## 2 Materials and Methods
### Micrometeorological Measurements  
The data used for the calibration and validation of SITE were obtained through a micrometeorological tower of 11 m height, equipped with an EC system installed in a conservation unit of the Caatinga biome named Serido Ecological Station (ESEC-Serido), near the town of Serra Negra do Norte, in the Rio Grande do Norte State. The ESEC-Serido area is managed by the Chico Mendes Institute for Biodiversity Conservation (ICMBio), comprising an area of 1163 ha of remnant Caatinga, which can be considered representative of the whole Caatinga biome. The micrometeorological tower belongs to the Brazilian National Institute of Semiarid (INSA) and is part of the National Observatory of Water and Carbon Dynamics in the Caatinga Biome (NOWCDCB) network. The data were collected from January to December 2014 and 2015.  
In the study region, annual accumulated rainfall in 2014 and 2015 was of 513 mm and 466 mm, respectively while the 30-year average in the region is of 758 mm. Highest rainfall amounts were observed during the months from March to May and the highest daily value was 42 mm in 2014 and 65 mm in 2015, which clearly indicates the occurrence of a more intense and longer dry season. Mean annual temperature was 28.9 \({}^{\circ}\)C, ranging from 24.7 \({}^{\circ}\)C in March to 32.2 \({}^{\circ}\)C in November. Mean soil temperature was 31.4 \({}^{\circ}\)C, and annual integrated Rg was 8030 MJ m\({}^{-2}\). The mean annual value for the VPD was 1.7 kPa. The highest daily VPD means were registered in the dry season (2.7 kPa) while the minimum means (0.2 kPa) were registered in the wet season.  
The measured data were grouped into two data sets: high and low frequency. The high frequency data obtained through the EC method consist of measurements of CO\({}_{2}\) and water vapor concentration and the three wind components (\(u_{x}\), \(u_{y}\), \(u_{z}\)) measured by an Integrated CO\({}_{2}\)/H\({}_{2}\)O Open-Path Gas Analyzer and 3D Sonic Anemometer (IRGASON, Campbell Scientific, Inc., Logan, UT, USA). Atmospheric pressure was measured using an Enhanced Barometer PTB110 (Vaisala Corporation, Helsinki, Finland) and air temperature was measured by a HMP155A probe (Vaisala Corporation, Helsinki, Finland). All these measurements were collected and stored at 10 Hz frequency in a memory card attached to a CR1000 model Datalogger (Campbell Scientific, Inc., Logan, UT, USA).  
The CNR4 Net-radiometer (Kipp and Zonen, Delft, The Netherlands) was used to obtain low frequency data such as net radiation (incoming and outgoing shortwave radiation,longwave radiation emitted and reflected by the surface and longwave radiation emitted by the atmosphere) and albedo. Air temperature and relative humidity were measured with a HMP45C probe (Vaisala Corporation, Helsinki, Finland). All sensors were installed at a height of 11 m above the surface, around 4.0 m above the average vegetation canopy of the region. Soil heat flux density (G) was obtained through the average value between the measurements of two HFP01 SC model plates (Hukseflux Thermal Sensors, Delft, The Netherlands), installed at a depth of 0.05 m. All data were sampled at a 5 s frequency and stored as half-hourly averages.