**4. Discussion**

Compared to the following season, the greater rainfall variability at the beginning of the wet season may be the primary driver for the interquartile range of the SOS to be greater than that observed for the EOS and LOS (Figure 3). The higher frequency and accumulation are the main characteristics of rainfall contributing to the higher AMPL and LOS values at the BP experimental site. The partial coefficient of air temperature in the BP experimental site was always lower. In tropical regions, the air temperature variability is low, with about four degrees of annual amplitude difference from the monthly normal. However, in the MSD experimental site, there was a greater partial correlation between air temperature (scenario 2). The climatological water deficit is calculated from the reference evapotranspiration and precipitation, providing an efficient measure of the water availability and demand of the environment. Air temperature, directly related to water vapour-pressure deficit, is one of the climatic factors contributing to the variability of reference evapotranspiration [62]. The average monthly precipitation is quite variable in the annual cycle, which has a greater impact on the water deficit.

In the Caatinga, vegetation's phenology and photosynthetic activity are associated with water availability [9]. In drier regions such as MSD and SD, intra-annual water availability is lower, leading to shorter growing season periods. The water stress makes the environmental conditions (Figures 2 and 3) harsh for leaf maintenance for vegetation. In contrast, BP with more regular water availability (Figures 2 and 3) shows larger LOS, which means more water and carbon exchange time at the soil–vegetation–atmosphere interface [63]. Carbon assimilation was related to EVI at Caatinga by Mendes et al. [64], demonstrating that the vegetation presents more significant photosynthetic activity and

productivity (seasonal and total) during high EVI value periods. The vegetation at BP showed a higher senescence rate than in the other areas. The phenological parameter is related to leaf loss in the dry season and the maximum use of water availability for its recovery after the dry period with accelerated regrowth and increased metabolic processes in the rainy season [65]. These vegetation strategies are adaptive to optimise the phenological, vegetative, and reproductive processes [10]. The study areas showed similarities in the seasonality of phenological parameters, corroborating the high association level with water availability.

Many studies have shown that rainfall seasonality regulates SDTF canopy seasonality [24,66], but there is a complex relationship between environmental drivers and the vegetation response [9,28], mainly for the Caatinga where the plant physiology is adapted to drought and elevated temperatures [67]. As expected for the Caatinga vegetation, the months with the highest water deficit have the lowest leaf cover (lowest EVI values), a result previously found by other studies in the region [7,33,68]. About 70% of the year has a water deficit [69]. According to Flerchinger et al. [70], about 90% of the rainwater in arid and semi-arid regions returns to the atmosphere through evapotranspiration. As there is not such a significant variability in the average air temperature between the tropics, the annual constancy provides a high evaporative demand from the atmosphere throughout the year [69], conditioning the water deficit to the seasonal fluctuation of the rainfall, adjusting the Caatinga phenological cycle to the water availability. Phenological transitions are an excellent indicator of climate change [71], and future scenarios estimate greater water demand from plants and the occurrence of droughts, providing a more significant water deficit [72,73], and with that, being able to alter the phenology of the plants. The water deficit slows down plants' growth, causes leaves, fruits, and flowers to fall, and, in the short term, tends to anticipate flowering and the beginning of fruiting, reducing plant cycles. In contrast, they tend to extend or even prevent the regular completion of the plant cycle [74].

The months with the highest spectral response of vegetation occurred in the rainy season when rain and soil moisture were predominantly distributed. The peak of the EVI was preceded by the month of greatest precipitation at SD and MSD. The rain at the BP experimental site shows a smoother and more regular distribution than in other areas, with the EVI closely following seasonal fluctuations in precipitation. The ecosystem accumulates sufficient water reserves in the soil and biomass for both sites under study, resulting in slower leaf fall during the dry season. The EVI followed the monthly rainfall distributions linearly, as shown in Figure 4. Likewise, the air temperature is observed after the maximum peak of the EVI. The air temperature also decreases the vegetation response, resulting in greater water stress for the research areas. Because of the high diversity of species at the Caatinga, the studied sites could present plants at different stages of adaptation or with physiological aspects related to water uptake or leaf abscission. The fact that MSD and SD present shorter LOS than BP could be related to vegetation strategies to use the water more efficiently during the available period. The peak of EVI values was higher at MSD and SD, and this would be related to more biomass production resulting from the water use efficiency (WUE) of the species composition.
