## 4 Discussion  
Comprehending the complex relationships between environmental factors and the components of energy and mass (CO\({}_{2}\)) balances is crucial to combat the effects and expansion of desertification. In this study, the seasonal impact of the desertification process in meteorological variables measured in situ and in components of energy and CO\({}_{2}\) balances simulated by an SVAT model in NEB desertification hotspots was analyzed. Our results showed that despite the Cabrobo and Ibimirim sites being located within or near desertification hotspots under the influence of the same type of climate, meteorological variables behaved differently in each site. This is probably a consequence of the different partitioning of the energy balance at the surface. In a region characterized by more degraded land (Cabrobo), sensible heat fluxes are expected to be higher than in less degraded lands (Ibimirim), while latent heat fluxes are expected to be lower. With a lower vegetation cover, the soil is increasingly exposed, increasing the risk of desertification [48; 49]. Thus, with a higher transfer of sensible heat flux, the air near the surface becomes warmer. The simulations we carried out showed that there was a more remarkable variability (excess of H and LE outliers) in the Cabrobo site, which might indicate that it is indeed a more unstable region regarding energy balance, compared with the Ibimirim site. Water availability conditions also play an important role in the development of vegetation, and our results showed that rainfall distribution was indeed a limiting factor for productivity in the studied sites, which is consistent with previous studies found in the literature [20; 21; 50; 51]. In one study [21], mean NEE values in the wet season of a preserved Caatinga site was \(-0.70\) g C m\({}^{-2}\) y\({}^{-1}\), while mean GPP was approximately \(1.68\) g C m\({}^{-2}\) y\({}^{-1}\). In the dry season, NEE values increased to approximately \(-0.26\) g C m\({}^{-2}\) y\({}^{-1}\), while GPP values decreased to \(-0.64\) g C m\({}^{-2}\) y\({}^{-1}\). [PERSON] et al. [21] also compared the net ecosystem CO\({}_{2}\) exchange (NEE) of the Caatinga biome with that of other savannas and tropical forests across the globe, showing that Caatinga could be considered mostly a carbon sink with mean assimilation of \(-157\) g C m\({}^{-2}\) y\({}^{-1}\) in 2014 and 2015. The SITE model's simulated results obtained in this study are similar to those found by [PERSON] et al. [21], especially in terms of the Cabrobo site results. Therefore, our results indicate that the SITE model can reasonably simulate seasonal variations in CO\({}_{2}\) fluxes in the studied regions. Results showed that even in the dry season, the NEE over the studied sites presented negative values (controlled by CO\({}_{2}\) assimilation), indicating that the Caatinga biome acted as a carbon sink in 2009 and 2014. The reduction in GPP and NEE (near 0) values with the decrease in precipitation is closely related to the reduction in leaf cover. In turn, this reduction is linked to leaf senescence of the Caatinga vegetation, which is a resilience mechanism in the face of drought that limits photosynthetic activity to the few semideciduous species that manage to keep their leaves all year round. Figure 11: Monthly boxplots of hourly simulated net ecosystem exchange (NEE) in the Cabrobó and Ibimirim sites in 2009 (wet year) and 2014 (dry year). Regarding NEE values, negative values represent carbon uptake, while positive values represent carbon release. During the dry season, trees are gradually impacted by the reduction in soil water content, which leads to the closure of stomates and to the reduction in stomatal conductance and leaf transpiration, which, in turn, limits CO2 assimilation and reduces net photosynthesis. The simulated values of the energy balance components agree with results from other studies carried out in Caatinga [24; 52].