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Overview - continuation

 

There is a great controversy about the amount of GHG which is exchanged between the atmosphere and the earth, mainly due to incipient methodology and incomplete knowledge about the coupling of the different components of each system. Research done in the last decade has shown that the food chain of many aquatic environments is not sustained by producing organisms (phytoplankton), but rather by decomposers (bacteria) and by the input of organic matter from the drainage basin (allochthonous material). Taking this into regard, we may conclude that photosynthesis is not the main source of carbon in these environments, being superseded by the surrounding land. And if primary production, based on photosynthesis, is smaller than the respiratory activity of bacteria, then these systems do not act as sinks of atmospheric carbon. On the contrary, they become sources of carbon dioxide. This approach on the functioning of aquatic environments is still relatively new and many studies and equipments are being developed for the measurement of the respiration rates of bacteria in comparison to photosynthetic production. As a result of this new limnological outlook, the status of hydroelectric power as “clean energy” has been questioned, since reservoirs assimilate large amounts of plant biomass which is flooded by them. It has been suggested that the decomposition of this huge reservoir of carbon in the aquatic environment is responsible for greenhouse gas emissions comparable to that of thermoelectric power plants of the same magnitude.

Indeed, recent research about the production and emission of GHG in reservoirs has shown that these systems have non-negligible emissions, particularly of methane (CH4), carbon dioxide (CO2) and nitrous oxide (N2O). Considering this, Brazil has invested in the development of national surveys of GHG emissions (
www.mct.gov.br). As regards the hydroelectric issue, at the first stage only the CH4 emissions in the reservoirs were taken into account, which are related to the deforestation and land use changes. FURNAS, through a partnership with COPPE, made a significant contribution to this survey and to the understanding of reservoir emissions, by studying the emissions in Serra da Mesa reservoir still in the flooding phase. In that study not only the CH4 emissions but also the dissolved carbon dioxide emissions were measured, including the measurement of dissolved gases in different depths. The results established that the carbon dioxide emissions are roughly ten times greater than the methane emissions, and that large amounts of both gases were retained in the hypolimnium, as a result of anaerobic decomposition of flooded vegetation. Another important contribution of that study was the use of the surface-water level regression for the estimate of the methane emission, since it was observed that no emission took place in areas with a depth greater than 40 meters. Due to this the estimates became more precise than the simplistic extrapolation of the observed values for the entire water body.

As well as COPPE, some other Brazilian research institutions have examined the matter of GHG emissions in reservoirs, most importantly the INPE-CENA (Lima & Novo, 1999; Lima, 2002) and the INPA (Fearnside, 2002). Due to the great variance in the average flow values measured by the different teams, resulting, mainly, from the diversity of methodology of data collection and by the often non-linear nature of emission processes, it is clear that research must be undertaken aiming to perfect and standardize the different methods. The state-of-the-art suggests that where the hydroelectric power generation is smaller than 0,1 W per square meter of reservoir área, there is the possibility that the GHG emissions are greater than those produced by a thermoelectric power plant of the same magnitude (Rosa & dos Santos, 2000).

The World Commission on Dams (www.dams.org) has underlined that in spite of the observation of GHG, one must also take into account the ways in which the system exchanged GHG with the atmosphere prior to the flooding of the reservoir. Therefore, it is necessary to establish a budget of how much carbon the reservoir will release in its life cycle, and how much the pre-existing system would release naturally in the same period. Considering this, our project has the following main questions to answer:

1 – What is the budget of GHG emissions in reservoirs?
2 – Does hydroelectric power act towards an increase in the greenhouse effect?
3 – What are the differences in the methods of estimating the flows of GHG?
4 – How great is our dependence upon the temporal and spatial resolution of the processes for the development of adequate models for understanding and predicting these processes?

The project involves the Department of Environmental Engineering (DEA.T) of Furnas Centrais Elétricas S.A, the Alberto Luiz Coimbra Institute – Graduate School and Research in Engineering – COPPE/UFRJ, the National Institute for Space Resarch – INPE, the Federal University of Juiz de Fora – UFJF, the International Institute of Ecology – IEE, and a foreign consultant, expert in measuring sediment exchanges of GHG with the water column, with experience at Chile, Europe, Africa, New Zealand, Canada and Japan.

This exchange of knowledge will open the door for the transfer of technology for measuring emissions from the sediment and for the development of models for the evaluation of GHG emissions in reservoirs that flood Cerrado areas. Each researcher will develop different aspects related to the carbon cycle and to the measurement of the emissions by reservoirs.

This project is designed to be developed over five years, in which measurements will take place in most of the reservoirs of the company. The studies will be developed in two reservoirs in each year, in this order:

1st year: UHE Serra da Mesa and APM Manso;
2nd year: UHE Itumbiara and UHE Corumbá;
3th year: UHE L.C.B. de Carvalho and UHE Mascarenhas de Morais; UHE Furnas;
4th year: UHE Funil and APM Manso;
5th year: development of models and final report.

The carbon emissions at APM Manso will be measured once again in the fifth year so that we may ascertain the effect of the passage of the year over the emission rates obtained earlier, while the reservoir was still in its stabilization phase.

The following documents will result from the project:

• field reports, presenting the results of each field trip, to be presented in January, May, and September;
• annual reports with conclusions about the emissions and most important factors influencing the carbon cycle in each reservoir; to be presented in March of each year;
• final report presenting the general conclusions of the project: the carbon budget in the reservoirs of FURNAS Centrais Elétricas S.A,

The following diagram shows the different activities that will take place in each reservoir.

The greenhouse effect

 

 

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