## 4 Energy Balance
#### CO\({}_{2}\) Balance  
The hourly averages of GPP and NEE simulated by the SITE model for the years 2009 and 2012 are presented in Figure 6, illustrating the observed effect of rainfall on both variables. The maximum daily mean gross CO\({}_{2}\) fixation (GPP) during 2009 was approximately 64% higher than in 2012. On the other hand, the maximum daily average net fixation was almost three times higher than the corresponding value in 2009. Figure 6: Hourly cycle of GPP (**A**) and NEE (**B**) simulated during the years 2009 and 2012. Figure 6 also shows that maximum assimilation of CO\({}_{2}\) (both gross and net) occurred between 09:00 h and 10:00 h local time, which corresponds to the period of stomatal opening in Caatinga species as reported by [20]. This coincides with the time of day when maximum CO\({}_{2}\) assimilation rates are observed. Maximum stomatal opening suggests that the SITE model satisfactorily represents the physiological mechanisms of Caatinga plants regarding CO\({}_{2}\) exchange. Nevertheless, in some experimental studies conducted in the region, the agreement between periods of maximum stomatal opening and maximum assimilation of CO\({}_{2}\) was not observed. Instead, the maximum assimilation (both gross and net) coincided with the peak availability of radiation [21; 39]. These studies suggest that, besides plant physiological mechanisms, meteorological variables, especially VPD and solar radiation, exert significant influences [20; 21]. Maximum NEE values were \(-8\) umol m\({}^{-2}\) s\({}^{-1}\) in the wetter year (2009) and \(-4\) umol m\({}^{-2}\) s\({}^{-1}\) in the dry year (2012). The maximum hourly NEE values in our study are similar to those reported by [4; 19; 21] in fragments of preserved Caatinga and by [63] in China's typical forests [64] in a savanna forest in southeastern Brazil. Ecophysiological studies indicate that CO\({}_{2}\) assimilation can be influenced by the diurnal cycle [65] of air temperature, RH and VPD. In this study, the decline in GPP and NEE after noon can be attributed to the increase in air and soil temperature [21]. Thus, it can be argued that the coupling of the physiological mechanisms of Caatinga plants to the diurnal cycles of environmental factors is crucial for determining CO\({}_{2}\) fluxes in the biome. A typical diurnal pattern was observed in all seasons, where GPP and NEE increased shortly after dawn, reaching their maximum values between 09:00 h and 10:00 h, and subsequently declining after noon to reach the lowest values (near zero) in the late afternoon. This positive behavior of GPP can be attributed to the absorption process associated with photosynthesis. The positive values of CO\({}_{2}\) flux and NEE during the night indicate that the ecosystem is releasing CO\({}_{2}\). CO\({}_{2}\) fluxes and GPP were higher in 2009. By analyzing the hourly cycle of NEE, one can observe that the ecosystem absorbs CO\({}_{2}\) (negative values) during the day and emits CO\({}_{2}\) (positive values) during the nighttime, while also responding to rainfall patterns. GPP showed high values until July, corresponding to the peaks of observed rainfall. GPP values decreased later on with the progression of the dry season, reaching their lowest values in November, indicating that water availability largely influences GPP patterns and the conditions for CO\({}_{2}\) capture. The mean diurnal value (06:00 h-17:00 h) of NEE was \(-3.98\) umol m\({}^{-2}\) s\({}^{-1}\) in 2009 and \(-1.91\) umol m\({}^{-2}\) s\({}^{-1}\) in 2012. The nocturnal values (05:00 h-24:00 h) were \(1.42\) umol m\({}^{-2}\) s\({}^{-1}\) in 2009 and \(1.19\) umol m\({}^{-2}\) s\({}^{-1}\) in 2012. These results indicate minimal variability in CO\({}_{2}\) emissions to the atmosphere during the night. During the day, there was a predominant uptake of CO\({}_{2}\) with a maximum peak observed at 10:00 h in both years.