# Evaluation of Water and Carbon Estimation Models in the Caatinga Biome Based on Remote Sensing
## 1 Introduction  
Solar and terrestrial radiation are the primary energy sources for the soil-plant-atmosphere continuum's physical, chemical, and biological processes. The interrelationship between heat, moisture, and radiation exchanges is essential in regulating the global climate [1]. Processes involving land cover changes may affect the climate as surface characteristics influence the atmosphere, thus influencing energy, water, and carbon exchanges [2; 3; 4; 5]. Evapotranspiration (ET) and gross primary production (GPP) are critical variables since they represent processes that are directly linked to the coupling between land surface and atmosphere. ET is necessary to understand the dynamics of hydrological processes, which is essential for water management, especially in regions with a semiarid climate [6]. Estimating GPP on a regional scale is still a challenge, and most studies utilize light use efficiency [7]. GPP and ET are the main regulators of water and carbon exchanges between the biosphere and atmosphere. The terrestrial biosphere influences the surface radiation balance (SRB) and the Earth's climate, mainly affecting atmospheric CO\({}_{2}\) and the surface energy balance. The dynamics of the SRB mainly depend on the vegetation type. However, this information is scarce for semiarid regions. Obtaining accurate information about surface energy fluxes in semiarid regions is essential because they have unique characteristics compared to other ecosystems [1]. It should be noted that the semiarid region covers around 11.5% of the Brazilian territory, with Caatinga originally composing \(\sim\)76% of the Brazilian semi-arid region [1]. The semi-arid areas are characterized by low and irregular rainfall, high temperatures, and solar radiation, which increase evaporation and soil desiccation, leading to water deficits during most of the year [8]. As can be seen, the estimation of water loss becomes relevant in aiding decision-making by stakeholders. Furthermore, environmental degradation potentially affects water and carbon fluxes [5; 9]. Monitoring and detecting environmental degradation-prone areas are just the first steps in providing valuable information for decision-makers to mitigate the effects and prevent climate disasters by the establishment of policies regulating land exploration and occupation [3]. According to the authors [10], among all Brazilian biomes, the greatest interest in studying carbon sequestration is directed to the Amazon Forest. Despite being one of the most threatened biomes due to hundreds of years of inefficient and unsustainable use of soils and natural resources, the Caatinga is the most neglected Brazilian biome in the most different sense. This problematic situation has come to be considered by various governmental and non-governmental sectors. In addition to the great need to preserve their natural systems, a serious insufficiency of scientific knowledge is still a challenge [11; 12]. Despite different studies carried out in the Caatinga region, there is much to be understood regarding the turbulent surface fluxes. It remains a challenge to understand how meteorological and environmental variables influence energy and mass fluxes at the biosphere-atmosphere interface, particularly in cases of suppression of native vegetation and the associated changes in the microclimate [1]. Despite the fact that Caatinga is an endemic Brazilian biome with expressive vegetation heterogeneity, few studies evaluate and validate data sets from the Moderate Resolution Imaging Spectroradiometer (MODIS). The MODIS products use simplified assumptions in algorithms to describe the processes of water and carbon [12; 13; 14; 15]. MODIS products are available in a cumulative composition of 8-day values with a pixel size of 500 m [16]. Several studies have evaluated the performance of MODIS products in recent years for different land covers [17; 18; 19; 20]; however, the validation of these products for the Caatinga biome is still lacking.