## 4 Discussion  
The meteorological patterns corroborate those in the same study region for other periods [16; 49; 55] and show the particularity of the site compared to others in the Amazon, such as in the region of Santarem-PA, which has greater distributions of air temperature density above 25 \({}^{\circ}\)C throughout the year [21] and has greater rainfall variability in the month of December. Seasonal changes in cloud cover are the main factors that determine the variation in solar radiation, even greater than the solar angle [69], which explains the increase in PPFD to the same extent as the decrease in precipitation. Analyzing ET and precipitation, there is an increase in ET even in the less rainy period, which can be explained by the fact that primary forest is still a very humid environment and by the local floristic composition, with trees with deep root systems that seek water in the deeper layers of the soil, thus continuing the physiological activity of transpiration and maintaining the flow of water in the biosphere-atmosphere system. The seasonal variability that occurs in energy flows during the rainy and dry periods is noticeable, and this variability is explained by the high incidence of cloudiness that occurs in the Amazon basin, which is partly responsible for the increase in diffuse radiation and the decreases in radiation balance values [70], while during the dry period, it is characterized by less cloud presence, which results in an increase in the amount of net energy available to carry out the different processes that occur at the surface-atmosphere interface, reaching its maximum. The pattern of energy fluxes accompanying energy supply corroborates other studies [16; 19; 57] for this type of forest, unlike other areas where energy supply does not correspond to increases in other flows, such as LE, due to plant stomatal control mechanisms that cause the lowest ET values to occur at the end of the dry season [71]. For the Amazon rainforest, [72] also found distinct patterns of light dependence for the period before and after 11:30 a.m., but for other types of forest, even in the Amazon, the greatest assimilation occurs strictly in the daytime period [17]. [PERSON] et al. [73] showed that the coefficients of determination between NDVI and GPP are extremely low (R\({}^{2}<0.2\)) for grasslands and evergreen broad-leaved forests, a pattern similar to the Caxiuana forest. For many forest sites, NDVI is more sensitive to vegetation change because its mathematical formulation is based on the ratio of the difference in infrared and the sum of red reflectance and the sum of red and infrared reflectance, which is not the case in this environment even with EVI, which is an enhancement of NDVI  
Figure 12: Annual carbon balance (**a**) and accumulated annual precipitation (**b**) at the Caxiuana site for the years 2005 (aquamarine line), 2006 (blue line), 2007 (green line) and 2008 (red line). because its mathematical formulation uses not only infrared and red reflectance but also surface cover and ground effects [61]. [PERSON] et al. [74] evaluated the spatial and temporal patterns of MODIS GPP at 40 sites located in the United States, comparing them with results from an ecosystem process model and with measurements of turbulent vortices (eddy covariance). It was found that there was an underestimation of MODIS GPP in highly productive months and an overestimation in low productivity months. This type of analysis is hampered in the study region and in almost the entire Amazon by the location's frequent cloud cover, which directly affects the images from optical sensors [75]. Although satellite remote sensing provides a viable and comprehensive means of observing environmental and phenological variations in the Amazon in a spatially wide-ranging and temporally frequent basis, the optical assessment of tropical areas is compromised by the high concentration of aerosols (biomass burning) and clouds [76; 77; 78], requiring the development of techniques for detecting clouds in images and improving estimates.