## 2 Materials and Methods
### Data Acquisition
#### 2.2.2 Spectral Vegetation Indices  
In this study, two sources of remotely-sensed data were considered. The 300 m resolution NDVI data, consisting of 10 days aggregates of NDVI, provided by the Proba-V platform (NDVI\({}_{\text{PV}}\)) and available for the whole world since 2014 were downloaded from Copernicus global land service portal [45]. Landsat 8 operational land imager (OLI) images (30 m resolution) were downloaded from the portal of the United States Geological survey [46] for the growing season (March to October) from 2014 to 2017. Landsat 8 images have a temporal resolution of 16 days. However, cloud cover rendered most of them useless [47; 48] for our study. Selected images included only available images with no or insignificant cloud cover in the footprint of the tower. Contrarily to NDVI\({}_{\text{PV}}\) data, Landsat images represent single date images. The Table 1 presents the different bands of Landsat 8 and those of Proba-V satellite.  
Landsat 8 surface reflectance data are available at [49]. In this study however, we conducted the preprocessing from raw images. Once downloaded, the Landsat images underwent a radiometric calibration to convert radiance values to top of atmosphere reflectance, followed by an atmospheric correction through dark object subtraction in order to remove atmospheric components such as scattering and absorption of solar energy in the atmosphere, and to obtain top of canopy reflectance [50]. The image processing was conducted with the ENVI 5.1 software.  
The corrected images were used to compute the different vegetation indices (Table 2, except NDVI\({}_{\text{PV}}\)). An average value for each index was computed for the pixels in the footprint of the tower (Figure 1) using the Raster package in R software [51].  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Name** & **Landsat Range (\(\mu\)m)** & **Proba-V Range (\(\mu\)m)** \\ \hline Ultra Blue (coastal/aerosol) & Band 1 (0.435–0.451) & – \\ Blue & Band 2 (0.452–0.512) & Band 1 (0.438–0.486) \\ Green & Band 3 (0.533–0.590) & – \\ Red & Band 4 (0.636–0.673) & Band 2 (0.615–0.696) \\ Near Infrared (NIR) & Band 5 (0.851–0.879) & Band 3 (0.772–0.914) \\ Shortwave Infrared (SWIR) 1 & Band 6 (1.566–1.651) & Band 4 (1.564–1.634) \\ Shortwave Infrared (SWIR) 2 & Band 7 (2.107–2.294) & – \\ Panchromatic & Band 8 (0.503–0.676) & – \\ Cirrus & Band 9 (1.363–1.384) & – \\ Thermal Infrared (TIRS) 1 & Band 10 (10.60–11.19 & – \\ Thermal Infrared (TIRS) 2 & Band 11 (11.50–12.51) & – \\ \hline \hline \end{tabular}
\end{table}
Table 1: Spectral range of Landsat and Proba-V bands.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Satellite** & **Index** & **Formula** & **Reference** \\ \hline Proba-V & NDVI\({}_{\text{PV}}\) & NDVI\({}_{\text{PV}}=(b_{3}-b_{2})/(b_{2}+b_{3})\) & [52] \\ Landsat 8 & NDVI & NDVI = \((b_{5}-b_{4})/(b_{5}+b_{4})\) & [52] \\ Landsat 8 & GNDVI & GNDVI = \((b_{5}-b_{3})/(b_{5}+b_{3})\) & [16] \\ Landsat 8 & EVI & EVI = \((2.5^{\circ}(b_{5}-b_{4}))/(b_{5}+6^{\circ}b_{4}-7.5^{\circ}b_{2}+1)\) & [53] \\ Landsat 8 & NDSVI & NDSVI = \((b_{6}-b_{4})/(b_{6}+b_{4})\) & [21] \\ Landsat 8 & SAVI & SAVI = \(((1+L)(b_{5}-b_{4}))/(b_{5}+b_{4}+L)\) & [54] \\ Landsat 8 & LSWI & LSWI = \((b_{5}-b_{6})/(b_{5}+b_{6})\) & [55] \\ Landsat 8 & MNDWI & MNDWI = \((b_{3}-b_{6})/(b_{3}+b_{6})\) & [56] \\ \hline \hline \end{tabular}  
* L is a constant dependent on the vegetation cover and takes values from 0 (for very green vegetation) to 1 (areas with no green vegetation), assumed 0.5 here, \(b_{1}\) to \(b_{6}\) represent band numbers defined in Table 1.  
\end{table}
Table 2: Vegetation indices adopted for this study.  
Despite the availability of flux data since 2008, only a timeframe of four years (2014-2017) was considered in order to match the timeframe of the remote sensing information available for this study. In total, for the growing seasons and excluding periods with large gaps of eddy covariance data, the number of NDVI\({}_{\text{PV}}\) and Landsat images used in the study were 70 and 39, respectively.