# WUE and CO2 Estimations by Eddy Covariance and Remote Sensing in Different Tropical Biomes
## 4 Discussion  
Evapotranspiration showed well-defined seasonality in all sites, varying at the annual scale with local precipitation, radiation availability and increasing temperatures. Highest evapotranspiration values were registered in the warmer months and/or in months with more radiation or precipitation. Evapotranspiration reached maximum values in the Cerrado and Pantanal sites in wetter months, reaching up to 7 mm day\({}^{-1}\) in November, with a mean value slightly above 3 mm day\({}^{-1}\). In the Caatinga site, even in the month with the highest evapotranspiration rates (October), overall values did not reach the same intensity of the other sites, showing the particularity of this site regarding its arid climate (BSh) according to [PERSON]'s classification ([PERSON] et al., 2020) [67]. Crucial studies have already been conducted to estimate the magnitude, seasonality and controls of ET at the local scale using eddy covariance measurements in Brazil ([PERSON] et al., 2009; [PERSON] et al., 2010) [6, 68]. These studies usually show that dry season evapotranspiration is higher than in the wet season, and Rn is the main ET control in tropical rainforests (such as the Amazon), while it is not true for the Caatinga, Cerrado and Pantanal sites. Our results showed that in these sites, evapotranspiration decreases throughout the dry season, reaching the lowest rates between August and September, when the Amazon features its highest values. Even during the dry season, high evapotranspiration rates can be explained by the predominance of pioneer tree species (_Combretum Lancolatum_ and _Vochysia divergens_) with high photosynthesis rates and stomatal conductance (gs) ([PERSON] et al., 2013; [PERSON] et al., 2016) [69, 70], associated with the ability of these species to extract water from deep storages containing similar water content to that of inundation areas and wet periods ([PERSON] et al., 2011; [PERSON] et al., 2011; [PERSON] et al., 2013; [PERSON] et al., 2021) [29, 69, 71, 72]. In the dry season of the Cerrado site, an inverse relationship was found between LE and Rn, which is typical of savanna ecosystems, where the root system does not reach deep water storages, but part of the vegetation is adapted and relies on senescence mechanisms of tree leaves and dormancy of grasses (3). A pulse in productivity (increased GPP) that is directly related to the increase in water availability (rainfall) can be observed, particularly from October on, which marks the beginning of the transition from the dry to the wet season. As expected, all sites reduced productivity in response to increased rainfall variability, with the most productive ecosystems being those with the highest precipitation rates (Amazon and Cerrado). The low rainfall rates observed in the Caatinga in October were sufficient to drive a sharp increase in productivity, which indicates that more frequent rainfall events in this environment could lead to more nutrient availability and the mitigation of water stress through leaf absorption mechanisms. A further indicator of these aspects can be observed through the remarkable variability in evapotranspiration during this month in the Caatinga. Furthermore, this biome presented the best correlations between GPP and evapotranspiration. The comparison between observed and remotely sensed data showed that both datasets represent the same seasonality, despite their weak correlation. Satellite data accurately represents the strong response of the Pantanal and Caatinga sites to local precipitation, while the results are not satisfactory regarding the dry period in the Amazon and Cerrado. The weak correlation between GPP data in tropical forests is well documented ([PERSON] et al., 2018) [55] and is generally attributed to light-use efficiency, which is underestimated by the MODIS GPP algorithm ([PERSON] et al., 2013) [73]. Additionally, potential uncertainties regarding the FPAR might also affect MODIS GPP estimates, which have also been discussed in the literature ([PERSON] et al., 2017) [60].