI–NDVI–RDVI–MSR |
Table 6.
The annual mean R2 of LAI obtained using different estimation methods. And the bold indicates the highest R2.
Table 6.
The annual mean R2 of LAI obtained using different estimation methods. And the bold indicates the highest R2.
| EVI | NDVI | RDVI | MSR | 2VIs | 3VIs | 4VIs | Proposed |
---|
grasses/cereal crops | 0.704 | 0.672 | 0.648 | 0.578 | 0.748 | 0.742 | 0.724 | 0.810 |
shrubs | 0.606 | 0.657 | 0.575 | 0.584 | 0.737 | 0.722 | 0.712 | 0.766 |
broadleaf crops | 0.627 | 0.614 | 0.594 | 0.547 | 0.691 | 0.663 | 0.646 | 0.704 |
savanna | 0.437 | 0.465 | 0.439 | 0.417 | 0.506 | 0.493 | 0.487 | 0.523 |
EBF | 0.330 | 0.265 | 0.316 | 0.268 | 0.325 | 0.307 | 0.295 | 0.415 |
DBF | 0.476 | 0.426 | 0.451 | 0.390 | 0.478 | 0.440 | 0.437 | 0.533 |
ENF | 0.445 | 0.398 | 0.420 | 0.350 | 0.490 | 0.443 | 0.407 | 0.533 |
DNF | 0.249 | 0.166 | 0.231 | 0.226 | 0.336 | 0.189 | 0.255 | 0.400 |
All | 0.767 | 0.765 | 0.764 | 0.728 | 0.783 | 0.789 | 0.767 | 0.830 |
Table 7.
The annual mean RMSE of LAI obtained using different estimation methods. And the bold indicates the least RMSE.
Table 7.
The annual mean RMSE of LAI obtained using different estimation methods. And the bold indicates the least RMSE.
| EVI | NDVI | RDVI | MSR | 2VIs | 3VIs | 4VIs | Proposed |
---|
grasses/cereal crops | 0.229 | 0.248 | 0.225 | 0.259 | 0.211 | 0.235 | 0.240 | 0.193 |
shrubs | 0.162 | 0.232 | 0.181 | 0.241 | 0.161 | 0.185 | 0.196 | 0.135 |
broadleaf crops | 0.341 | 0.357 | 0.327 | 0.385 | 0.298 | 0.346 | 0.345 | 0.271 |
savanna | 0.629 | 0.719 | 0.645 | 0.679 | 0.625 | 0.707 | 0.729 | 0.543 |
EBF | 0.632 | 0.768 | 0.678 | 0.671 | 0.680 | 0.790 | 0.764 | 0.539 |
DBF | 0.335 | 0.429 | 0.339 | 0.439 | 0.337 | 0.379 | 0.400 | 0.298 |
ENF | 0.576 | 0.685 | 0.615 | 0.658 | 0.529 | 0.653 | 0.640 | 0.473 |
DNF | 0.239 | 0.320 | 0.258 | 0.259 | 0.173 | 0.226 | 0.206 | 0.139 |
All | 0.559 | 0.590 | 0.551 | 0.616 | 0.552 | 0.558 | 0.591 | 0.501 |
Table 8.
Mean absolute value of the difference between the MERSI-II LAI and MODIS LAI (). And the bold indicates the minimum error.
Table 8.
Mean absolute value of the difference between the MERSI-II LAI and MODIS LAI (). And the bold indicates the minimum error.
DOY | EVI | NDVI | RDVI | MSR | 2VIs | 3VIs | 4VIs | Proposed |
---|
017 | 0.268 | 0.310 | 0.283 | 0.317 | 0.286 | 0.282 | 0.281 | 0.127 |
081 | 0.251 | 0.310 | 0.278 | 0.294 | 0.254 | 0.297 | 0.271 | 0.133 |
193 | 0.540 | 0.539 | 0.525 | 0.574 | 0.530 | 0.534 | 0.548 | 0.296 |
289 | 0.549 | 0.601 | 0.548 | 0.549 | 0.540 | 0.547 | 0.552 | 0.279 |
mean | 0.376 | 0.387 | 0.374 | 0.532 | 0.367 | 0.369 | 0.432 | 0.182 |
Table 9.
STD of the difference between the MERSI-II LAI and MODIS LAI (). And the bold indicates the minimum error.
Table 9.
STD of the difference between the MERSI-II LAI and MODIS LAI (). And the bold indicates the minimum error.
DOY | EVI | NDVI | RDVI | MSR | 2VIs | 3VIs | 4VIs | Proposed |
---|
017 | 0.509 | 0.546 | 0.513 | 0.547 | 0.521 | 0.507 | 0.507 | 0.320 |
081 | 0.485 | 0.525 | 0.505 | 0.550 | 0.480 | 0.500 | 0.505 | 0.320 |
193 | 0.806 | 0.824 | 0.808 | 0.822 | 0.793 | 0.791 | 0.800 | 0.560 |
289 | 0.879 | 0.886 | 0.874 | 0.860 | 0.850 | 0.841 | 0.846 | 0.553 |
mean | 0.637 | 0.650 | 0.640 | 0.755 | 0.623 | 0.626 | 0.704 | 0.402 |
Table 10.
The ratio of different LAI difference categories.
Table 10.
The ratio of different LAI difference categories.
| Category 1 (<−1.5) | Category 2 (−1.5 – −0.5) | Category 3 (−0.5 – 0.5) | Category 4 (0.5 – 1.5) | Category 5 (>1.5) |
---|
MERSI-MODIS | 2.64% | 9.18% | 83.27% | 4.64% | 0.27% |
MERSI-GLASS | 0.91% | 21.11% | 76.84% | 1.11% | 0.03% |