Water and carbon dioxide fluxes in contrasting land covers typical from Brazilian Cerrado: Modeling and methodological challenges using eddy covariance data

Abstract

The tropics play a pivotal role in the terrestrial energy and water cycles, as well as regulating the carbon cycle. The increasing pressures over the remaining natural vegetation areas in Brazilian tropical forests, allied to climate change, are likely expected to alter these cycles. Despite the existence of studies that have already observed changes on water and energy fluxes, questions regarding heat and mass exchange mechanisms and the biophysical processes in tropical ecosystems and crops for food and energy production remain. Studies involving carbon exchanges and water fluxes in the Cerrado ecoregion are mostly related to agricultural land uses (e.g., pasture, eucalyptus, and sugarcane). Thus, empirical answers from undisturbed areas of this ecoregion are important to understand the whole of pristine vegetation in carbon and water flux related processes in tropical ecosystems, which generally lacks on-site observations. Here, the complex land use pattern (contrasting land use in the footprint area) of an experimental site challenges the data processing and the representativeness of a dataset obtained using eddy covariance technique. However, these challenges may also create scientific opportunities to obtain responses from contrasting land uses at the same measurement tower if a consistent data processing along fluxes calculations is performed. The purpose of this study is to compare contrasting land uses responses concerning the water and carbon dioxide fluxed observed from an eddy covariance experiment deployed in a complex site, which measures an undisturbed tropical woodland and a mixed agricultural site (pasture, sparse trees, and sugarcane). This complex landscape created methodological challenges concerning the flux footprint representativeness for data filtering to allow modelling water and carbon dioxide fluxes. Thus, this study also evaluated a workflow to calculate fluxes considering a dynamic metadata that varied canopy height, displacement height, and roughness length binned by the wind direction. The evapotranspiration in wooded Cerrado is higher than the agricultural land along the entire year, mainly due to the increased transpiration along the whole year including the dry season. In addition, this remarkable plant activity difference between the observed land covers can also be seen in the carbon dioxide flux, as its absorption tend to be higher in the wooded Cerrado than what was observed in the agricultural site. Thus, through the sampling context of this site-specific studies, it is possible to assume that the plants water-use strategies are driven by vegetation height, and the ecosystem carbon flux is controlled by vegetation structure and water availability.