## 2 Materials and Methods
### Soil-Vegetation-Atmosphere Transfer Model  
In order to determine the components of energy and CO\({}_{2}\) balances, we used the simple tropical ecosystem (SITE) model, which represents soil-vegetation-atmosphere processes and was first designed as a tool to be used in tropical ecosystems [34]. The SITE model comprises different surface processes such as infrared radiation budget above the canopy, solar radiation budget, aerodynamic processes, canopy physiology and transpiration, canopy water budget, mass and energy transfer in the atmosphere, two-layer soil heat fluxes, soil water, carbon flux, and balance. There is a great variety of available models similar to SITE [29], as reported in another study. However, for this particular study, the SITE model is more advantageous because it has been previously calibrated for the Caatinga environment [31], presenting satisfactory results, with correlation coefficients statistically significant at a 95% confidence level, which is higher than 0.70 for energy and CO\({}_{2}\) balance components. These results were obtained through the assimilation of in situ measurements of biome information such as specific leaf area, leaf width, maximum capacity of rubisco enzyme, coefficient of stomatal conductance, \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Station** & **Latitude** & **Longitude** & **Elevation** & **Gaps** \\ \hline Born Jesus do Piaui (PI) & 09\({}^{\circ}\)04\({}^{\prime}\)28\({}^{\prime\prime}\) S & 44\({}^{\circ}\)21\({}^{\prime}\)31\({}^{\prime\prime}\) W & 277 m & 7\% \\ Cabrobó (PE) & 08\({}^{\circ}\)30\({}^{\prime}\)51\({}^{\prime\prime}\) S & 39\({}^{\circ}\)18\({}^{\prime}\)36\({}^{\prime\prime}\) W & 325 m & 3\% \\ Ibimirim (PE) & 08\({}^{\circ}\)32\({}^{\prime}\)26\({}^{\prime\prime}\) S & 37\({}^{\circ}\)41\({}^{\prime}\)25\({}^{\prime\prime}\) W & 395 m & 2\% \\ Jaguaribe (CE) & 05\({}^{\circ}\)53\({}^{\prime}\)26\({}^{\prime\prime}\) S & 38\({}^{\circ}\)37\({}^{\circ}\)19\({}^{\prime\prime}\) W & 123 m & 3\% \\ Patos (PB) & 07\({}^{\circ}\)01\({}^{\prime}\)28\({}^{\prime\prime}\) S & 37\({}^{\circ}\)16\({}^{\prime}\)48\({}^{\prime\prime}\) W & 242 m & 7\% \\ Sobral (CE) & 03\({}^{\circ}\)41\({}^{\prime}\)10\({}^{\prime\prime}\) S & 40\({}^{\circ}\)20\({}^{\prime}\)59\({}^{\prime\prime}\) W & 69 m & 15\% \\ Tauú (CE) & 06\({}^{\circ}\)00\({}^{\prime}\)11\({}^{\prime\prime}\) S & 40\({}^{\circ}\)17\({}^{\prime}\)34\({}^{\prime\prime}\) W & 402 m & 3\% \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Location of the seven municipalities located within or in the surroundings of each desertification hotspot in the NEB, as shown in Figure 1: PI, Piaui state; PE, Pernambuco state; CE, Ceará state; PB, Paraiba state. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & \multicolumn{3}{c}{**Averages of Climate Variables**} \\ \hline
**Station** & \begin{tabular}{c} **Annual Accumulated** \\ **Precipitation (mm)** \\ \end{tabular} & \
\begin{tabular}{c} **Air Temperature** \\ **(\({}^{\circ}\)C)** \\ \end{tabular} &
\begin{tabular}{c} **Relative** \\ **Humidity (\%)** \\ \end{tabular} \\ \hline Born Jesus do Piaui (PI) & 598.1 & 27.3 & 62.3 \\ Cabrobó (PE) & 407.6 & 27.1 & 59.1 \\ Ibimirim (PE) & 380.9 & 25.9 & 59.1 \\ Jaguaribe (CE) & 407.8 & 29.1 & 55.0 \\ Patos (PB) & 654.6 & 27.9 & 52.0 \\ Sobral (CE) & 505.1 & 27.6 & 69.0 \\ Tauú (CE) & 507.5 & 27.1 & 55.9 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Averages of climate variables of the seven municipalities according to Table 1.  
etc. Thus, the same specific parametrization used in another study [31] was used in the simulations of this study. In order to more objectively represent results, we selected two contrasting years regarding annual rainfall volumes, since it is the main meteorological aspect driving ecosystem activity in the region. The choice of the years was based on another study [2], which evaluated vegetation-rainfall relations in NEB desertification hotspots and showed that 2009 was an anomalously wet year and 2014 was an anomalously dry year in the Cabrobo hotspot.