Closure and partitioning of the energy balance in a preserved area of a Brazilian seasonally dry tropical forest

Abstract

The energy balance closure obtained through the eddy covariance method is a problem which persists, despite advances in the development and improvement of instruments and recent efforts in the description of corrections and in the characterization of measuring uncertainties. In most places the sum of sensible and latent heat fluxes (H and λE) is less than available energy, i.e. the difference between net radiation (Rn) and soil heat flux (G). This study analyzed the annual and seasonal behavior of the energy partitioning and energy balance closure in the Caatinga Biome, which is a seasonally dry tropical forest located in the semiarid lands of Brazil, using the eddy covariance method. Results showed high seasonal variability in the energy partitioning. During the dry season, approximately 70% of Rn was converted into H and less than 5% of it was converted into latent heat flux (λE). During the wet season, the Rn portion converted into H and λE was similar: ˜ 40%. In annual terms, the Rn portion converted into H and λE was of the order of 50% and 20% respectively. The degree of the energy balance closure varied depending on the method used. When the closure was calculated using orthogonal regressions, the slope varied from 0.87 to 0.90 in 2014 and from 0.92 to 1.00 in 2015. However, when the closure was calculated by the energy balance ratio method, values varied from 0.70 to 0.79 in 2014 and from 0.73 to 0.82 in 2015. The closure was better in 2015 if compared to 2014 possibly due to the more intense turbulence observed in 2015 because friction velocity was higher than in 2014. The better closure in 2015 may also be associated with large eddies, which were more frequent in 2014 as evidenced by the correction coefficients for vertical wind velocity and water vapor and vertical wind velocity and sonic temperature. The energy balance closure was also analyzed considering atmospheric instability conditions and the best results were found under very unstable conditions, while the least expressive results were found under stable conditions. Under these conditions negative values of the energy balance ratio were also observed during dry and transition seasons, indicating that fluxes were reversed during these periods.