[1] The emissions of carbon dioxide (CO 2 ) and methane (CH 4 ) from the Petit Saut hydroelectric reservoir (Sinnamary River, French Guiana) to the atmosphere were quantified for 10 years since impounding in 1994. Diffusive emissions from the reservoir surface were computed from direct flux measurements in 1994, 1995, and 2003 and from surface concentrations monitoring. Bubbling emissions, which occur only at water depths lower than 10 m, were interpolated from funnel measurements in 1994, 1997, and 2003. Degassing at the outlet of the dam downstream of the turbines was calculated from the difference in gas concentrations upstream and downstream of the dam and the turbined discharge. Diffusive emissions from the Sinnamary tidal river and estuary were quantified from direct flux measurements in 2003 and concentrations monitoring. Total carbon emissions were 0.37 ± 0.01 Mt yr À1 C (CO 2 emissions, 0.30 ± 0.02; CH 4 emissions, 0.07 ± 0.01) the first 3 years after impounding (1994)(1995)(1996) and then decreased to 0.12 ± 0.01 Mt yr À1 C (CO 2 , 0.10 ± 0.01; CH 4 , 0.016 ± 0.006) since 2000. On average over the 10 years, 61% of the CO 2 emissions occurred by diffusion from the reservoir surface, 31% from the estuary, 7% by degassing at the outlet of the dam, and a negligible fraction by bubbling. CH 4 diffusion and bubbling from the reservoir surface were predominant (40% and 44%, respectively) only the first year after impounding. Since 1995, degassing at an aerating weir downstream of the turbines has become the major pathway for CH 4 emissions, reaching 70% of the total CH 4 flux. In 2003, river carbon inputs were balanced by carbon outputs to the ocean and were about 3 times lower than the atmospheric flux, which suggests that 10 years after impounding, the flooded terrestrial carbon is still the predominant contributor to the gaseous emissions. In 10 years, about 22% of the 10 Mt C flooded was lost to the atmosphere. Our results confirm the significance of greenhouse gas emissions from tropical reservoir but stress the importance of: (1) considering all the gas pathways upstream and downstream of the dams and (2) taking into account the reservoir age when upscaling emissions rates at the global scale.

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RESUMODurante a década de 90 surgiram intensas especulações a respeito de que reservatórios de hidrelétricas poderiam estar contribuindo para a intensifi cação do efeito estufa através da emissão de Gases de Efeito Estufa (GEE). Muita polêmica tem sido estabelecida recentemente a partir de estudos realizados nos reservatórios amazônicos, especialmente a partir de estudos teóricos e baseados em extrapolações desprovidas de critérios científi cos estabelecidos. Uma das questões mais polêmicas é a estimativa de emissão de gases à jusante de represas, logo após a passagem da água pelas turbinas, em particular do metano (CH 4 ) cujas concentrações são mais elevadas em maiores profundidades. Um dos problemas é que a concentração de CH 4 a ser utilizada nos cálculos de fl uxo ebulitivo deveria ser a concentração média deste gás na faixa de captação de água e não a concentração deste gás na profundidade maior, como tem sido usado. A intensidade da emissão de GEE não é invariante no tempo, havendo fl utuações em períodos de duração irregular. Estas fl utuações são infl uenciadas por muitos fatores, como temperatura, regime de ventos, exposição ao sol, parâmetros físicos, químicos e biológicos da água. Existem difi culdades para se estabelecer uma extrapolação que realmente represente a heterogeneidade espacial dos reservatórios e que possa captar a variação temporal dos fl uxos. Estudos adicionais são indispensáveis para reduzir as dúvidas a respeito da emissão de GEE pelos reservatórios de hidrelétricas. A compreensão das diferentes formas de fl uxo de carbono, em diferentes escalas espaciais (nível do reservatório, nível da coluna d´água) e temporais (antes e depois da inundação) é indispensável para compreender a real contribuição do reservatório na emissão de GEE. Muitos ambientes naturais emitem CH 4 , especialmente pântanos e outras áreas úmidas ou habitats de fl orestas em climas tropicais. Estas emissões devem ser consideradas e descontadas em cálculos de emissões de CH 4 posteriores a inundação do reservatório neste tipo de ambiente. Este método garante que os dados obtidos após a inundação representem realmente o aumento na emissão de CH 4 provocado pela inundação da área pela represa. A emissão de CH 4 pelas hidrelétricas é sempre desfavorável para a hidroeletricidade, pois mesmo que o carbono origine-se de fontes naturais, ele se torna um gás de maior Potencial de Aquecimento Global (Global Warming Potential -GWP) no computo fi nal. Já a emissão de CO 2 em parte pode ser originada da atmosfera e ser incorporada ao sistema do reservatório pela ciclagem natural do carbono em ciclo curto de tempo. Palavras-chave: Hidroeletricidade, carbono, metano, dióxido de carbono, emissão de gases ABSTRACT GREENHOUSE GAS EMISSIONS OF DIFFERENT BRAZILIAN HYDROELECTRIC DAMS. Over the 90s surged a growing concern about the participation of hydroelectric dams in the global warming through considerable emission of greenhouse gases. Recently, the results of analysis of some hydroelectric dams in the Amazon have become a matter of c...

Abstract:Methane (CH 4 and carbon dioxide (CO 2 emissions from Brazilian reservoirs were assessed. Point measurements were made during 1998 and 1999 (using inverted funnels for bubbles and air and water concentration gradients for diffusion) in the 559 km 2 Samuel reservoir, which was initially flooded in 1988, and the 2430 km 2 Tucuruí reservoir, which was flooded in 1984, and the data were evaluated with respect to historical measurements in other Brazilian reservoirs. Bubble emissions of CH 4 were higher in Samuel (ranging from 2 to 70 mgCH 4 m 2 day 1 ) than in Tucuruí (ranging from 0Ð5 to 30 mgCH 4 m 2 day 1 ), with the highest values occurring the shallowest regions in each reservoir. CH 4 from diffusion for the Tucuruí reservoir ranged from 5 to 30 mgCH 4 m 2 day 1 , which is lower than that for the Samuel reservoir, which ranged from 10 to 80 mgCH 4 m 2 day 1 . The smaller emissions in Tucuruí compared with Samuel are attributed to a larger depletion in the source organic material that was present when the reservoir was filled. The CO 2 concentration was similar for each reservoir, and ranged from 1000 to 10 000 mgCO 2 m 2 day 1 .

In the September 2004 edition of Climatic Change (N • 66), an editorial comment was published by Philip Fearnside, jointly with an article written by ourselves (Rosa et al., 2004) reporting on a broad-ranging project through which we measured CO 2 and CH 4 emissions at eleven Brazilian hydroelectric dams, from Itaipu, with the largest installed capacity in Southern Brazil, northwards to Tucuruí in Amazonia.This comment reflects a long-standing controversy over the estimates presented by Fearnside and the experimental results that we have obtained so far on greenhouse gases emissions by hydroelectric dams in Brazil (Rosa et al., 1996;Fearnside, 1996).The key issue in this latest comment by Fearnside is based on the fact that the water intake for the Tucuruí hydroelectric dam turbines is located at an average depth of 35 m, with a high methane concentration at a pressure of around three atmospheres 1 and an estimated temperature of 15 • C. This water pressure is suddenly eased after running through the turbines to the tailrace, flowing downstream from the hydroelectric dam, while it is drawn into the spillway at a depth of no more than twenty meters, far shallower than the water intake point for the turbines.The author of these comments uses the data published by us (Rosa et al., 1997) on the methane concentration measured by Tundisi 2 , who worked in cooperation with our research group. Fearnside commits a series of errors: he is confused about the transformation of units to express the Henry's constant, obtaining a value that is completely wrong; he errs when calculating the methane concentration in the water Climatic Change (2006) 75: 91-102