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Hydroelectric reservoirs can release significant quantities of CO2, but very few resultsare available from the tropics. The objective of the present study was to estimate theemission of CO2 from the Balbina hydroelectric reservoir in the central Brazilian Amazon.Diffusive and ebullitive emissions were estimated at regular intervals, both above andbelow the dam, using a combination of static chambers and submerged funnels. Gasreleases immediately below the dam were calculated as the difference between gas flux atthe entrance and the outflow of the hydroelectric turbines. An inundation model derivedfrom a bathymetric map and daily stage readings was used for spatial and temporalinterpolation of reservoir emissions. Annual emissions of CO2, upstream and downstreamof Balbina dam for 2005, were estimated as 2450 and 81 Gg C, respectively, for atotal annual flux of 2531 Gg C. Upstream emissions were predominantly diffusive withonly 0.02 Gg C yr.1 resulting from ebullition. On average, 51% of the downstreamemission was released by degassing at the turbine outflow, and the remainder was lost bydiffusion from the downstream river. The total annual greenhouse gas emission fromBalbina dam, including the CO2 equivalent of previously estimated CH4 emissions, was3 Tg C yr.1, equivalent to approximately 50% of the CO2 emissions derived from theburning of fossil fuels in the Brazilian metropolis of São Paulo.

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