Estudo das trocas de energia sobre a Floresta Amazônica

Abstract

Uncertainties on the determination of the components of the energy balance at the surface imply in limitations on the applicability of these measurements for studies of soil-vegetationatmosphere interactions and for the validation of climate and surface models. In this thesis, many aspects of the measurement of the energy balance components, especially of turbulent fluxes, were investigated, in an area of terra firme forest, at the Biological Reserve of Cuieiras of INPA, located in Central Amazonia. In general, measurements of sensible heat flux (H) and latent heat flux (LE) obtained by the eddy covariance technique (CVT) are systematically underestimated in forest sites, in relation to the measured amount of available energy. To investigate this problem of energy balance closure, besides measurements of latent and sensible heat fluxes in the air, other terms were considered, such as sensible heat fluxes in the soil (G), energy storage rates in the air (Sa), biomass (Sb) and in the soil (Sg), in the layer among the ground of the forest the height of the sensor of measurement of the turbulent fluxes, and the energy rates consumed during photosynthesis and released during plant respiration and decomposition processes (P), for 10 days of the dry and rainy seasons. On daily scales, Sa, Sb, Sg and P were lower than 3% of the net radiation (Rn), however when measurements at hourly scales are considered, an increase on the energy closure of up to 8% was obtained. Additionally, with the use of the scintillometry technique, values of H higher than the ones obtained by the CVT were observed, with the biggest differences occurring for 10-min averages and lowest for 1-hour intervals, what indicates a limitation of the CVT technique to measure fluxes in relatively long temporal scales. In this sense, for a long term analysis of the flux variability and of the partition of energy, corrections to H and LE were necessary. The amount and seasonal variability of turbulent fluxes (H and LE) were controlled by the energy available at the surface, with 75% of the net radiation being used for evapotranspiration and 25% for the heating of the air. The evapotranspiration (ETP) varied seasonally with higher values in the dry season, when the available energy and vapor pressure deficit (VPD) were its main controllers. However when the dry season presented a considerable reduction of the soil water content (Ssolo) and of the surface conductance (gs), the high vapor pressure deficit was not enough to maintain an elevated ETP. A study of the spatial variability of the fluxes was also done, comparing measurements of the energy fluxes in two topographic positions (plateau and valley) at the site. The components of the energy balance presented significant differences between the locations and between the seasons, especially in the case of H and LE fluxes, where LE was up to 65% higher in the plateau during the dry season. P was similar at both places, while the remaining terms were significantly higher in the valley and in the dry season.