Validation of LAI, chlorophyll and FVC biophysical estimates from sentinel-2 level 2 prototype processor over a heterogeneous savanna and grassland environment in South Africa

Abstract

The Sentinel-2 Level 2 Prototype Processor (SL2P) allows the generation of biophysical estimates at high spatiotemporal resolution from Sentinel-2 imagery and could be a solution for generating products in natural environments. This study validated the SL2P estimates of leaf area index (LAI), fractional vegetation cover (FVC) and canopy chlorophyll content (CCC) over the savanna and grassland environments using field measurements. The performance of the SL2P estimates in Marakele and Golden Gate Highlands National Parks were comparatively poor and linearly biased coupled with moderate-to-high errors. The SL2P estimates in the two study sites had low accuracy with relative root mean squared error’s in the range 61.63% to 85.26% and possible systematic underestimations with pBias's ranging from 32.17% to 63.16%. These findings gave insight about the performance of the SL2P estimates over the considered heterogenous environments, and suggest the need for extensive validation and re-calibration of the system using long-term field measurements.

Keywords:

• Leaf area Index (LAI)
• fractional vegetation cover (FVC)
• canopy chlorophyll content (CCC)
• error
• Sentinel-2

Acknowledgments

The Sentinel-2 data used in this study were downloaded from the European Space Agency Copernicus Open Access Hub. We sincerely thank the field assistants (namely Phomolo Seriba, Katlego Mashiane, Steven Khosa and Brian Mabunda) in the Golden Gate Highlands National Park and Marakele National Park for their collaborative effort in collecting grass LAI, FVC and LCC ground measurements. Furthermore, we sincerely thank the Agricultural Research Council of South Africa for their assistance with filed data collection equipment. Last but not least, we thank the anonymous reviewers for their helpful comments on the initial version of this manuscript.

Disclosure statement

The authors declare no conflict of interest.

Author contributions

Conceptualization, P.T. and A.R.; methodology, P.T. and A.R.; Formal analysis, P.T. and M.Q; validation, P.T., M.Q., and M.M.; resources, G.C.; writing—original draft preparation, P.T.; writing—review and editing, A.R., M.Q., M.M., G.C; project administration, P.T.

Data availability statement

We understand that the publication of the data is becoming a good practice in research. However, we plan to share all our data in future, but at this stage we are still going to further analyse it for locally parameterized types of models, looking at both empirical and the inversion of the physically-based models.

Additional information

Funding

This research was funded by Research development programme of the University of Pretoria, as well as the Southern African Science Service Centre for Climate Change and Adaptive Land Management (SASSCAL) of the National Research Foundation (NRF) of South Africa, grant number 118590.