## 4 Discussion
The interannual variability of CO\({}_{2}\) uptake by terrestrial sinks is mainly associated with changes in land use and meteorological factors, as the carbon balance is strongly related to its high spatial and temporal variability. Among them, precipitation plays an important role due to its remarkable seasonality in semi-arid regions [19; 50]. In these ecosystems, the availability of natural resources such as water, plant biomass, nutrient dynamics in the litter, and soil are modulated by the occurrence of rainfall. [51]. Satellite derived greenness-associated vegetation indices (such as the NDVI and EVI) have commonly been used as proxies in several biochemical characterization studies and on studies regarding their correlation with biophysical variables [30]. Our results show that there is a good correlation between GPP and EVI/NDVI patterns in the Caatinga, satisfactorily responding to surface energy and water availability variations. This kind of response is important because a prolonged leaf senescence period might actually be a reaction of the vegetation due to water availability at the end of its growing stages, which might shift a prolonged period behaving as carbon sink in years with longer than usual growing stages. During periods of abundant rainfall, the availability of nutrients in the soil is greater, resulting in a rapid and efficient uptake of nutrients by plants allocated for leaf development, reflecting higher productivity [52]. Rain events during the dry season, when leaves are still present in the canopy, may be more productive than rain events at the end of the dry season, when a second leaf emergence period might be necessary [53]. Such a combination of climate events and conditions could influence on vegetation mortality and on species dynamics during forest succession, which may be possible to monitor through satellite observations such as the products used in this study. We also found that WUE reduces with monthly rainfall, and sharply rises with consecutive dry days. At the annual scale, WUE was higher in the drier year (2015). The relative magnitudes of GPP and ET represent the sensitivity of different biological processes to drought in different seasons and years. Although both GPP and ET reduced during the dry season, the response of the latter was much more prominent than the first, leading to a higher WUE in 2015. The expressive increase of 31% in WUE (Figure 4b) during 2015 is associated to a reduction of 49% in the number of rain days, which in turn represented a reduction of 34% in mean annual ET. These results corroborate other studies found in the literature [54; 55; 56; 57]. [PERSON] [58] suggested that plants may prioritize resistance to higher temperatures via an uneven opening of stomates to improve leaf transpiration and dissipate heat while maintaining low WUE. Our study also indicates that WUE in the Caatinga is inversely related to evapotranspiration in both wet and dry seasons, which is not observed with air temperature. The negative relationship between WUE and VPD found in our study was also previously reported in the literature at the ecosystem level or even at the leaf level [59; 60]. These results have important implications on the understanding of climate change effects on water and carbon exchange processes in the tropical Caatinga environment, since future climate projections for the region indicate an increase in temperature and a reduction in rainfall [61]. Such a scenario, coupled with the observed trend of longer periods with consecutive dry days, would lead to more frequent and more intense drought events with the potential total aridification of the region [29], which would lead to an increase in WUE of the Caatinga at the ecosystem level.