## 3 Results and Discussion
### Daily Variations and Seasonal Dynamics of Simulated CO\({}_{2}\) Fluxes  
Figure 7 shows the daily variations in observed and simulated GPP and NEE, considering both non-calibrated simulations and the simulations with the best calibration for the wet and dry seasons. The observed GPP and NEE values presented a pronounced daily cycle, with larger amplitude during the wet season and smaller amplitude during the dry season (Figure 7), indicating that carbon uptake was more intense in the wet season due to the higher water availability. The observed GPP and NEE values sharply increased after sunrise, reaching maximum values between 09h and 11h and declining after noon, reaching its lowest values (near zero) at the end of the afternoon. The non-calibrated simulations greatly overestimated the mean daily cycle values during both seasons (Figure 7). This illustrates the effect of neglecting the contribution of the physiological and morphological parameters of plants in this environment when predicting CO\({}_{2}\) fluxes. The main parameter influencing NEE and GPP was V\({}_{\text{max}}\). Using a value of 90 umol CO\({}_{2}\) m\({}^{-2}\) s\({}^{-1}\) for the wet season and 60 umol CO\({}_{2}\) m\({}^{-2}\) s\({}^{-1}\) for the dry season resulted in a fair agreement with observed values and a reasonable consistence with values obtained in previous ecophysiological studies involving species of the Caatinga [4; 62; 64]. In comparison with data from others research, we can observe that most of the modeling studies neglect the phenological variation of the ecosystems, usually considering a single value for Vmax. For example, the values used in the Biosphere Energy Transfer and Hydrology model--BETHY [72] and in the global-scale mechanistic model HYBRID [73], 65 and 51 \(\upmu\)mol CO2 m-2 s-1, respectively, are similar to the dry season Vmax used in the present study. In contrast, the values of Vmax defined for the joint UK land environment simulator model--JULES [74; 75] and community land model--CLM [76] are lower (48 and 31 \(\upmu\)mol CO2 m-2 s-1, respectively) if compared to Vmax values used in other models. As reported in the literature [64] after calibrating Vmax (approximately 90 \(\upmu\)mol CO2 m-2 s-1) values for the Caatinga in dynamic global vegetation models (DGVM), simulation results reached 72% of total observed GPP. Simulations of GPP and NEE were also sensitive to changes in the coefficient of stomatal conductance, specific leaf area, typical dimension of leaves and leaf width (Table 2). The study [77] used a dimensional model for forest canopy radiation absorption, photosynthesis, and transpiration (MAESTRA) to estimate GPP in the Amazon Forest, showing that modeled GPP was sensitive to changes in total canopy leaf area and Vmax, and that soil moisture, in addition to vapor pressure, controlled canopy CO2 fluxes during drought periods. The simulation also accurately retrieved the time at which the daily NEE peak occurs, between 09 h and 11 h. As presented in the previous study [35] using the SITE model, obtained a correlation coefficient of 0.88 between simulated and observed hourly CO2 fluxes in a primary tropical evergreen forest, while in the literature [78] using two versions of the SSiB model in Amazon region, obtained correlation coefficients of 0.73 and 0.79 between observed and simulated hourly CO2 fluxes. Figure 7: Simulations without calibration (dashed blue lines), simulations with calibration (dashed red lines, simulation M—wet season and simulation G—dry season) and eddy covariance observations (solid black lines) of mean daily cycle of gross primary production (GPP) and net ecosystem CO2 exchange (NEE) during the wet season (**A**,**C**) and dry season (**B**,**D**) of 2014. For details on the calibrated parameters of simulations M and G, see Table 2. Regarding NEE, carbon uptake was denoted as negative values and carbon release was denoted as positive values. For GPP, carbon uptake was denoted as positive values. Vertical bars indicate the standard deviation of fluxes.