Temporal and spatial variations of energy balance closure across FLUXNET research sites

There is always a discrepancy between available energy and output energy in the surface energy budget of FLUXNET research sites. Using the daily data retrieved from the FLUXNET database, the energy balance closure (EBC) of around 150 sites covering nine land covers and five Koppen climate zones [i.e. tropical area (A), dry area (B), mild temperate area (C), snow area (D) and polar area (E)] was analyzed. The temporal and spatial variations of EBC in different land covers and climate zones were summarized, and the relationships between EBC and environmental variables were explored. The possible differences in the EBCs of sites with open path (OP) and closed path (CP) gas analyzers were also examined at different precipitation levels in various climate zones. The results showed that EBC was positively related to air temperature (Ta) and vapor pressure deficit (VPD) below certain thresholds. Better EBCs were observed in land covers with stable evaporative fraction (i.e. the ratio of latent heat flux to the sum of latent and sensible heat fluxes). For land covers with seasonal varying evaporative fraction, the larger evaporative fraction in summer corresponded with better EBCs. OP systems resulted in better EBCs at various precipitation levels, and EBCs decreased with increasing precipitation for both OP and CP systems. In addition, the relationship between EBC and CO2 fluxes was different among the different land covers. There was a positive relationship for most land covers but not for savannah, shrub land, and evergreen broadleaf forest. The relationships between EBC and CO2 as well as EBC and other environmental variables are cross-influenced, which could be related to the stomata aperture and metabolism of the vegetation. Overall, this study summarized patterns of EBCs that could be used to correct eddy covariance data and energy balance closure related models. It further enhanced our understanding of the potential link between EBC and vegetation physiology that could facilitate the modeling and prediction of biophysical processes related to water, energy, and carbon fluxes from the leaf to ecosystem levels.