# Estimation of Gross Primary Productivity (GPP)
## 2 Materials and Methods
### VIs  
In this study, we used the European Space Agency Sentinel-2 TOC (Top Of Canopy) satellite images provided by the Terrascope platform ([[https://www.terrascope.be](https://www.terrascope.be)]([https://www.terrascope.be](https://www.terrascope.be))). This platform provides access to a data set of processed Sentinel-2 images covering Belgium. Sentinel-2 carries a Multispectral Instrument (MSI) that samples 13 spectral bands in the visible and NIR regions at 10, 20 or 60 m resolutions depending on the spectral band ([[https://sentinel.esa.int/web/sentinel/home](https://sentinel.esa.int/web/sentinel/home)]([https://sentinel.esa.int/web/sentinel/home](https://sentinel.esa.int/web/sentinel/home))). The whole time-series of Sentinel-2 data from 1 January 2016 to 31 December 2018 were used in this study. The long cloudy and rainy periods in this area, mainly in 2016, affected the quality of satellite time-series. After the quality check, 15 images were retained for 2016, 24 for 2017 and 23 in 2018. From the Sentinel-2 time-series an area of 8 ha corresponding to the area of the plantation was extracted first from the Sentinel-2 time-series for each available day (Figure 1, red polygon). A total of about 800 pixels were extracted for the bands at a resolution of 10 m and 200 pixels were extracted for the bands at a resolution of 20 m (see Table 1 for more details). Secondly, using the cloud mask available for this data set, each pixel was quality checked and only cloud-free days were selected for further analysis [82]. Third, the values of the retained good pixels in the extracted area were averaged for each band, and VIs were derived. Eight VIs were calculated over all available images of the time-series: the Normalized Difference Vegetation Index (NDVI), the Enhanced Vegetation Index (EVI), the MERIS Terrestrial Chlorophyllll Index (MTCI), the Chlorophyll Red Edge Index (ChlRedEdge), the Modified Chlorophyllll Absorption Reflectance Index (MCARI), the Pigment Specific Simple Ratio (PSSR), the Structure Insensitive Pigment Index (SIPI) and the green NDVI (gNDVI). The vegetation indices were calculated with Sentinel-2 bands B02, B03, B04, B05, B06 and B08, using the formulas reported in Table 1. The Sentinel-2 bands have the following band centers; B02: 490 nm; B03: reflectance at 560 nm; B04: reflectance at 665 nm; B05: reflectance at 705 nm; B06: reflectance at 740 nm; B08: reflectance at 842 nm. B02, B04 and B08 have a 10 m spatial resolution, while B05 and B06 have a 20 m resolution.  
The NDVI and the EVI are the most known and used spectral indices for monitoring canopy greenness, development and phenology [38]. However, both the NDVI and the EVI have shown to fail in predicting the canopy phenology and productivity in specific periods of the year, especially when canopy greenness and physiology are decoupled, i.e., during drought [45] and senescence [47]. For that reason, in this study, we also considered VIs that correlate with canopy pigments and, therefore, are indirect proxies of canopy biochemistry. In particular, the gNDVI is more sensitive to chlorophyll. The ChlRedEdge and the MTCI indirectly correlate with chlorophyll content [52, 53, 54]. The PSSR was reported to have a strong relationship with chlorophyll content [29, 55] while MCARI correlates to chlorophyll concentration [56, 57]; the Structure Insensitive Pigment Index (SIPI) is sensitive to the ratio of carotenoids to chlorophyll [61].