# Progress in Remote Sensing of Photosynthetic Activity over the Amazon Basin
## 4 Case Study of Seasonal Variation in Light-Use Efficiency and SIF across the Amazon Basin
### Data Retrieval
#### 4.1.3 Eddy-Flux Measurements  
GPP derived from eddy covariance measurements of CO\({}_{2}\) exchange made at four forested Amazonian tower sites established during the 'Large-Scale Biosphere Atmosphere Experiment in Amazonia' (LBA) [46] provides an independent measure of photosynthetic activity to compare with values of PRI acquired from satellites. Five other tower sites established during the LBA campaign collected data from non-forest vegetation [47].  
The Santarem K67 tower was the most northern of the four tropical rainforest sites (Figure 4); it was located near the confluence of the Tapajos and Amazon rivers. The Caviana National Forest tower (CAX) was situated approximately 350 km to the west of the city of Belem in Para State, close to the Baia de Caxiauana. For more details, see [48]. The Reserva Jara (RJA) site, located in the Rondonia State in Brazil, lies approximately 100 km north of Ji-Parana, while the Bananal Island (BAN) site was located about 260 km west of Palmas in the Tocantins State. The site is seasonally flooded and represents a type of vegetation somewhere between a tropical rainforest and savanna (Brazilian Cerrado).  
Detailed descriptions of the types of instrumentation, the procedures for processing eddy covariance data and averaging the results at hourly and monthly time-steps are provided from the references listed in Table 1.  
Figure 4: (a) Location of the four forested Brazilian flux tower sites used in this study: Santarem forest (K67), Caixiauá forest (CAX), Reserva Jará forest (RJA) and Bananal island (BAN); (b) Average monthly precipitation (bars) and photosynthetically-active radiation (line) measured at the flux towers for periods between two and four years (see Table 1).  
Monthly averages of GPP and incident PAR were obtained for each of the forested site from the integrated Brazilian Flux LBA project CD-32 database. The fraction of visible light (PAR) absorbed by the vegetation (f\({}_{\text{PAR}}\)) was calculated from the simple [PERSON]'s law:  
\[\text{f}_{\text{PAR}}=1-\text{exp}(\text{LAI}(\text{-k})), \tag{4}\]  
where LAI is the projected leaf area index (m\({}^{2}\cdot\)m\({}^{-2}\)) and k is a light extinction coefficient of 0.5. Eight-day composites of MODIS Terra MOD15A2 LAI products (Collection 5) were the basis for calculating f\({}_{\text{PAR}}\) at each site.  
MODIS selects the maximum LAI value recorded over eight-day sampling intervals as a representative value for each pixel (Table 1). The maximum values for the 3 \(\times\) 3 km grid around each tower site were then averaged for comparison with PRI and SIF. For each site, monthly values of \(\varepsilon\) (Equation (1)) were calculated from eddy-flux-derived GPP to compare with monthly estimates of PRI. The requisite PRI observations were averaged from a 3 \(\times\) 3 MAIAC pixel cluster (9 km\({}^{2}\)) with the tower located in the central pixel. Each forested site was fairly homogeneous, minimizing the variance around the averaged PRI value. A similar averaging approach using a 3 \(\times\) 3 MODIS pixel window was also used by [PERSON] and collaborators [55] to obtain NDVI and EVI values for tower sites in the Amazon.