## 2 Materials and Methods
### Description of SITE Model and Site Specific Biophysical Parameters  
The 30 min micrometeorological observations measured at the study site were used to drive SITE simulations (SITE, version 1.1-0d). Observations included air temperature, precipitation, horizontal wind speed, downward shortwave and longwave radiation and specific humidity. The model was developed by [PERSON] and [PERSON] (2004), and is available at [50]. SITE is available as a FORTRAN code and structured with a canopy layer and two layers of soil using data collected above the canopy in 1 h periods, such as air temperature and specific humidity, horizontal wind speed, incident shortwave radiation, net longwave radiation, albedo, precipitation and atmospheric pressure. The model also uses parameters of biophysical characteristics of the vegetation. The main output variables of the model are net radiation (Rn, W m\({}^{-2}\)), latent heat flux (H, W m\({}^{-2}\)), sensible heat flux (LE, W m\({}^{-2}\)), soil heat flux (G, W m\({}^{-2}\)), gross primary production (GPP, g C m\({}^{-2}\) h\({}^{-1}\)) and net ecosystem exchanges (NEE, kg C ha\({}^{-1}\) h\({}^{-1}\)). SITE is a dynamical point model that uses an integration time step (dt) of one hour, representing a land portion entirely covered by an evergreen broadleaf forest. The biophysical parameters used for the studied area were determined by in situ analyses and previous studies carried out in the Caatinga vegetation, shown in Table 1. The SITE model uses a parameterization for the carbon balance adapted from the integrated biosphere simulator--IBIS model [7]. The hydraulic parameters of the soil were calculated as adapted from [51]. For more details on the dynamics of the model see the study [35]. A series of 730 days of data, collected from January to December 2014 and 2015, was used to evaluate the model. Moreover, a series of 120 days of data collected during the wet season (February to May 2014) and 92 days during the dry season (August to October 2014) was used to calibrate daily variations in the model. Validation of the model was performed using a dataset measured by the EC technique. The measured data used for calibration and validation of the model were: net radiation (Rn), latent heat (H), sensible heat (LE), soil heat (G) and CO\({}_{2}\) fluxes. The SITE model was used using site-specific data on temperature, horizontal wind speed, specific humidity, photosynthetically active radiation (PAR) and rainfall in 2014 and 2015. The daily cycle and seasonal dynamics of simulated Rn, LE, H, G, GPP and NEE from the SITE model were compared with observed data form the wet and dry seasons. The SITE model was originally developed to study the response of tropical ecosystems to varying environmental conditions. Since our study area is a seasonally dry tropical forest with a long dry season, we carried out calibrations tests to assess the variability in energy flux and CO\({}_{2}\) caused by the variation in biophysical parameters. Based on previous sensitivity analysis [52; 53; 54], we chose to calibrate only the parameters most likely to influence the result of the model: initial fraction of moisture in the soil (\(\mathrm{\theta g/\theta d}\)), coefficient of stomatal conductance (m), maximum capacity of the Rubisco enzyme (Vmax), typical dimension of leaves (d\({}_{\mathrm{u}}\)), typical dimension of stems (d\({}_{\mathrm{s}}\)), leaf width (w), and specific leaf area (SLA). Parametrization of the model followed the original parametrization included in the model SITE. Table 1 shows the parameters adopted for the Caatinga biome. For the calibration of the model parameters we adopted the sequential method, in which parameters are calibrated separately for each step, according to the hierarchy used by the SITE model in the calculations: infrared radiation balance in the canopy and balance of solar radiation, aerodynamic processes, plant physiology, transpiration, balance of water intercepted by the canopy, transport of mass and energy fluxes, soil heat flux and soil moisture and carbon balance.