Corinne Galy-Lacaux

Carbon dioxide and methane emissions and the carbon budget of a 10-year old tropical reservoir (Petit Saut, French Guiana)

emissions, 0.07 ± 0.01) the first 3 years after impounding (1994–1996) and then decreased to 0.12 ± 0.01 Mt yr 1 C( CO2, 0.10 ± 0.01; CH4, 0.016 ± 0.006) since 2000. On average over the 10 years, 61% of the CO2 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. CH4 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 CH4 emissions, reaching 70% of the total CH4 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.

Gaseous emissions and oxygen consumption in hydroelectric dams: A case study in French Guyana

Methane, carbon dioxide, and hydrogen sulfide emissions from the hydroelectric dam of Petit Saut on the Sinnamary River in French Guyana have been measured over a 2 year period. Since the beginning of the reservoir filling (January 1994), 300 km 2 of tropical forest have been submerged. Emissions of CH 4 by diffusion and by bubbling into the atmosphere or by degassing of the water released into the river, as well as the stock of dissolved gases in the lake, and their temporal evolutions were determined. Maximum emissions of 800 t CH per day were reached in February 1995, corresponding to dissolved CH 4 concentrations of 14 mg 4 L -1 in the water column. The biological oxidation of methane results in a strong oxygen consumption in lake and river waters. Total emissions of CH 4 and CO 2 from January 1994 to December 1995 were calculated from the whole data set, which also allows us to calculate the total carbon loss since reservoir filling. About 10% of the carbon stored in soil and vegetation was released in gaseous form within 2 years.

Long‐term greenhouse gas emissions from hydroelectric reservoirs in tropical forest regions

The objective of this work is to quantify long-term emissions of two major greenhouse gases, CO2 and CH4, produced by the decomposition of the flooded organic matter in tropical artificial reservoirs. In a previous paper [Galy-Lacaux et al., 1997], gas emissions from the tropical reservoir of Petit Saut (French Guiana) were quantified over the first two years after impounding. This work presents emission fluxes and distributions of dissolved methane and carbon dioxide measured in the reservoir of Petit Saut over three and a half years, since the beginning of impounding (1994) and during operation (1995–1997). To assess long term emissions, an experimental campaign was conducted on four hydroelectric reservoirs (Taabo, Buyo, and Ayame I and II) built between 1960 and 1980 in the Ivory Coast. Average dissolved CH4 concentration in the water column of the Petit Saut reservoir first increased, up to a maximum of 14 mg L−1, in May 1995. Then the time course of dissolved CH4 over the three and a half year period, showed periodical variations. These changes were related to changes in the inlet water flow and the residence time of water in the reservoir. In the older African reservoirs, average dissolved methane concentrations were lower and ranged between 0.20 and 0.32 mg L−1. The whole data set allows us to propose an analytical algorithm in order to predict the time course of dissolved CH4 concentration in the Petit Saut reservoir. Temporal variations of total CH4 and CO2 emissions from the reservoir over three and a half years were extrapolated with this algorithm to calculate long term carbon losses. Over a 20-year period the estimated carbon losses in the form of CO2 and CH4 were dominated by the outlet fluxes of dissolved gases (2160 ± 400 Gg (C)), and they correspond to a total net carbon loss of 3.2 Tg (C). The contribution of the Petit Saut reservoir to greenhouse gas emission, over 20 years, is estimated to be 66 ± 20 Tg of CO2 equivalent (56 Tg as CH4 and 9.7 Tg as CO2).

Emissions of greenhouse gases from the tropical hydroelectric reservoir of Petit Saut (French Guiana) compared with emissions from thermal alternatives

Greenhouse gas (GHG) emissions of CH4 and CO2, resulting from decomposition of flooded organic matter from the hydroelectric reservoir of Petit Saut in the tropical rain forest of French Guiana have been monitored since reservoir impoundment in January 1994. This data set along with complementary data taken from older reservoirs in forested regions of the southern Ivory Coast provides an estimate of long-term GHG emission trends from a tropical reservoir. The trends are used to calculate the contribution of this reservoir to global warming on a 100 year timescale, assumed to be consistent with the life cycle of the reservoir. Calculations are based on the concept of global warming potential (GWP). Natural emission of greenhouse gases (CH4 and N2O) from soils of the reservoir before impoundment is estimated through field measurements and literature data. Then net GHG emissions from the reservoir on a 100 hundred year timescale (30 million tons of equivalent CO2, with an uncertainty range of 7–54 Mt CO2eq) are compared with predicted emissions from thermal power plants of equivalent power (115 MW). The final comparison takes into account the actual energy production of the dam power station at only 50% of the installed capacity. Emission from this reservoir, whose power density is low (0.315 MW km−2 flooded), would be similar to emissions from a gas power plant (33 Mt CO2eq) producing the same energy amount and less than emissions from other thermal alternatives, among which the most polluting are coal plants. Such a result, however, strongly depends on the choice of the integration time.

Long Term Greenhouse Gas Emissions from the Hydroelectric Reservoir of Petit Saut (French Guiana) and Potential Impacts

This paper summarizes, in a first part, results of greenhouse gas emissions from the hydroelectric reservoir of Petit Saut in French Guiana obtained during the three first years after impoundment (1994–1997). Results from three years of measurements have been extrapolated to estimate trends in methane emissions and the carbon budget of the reservoir over a 20-year period. Extrapolations were made using the global warming potential concept to calculate cumulative greenhouse gas emissions at a 100-year time horizon and to compare these emissions to potential emissions from thermal alternatives. In a second part, we analyze new data from long term continuous observations (1994–2003) of methane concentrations in the reservoir and flux data obtained during a recent campaign in May 2003. These data confirm predicted trends and show some suitable adjustments. They constitute a unique data base which is used for the development of a model to simulate both water quality and greenhouse gas emissions from tropical artificial reservoirs.

Impact of Methane Oxidation in Tropical Reservoirs on Greenhouse Gases Fluxes and Water Quality

This chapter presents a summary of water quality data (physico-chemical) from 10 years of measurements in the Petit Saut hydroelectric reservoir in French Guiana. Methane oxidation in and downstream of the reservoir are of particular interest. In the first part of the paper we discuss both the primary factors influencing the water quality and the patterns of stratification, methane production and oxidation in the reservoir. Secondly, we present data of methane emissions and oxidation downstream of the dam. We demonstrate that the oxidation of the dissolved CH4 was a major oxygen consumer downstream of the dam. The results indicate that the aerating weir built in the plant outlet canal guarantees the minimum regulatory concentration of 2 mg·L−1 of dissolved oxygen as delineated by the scientific community of Petit Saut, following observations of the resistance to hypoxia in a tropical environment. This long term database, which helped in detecting changes over time (dissolved gases concentrations, CH4 oxidation velocity) will be used to improve the models developed to simulate both water quality and greenhouse gas emissions in a tropical reservoir environment.

Monitoring of ambient fine particulate matter concentrations from space: application to European and African cities

Air pollution is a major issue for global environment as well as human health and well-being. Recently, satellites which are equipped with relevant air quality instruments have been placed into orbit. In this paper, we first present a review on satellite remote sensing of particulate pollution. We then present new results for Europe and on African cities particulate air pollutants using POLDER satellite data. Based on satellite AOD observations, we show that the number of days exceeding the 15.4 μg/m3 threshold is twice frequent in Ouagadougou, Burkina- Faso than in Paris, France. At the regional scale, we observe that the northern coast of the golf of Guinea is dramatically impacted by poor air quality.