Medições de gases de efeito estufa e variáveis ambientais em reservatórios hidrelétricos na Amazônia Central

Abstract

Lakes and reservoirs are significant sources of carbon dioxide (CO2) and methane (CH4) to the atmosphere. Hydroelectric reservoirs in the Amazon region emit more greenhouse gases (GHG) than reservoirs in other regions. However, few studies with direct in situ measurements of GHG have been performed in Amazonian tropical reservoirs. Therefore, in this study, direct in situ measurements were performed at the Balbina reservoir in Amazonas State, and at the Curuá-Una reservoir in the State of Pará. The CO2 flux measurements were performed using an eddy covariance system (Curuá- Una) and a floating chamber (Balbina). CO2 concentration measurements in air were made using a Li-7500A at Balbina and CO2 / CH4 measurements were made using an EC 150 and GGA-Los Gatos, respectively, and pCO2 was measured in the water with a CSense sensor at Curuá-Una. Gas measurements were related to meteorological variables (magnitude and direction of wind, precipitation, air temperature and atmospheric pressure) and environmental variables (water temperature, radiation, reservoir level). The study at Balbina, was performed between 15 and 20 July 2013, during the transition from wet to dry seasons, and had relatively low CO2 flux compared to another study from the same reservoir. This study showed that fluxes did not present a statistically significant relationship with the meteorological variables, however the current study identified the presence of a breeze contributing to the night concentration of CO2 over the reservoir. For Curuá-Una, an autonomous floating micrometeorological platform was used which conducted measurements during July 2015 to July 2016. Our results for Curuá-Una reveal that the turbulent fluxes of momentum, sensible and latent heat present daily and intraseasonal variability, the concentration downstream of CO2 and CH4, are influenced by variation in the level of the reservoir, CO2 flux was mainly negative throughout the day. pCO2 presented daily variability and high values with a significant difference between the measurements taken with the C-sense sensor and the headspace method. The basin of the Curuá-Una River, although small, is influenced by large-scale events such as El Nino and La Nina. The results presented here suggest that the dynamics of GHG emissions are affected by the size of the reservoir, allowing the influence of breezes and large scale events such as the El Niño of 2015-2016 which might increase emissions. The wide availability of radiation is responsible for much of the instability in the overlying layer of the water surface. The Curuá-Una reservoir, can act as a heat sink during the rainy season and as a heat source during the dry season, thus affecting the local heat balance of water and atmosphere and indirect GHG emissions.