# 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
### Results  
#### 4.2.1 Photochemical Reflectance Index and Sun-Induced Fluorescence  
PRI values, averaged for the period 2000 to 2012, were generally higher when acquired in the forward direction (Figure 5) compared with those acquired from the backscatter direction (Figure 6). PRIf values were lowest in the northern Amazonia during the months of February, March and April and southern Amazonia during July and August; however, the seasonal variability was small. Seasonality was much more pronounced with backscattered observations than those obtained from the forward-scattered direction. Areas with the lowest PRI\({}_{\text{b}}\) values were concentrated in the northern Amazonia from February to April (when the dry season peaks) and from July to September (droist months in the southern Amazonia). The differences between PRI\({}_{\text{b}}\) and PRI\({}_{\text{f}}\) illustrate this more clearly (Figure 7). The lowest values of PRI\({}_{\text{dif}}\) in the southern part of the study area occurred during the dry season, peaking in August. The smallest values of PRI\({}_{\text{dif}}\) were consistent with remotely-sensed estimates of precipitation averaging less than 100 mm per month [56]. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**ID/Site Name** & **Nearest City** & **Lat/Long.** & **Biome Type** & **Measurement Period** & **Principle Investigators** & **Data Reference** \\ \hline K67/Tapajos & Santarém/Belterra, Para State, Brazil & 2.855/54.97W & Tropical rainforest & January 2002 & Wofsy[PERSON], 2004 & [49, 50] \\ \hline CAX/Caxiauana & Belem, Para State, Brazil & 1.725/51.46W & Tropical rainforest & January 1999 & [PERSON], [PERSON], 2003 & [48, 51] \\ \hline RJA/Reserva Jaru & Ji-Paraná, Rondonia State, Brazil & 10.085/61.93W & Tropical dry forest & October 2003 & [PERSON], [PERSON] 2006 & [52, 53] \\ \hline BAN/Bananal Island & Pium, Tocantins State, Brazil & 9.825/50.13W & Seasonally flooded forest-Savanna & October 2003 & [48] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Information on the four Brazilian flux sites used in this study [47]. Figure 5: Monthly averages of the Photochemical Reflectance Index (forward scattered) are indicative of seasonal variation for the years 2000 to 2012 over forested areas in the Amazon Basin. Areas in grey represent non-forest cover. Figure 6: Monthly averages of the Photochemical Reflectance Index (backward scatter) are indicative of seasonal variation for the years 2000 to 2012 over forested portions of the Amazon Basin. Figure 7: Monthly averages of the difference between forward- and back-scattered Photochemical Reflectance Indices for the years 2000 to 2012 for forested areas in the Amazon Basin. Figure 8 shows the seasonal departures of GOME-2 SIF estimates from the annual mean on a 0.5\({}^{\circ}\)\(\times\) 0.5\({}^{\circ}\) grid for the combined months of June, July and August (JJA). Extremely negative values, ranging from \(-\)0.2 to \(-\)0.5, characterized most of the JJA period. At the other extreme, December, January and February (DJF) were characterized by positive values (ranging from 0.2 to 0.5) for most of the area. In contrast to PRI, SIF exhibited a more spread out behavior, extending well into northern forests rather than being concentrated mostly in the southern region. Additionally, SIF showed patches of positive values for the dry season (from 0.3 to 0.45) in the northern most point of the basin (upper left corner of Figure 8) and negative values for the same area during the wet season (ranging from \(-\)0.2 to \(-\)0.5). These seasonal patterns of SIF are consistent with results report by [27] using data retrieved from GOSAT. In contrast to PRI, the seasonality of NDVI derived from MAIAC reflectance measurements was muted across the Amazon Basin (Figure 9). NDVI is insensitive to subtle variation in canopy physiology and more responsive to structural variation in LAI. There is an ongoing debate over whether the productivity of evergreen forests in the Amazon is limited by radiation or by water.