## 3 Results and Discussion
### Daily Variations and Seasonal Dynamics of Simulated CO\({}_{2}\) Fluxes
The NEE is the difference between soil heterotrophic respiration and net primary production, thus negative NEE values indicate assimilation of carbon by the ecosystem. Furthermore, net primary production is GPP (gross photosynthesis) minus autotrophic respiration. NEE is highly dependent on stomatal opening, since stomatal conductance is commonly the main determining factor in CO\({}_{2}\) fixation [79; 80]. In addition, biochemical models of photosynthesis (P\({}_{\rm N}\)) show that P\({}_{\rm N}\) represents the minimum value of two limiting factors: maximum carboxylation rates of Rubisco (V\({}_{\rm max}\)) and electron transport rate (J\({}_{\rm max}\)) [81]. Thus, the decline in GPP and NEE after 11h can be explained by the influence of environmental factors on the stomatal opening and biochemical parameters of photosynthesis such as kinetics of the Rubisco enzyme [4; 80]. One noteworthy exception in the accuracy of the model refers to the nighttime period (Figure 7C,D). Values of CO\({}_{2}\) fluxes during nighttime were overestimated by 48% (wet season) and 56% (dry season), leading to higher RMSE (2.25) and MAE (1.99) of NEE relatively to GPP, which presented RMSE of 1.53 and MAE was 1.24 (Table 3). Have been previously reported [9] that the sensitivity of soil-surface CO\({}_{2}\) fluxes to volumetric water content above 10 cm is probably closely related to a high percentage of root and microbial biomasses in the 0-10 cm profile of tallgrass prairie. Therefore, the overestimation of simulated night CO\({}_{2}\) fluxes can be explained by the combination of root metabolism, root exudation, and, consequently, microbial activity in the rhizosphere. Figures 8 and 9 shows of monthly mean values of CO2 fluxes in 2014 and 2015, respectively, evidencing the existence of a clear seasonal variability. Table 4 shows means seasonal values of observed (and simulated) for the years 2014 and 2015 of GPP and NEE. The GPP and NEE increased at the onset of the wet season and reaching peak values in April 2014 and 2015, which GPP declined until reaching values lower than 0.2 g C m\({}^{-2}\) h\({}^{-1}\) in the dry season. Mean seasonal GPP in 2014 varied from 0.11 g C m\({}^{-2}\) h\({}^{-1}\) (0.17 g C m\({}^{-2}\) h\({}^{-1}\)) (dry season) to 0.26 g C m\({}^{-2}\) h\({}^{-1}\) (0.29 g C m\({}^{-2}\) h\({}^{-1}\)) in the wet season (Table 4). In 2015, GPP values presented a similar trend, ranging from 0.10 g C m\({}^{-2}\) h\({}^{-1}\) (0.12 g C m\({}^{-2}\) h\({}^{-1}\)) (dry season) to 0.20 g C m\({}^{-2}\) h\({}^{-1}\) (0.25 g C m\({}^{-2}\) h\({}^{-1}\)) in the wet season (Table 4). The simulation was able to capture the GPP and NEE seasonal variability. This result may indicate an efficiency of the SITE model in representing of gross primary production (GPP) and net ecosystem CO\({}_{2}\) exchange (NEE) dynamics in the Caatinga environment. In the wet season, climate, canopy, and soil water conditions were optimal for the Caatinga vegetation. Thus, the GPP and NEE peaked and surface energy exchange was driven mainly by LE, while H was low. Photosynthetic activity decreased at the onset of the dry season because the soil was dry, leaf senescence occurred and H dominated surface energy exchange. In other words, water stress modulates the variations in canopy CO\({}_{2}\) fluxes throughout the seasons. The increase in rainfall rates in the wet season favors CO\({}_{2}\) uptake resulting in increased GPP and NEE in the Caatinga. In contrast, a reduction in soil moisture content induces stomatal closure and might affect some photosynthetic traits such as the Rubisco kinetics--V\({}_{\rm max}\)[82]. Furthermore, the effects of drought in the GPP and NEE could not be captured by the SITE model unless the calibration for the dry season was carried out. These results demonstrate that net CO\({}_{2}\) fluxes are very sensitive to the physiological processes that control surface energy exchange.