Fodder Biomass Monitoring in Sahelian Rangelands Using Phenological Metrics from FAPAR Time Series
Abstract
:1. Introduction
2. Materials and Methods
2.1. Study Area and Ground Control Sites
2.2. Biomass Data Collection
Ecoregion | Main Vegetation Type and Woody Species | Annual Rainfall (mm) | Woody Leaf Biomass (kg·DM/ha) | Herbaceous Biomass (kg·DM/ha) | Woody Cover (%) |
---|---|---|---|---|---|
ECOnorth (Sandy Pastoral) | Pseudo-steppe: Boscia senegalensis, Balanites aegyptiaca, Guiera senegalensis, Calotropis procera, Combretum glutinosum, Sclerocarya birrea | 345 | 490 | 905 | 5.5 |
ECOeast (Ferruginous Pastoral) | Shrub savannah: G. senegalensis, C. glutinosum, Pterocarpus lucens, Grewia bicolor, B. senegalensis, Adenium obesum | 488 | 1219 | 1257 | 17.7 |
ECOwest (Pastoral-Agricultural) | Shrub/tree savannah: C. glutinosum, G. senegalensis, G. bicolor, B. senegalensis, C. micranthum, Commiphora africana | 524 | 1204 | 1867 | 16.9 |
ECOsouth (Eastern Transition) | Tree savannah/woodland: C. glutinosum, Strychnos spinosa, Acacia macrostachya, Crossopteryx febrifuga, Terminalia avicennioides, Maytenus senegalensis | 633 | 2611 | 1937 | 33.1 |
2.2.1. Herbaceous Biomass Collection
2.2.2. Woody Leaf Biomass Collection
2.2.3. Filtering the Ground Dataset
2.3. Satellite Data
2.3.1. CSE Biomass Product
2.3.2. Phenological Metrics from FAPAR Time Series
No. | Variables | Abbreviation | Short Definition |
---|---|---|---|
1 | Start of season | SOS | Time when the left edge has increased to 20% of the amplitude |
2 | End of season | EOS | Time when the right edge has decreased to 50% of the amplitude |
3 | Length of season | LOS | Time from the SOS to the EOS |
4 | Middle of season | PMID | Computed as the mean value of the times when the signal is higher than 80% of the amplitude |
5 | Base value | BVAL | Averaged minimum values over the annual cycle |
6 | Maximum value | PEAK | Highest FAPAR value over the season |
7 | Amplitude | AMPL | Difference between the maximum and BVAL |
8 | Large seasonal integral | LINTG | Integral of the signal from the SOS to the EOS |
9 | Small seasonal integral | SINTG | Integral of the signal above the BVAL from the SOS to the EOS |
10 | Left derivative | LDERIV | Rate of increase at the SOS between the left 20% and 80% of the amplitude |
11 | Right derivative | RDERIV | Rate of decrease at the EOS between the right 20% and 80% of the amplitude |
2.4. Modeling Total Biomass Production
2.4.1. Reduction of Explanatory Variables and Model Development
Criterion | Annotation | Formula | Decision Rule |
---|---|---|---|
Ajusted coefficient of determination | Adj. R2 | Performance increases with |Adj. R2| | |
Akaike Information Criterion | AIC | n.log + 2(p+1) | Performance increases with lower AIC |
Variation Inflation Factor | VIF | VIF > 10 points to co-linearity | |
Variable Importance in the projection | VIP | Variable is important if VIP ≥ 0.8 | |
Mean Absolute Error | MAE | A low MAE shows higher reliability | |
Normalized Mean Absolute Error | NMAE | A low NMAE shows higher reliability |
2.4.2. Bootstrap Resampling and Model Verification
Variable | Simple Statistics | Pearson Correlation Statistics | ||
---|---|---|---|---|
Mean | SD | R | p-value | |
LINTG | 6.09 | 2.88 | 0.79 | <0.0001 |
PEAK | 0.65 | 0.19 | 0.77 | <0.0001 |
SINTG | 5.25 | 2.34 | 0.76 | <0.0001 |
AMPL | 0.59 | 0.17 | 0.72 | <0.0001 |
LOS | 109 | 30 | 0.63 | <0.0001 |
BVAL | 0.06 | 0.05 | 0.59 | <0.0001 |
SOS | 196 | 20 | −0.52 | <0.0001 |
RDERIV | 14.03 | 6.10 | 0.45 | <0.0001 |
EOS | 304 | 21 | 0.38 | <0.0001 |
PMID | 258 | 14 | 0.23 | 0.0002 |
LDERIV | 20.85 | 6.21 | −0.11 | 0.0825 |
3. Results
3.1. Relationship between Total Biomass and Phenological Variables
3.2. Importance of the Explanatory Variables in Total Biomass Prediction
3.3. Selection and Verification of the Estimation Models
Region | Estimation Model of Total Biomass (B) | Adj. R2 | MAE | NMAE | n/n_test | |
---|---|---|---|---|---|---|
(kg·DM/ha) | (%) | |||||
Study area | Model_SA | B = 424.13 × LINTG − 100.91 × LOS + 39.80 × RDERIV + 293.71 | 0.67 | 608.13 | 26.0 | 263/39600 |
Model_EW | B = 4594.18 × PEAK − 129.09 × SOS + 1866.17 | 0.62 | 641.04 | 27.3 | 263/39600 | |
Ecoregion | ECOnorth | B = 1703.10 × PEAK + 1644.92 × BVAL + 432.94 | 0.24 | 426.85 | 31.0 | 121/18600 |
ECOeast | B = 463.02 × LINTG − 296.29 × LOS − 152.37 × SOS + 5969.39 | 0.49 | 575.45 | 23.1 | 65/9800 | |
ECOwest | B = 3341.72 × PEAK + 282.87 × PMID − 7125.91 | 0.15 | 589.29 | 19.1 | 44/6600 | |
ECOsouth | B = 603.53 × LINTG + 52.83 × RDERIV − 325.30 × LOS + 1944.04 | 0.31 | 512.96 | 11.3 | 33/5000 |
3.4. Comparison with the NDVI-Based CSE Biomass Product
3.5. Testing the Multiple-Predictor Model for Early Warning
4. Discussion
5. Conclusions
Acknowledgments
Author Contributions
Conflicts of Interest
Appendix
References
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Diouf, A.A.; Brandt, M.; Verger, A.; Jarroudi, M.E.; Djaby, B.; Fensholt, R.; Ndione, J.A.; Tychon, B. Fodder Biomass Monitoring in Sahelian Rangelands Using Phenological Metrics from FAPAR Time Series. Remote Sens. 2015, 7, 9122-9148. https://doi.org/10.3390/rs70709122
Diouf AA, Brandt M, Verger A, Jarroudi ME, Djaby B, Fensholt R, Ndione JA, Tychon B. Fodder Biomass Monitoring in Sahelian Rangelands Using Phenological Metrics from FAPAR Time Series. Remote Sensing. 2015; 7(7):9122-9148. https://doi.org/10.3390/rs70709122
Chicago/Turabian StyleDiouf, Abdoul Aziz, Martin Brandt, Aleixandre Verger, Moussa El Jarroudi, Bakary Djaby, Rasmus Fensholt, Jacques André Ndione, and Bernard Tychon. 2015. "Fodder Biomass Monitoring in Sahelian Rangelands Using Phenological Metrics from FAPAR Time Series" Remote Sensing 7, no. 7: 9122-9148. https://doi.org/10.3390/rs70709122