In the first-ever study of its kind, Hydro-Quebec set out to determine net greenhouse gas emissions from its Eastmain 1 Reservoir. The results indicate that, over 100 years, this hydro project emits only 16 percent of the CO2 equivalent produced by a combined-cycle natural gas plant, and this amount would be lower when extrapolated to the entire watershed.
By Alain Tremblay
This article has been evaluated and edited in accordance with reviews conducted by two or more professionals who have relevant expertise. These peer reviewers judge manuscripts for technical accuracy, usefulness, and overall importance within the hydroelectric industry.
Over the past two decades, anthropogenic GHG emissions have increased to a critical level. Lakes, rivers, wetlands and reservoirs are sources of greenhouse gases (GHGs).1,2,3 In Canada, hydro plants represent about 60 percent of electricity generation capacity. Boreal run-of-river plants do not emit GHGs, and those with reservoirs emit about 40 to 100 times less GHGs per terawatt-hour (TWh) than thermal plants.1 Nevertheless, there is concern regarding the contribution of freshwater reservoirs to the increase in atmospheric GHGs.1,4
The major GHGs related to reservoir creation are carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O).5 Water residence time, reservoir shape and volume, and amount and type of vegetation flooded affect the duration and quantity of emissions.1 N2O emissions from reservoirs typically are low, unless there are significant sources of nitrogen from the watershed.1,5
For boreal and temperate regions, most of the data available account for gross emissions measured at the surface of water bodies and from reservoirs greater than 10 years old.1,6 Net emissions are emissions resulting from reservoir creation that account for GHG produced or absorbed by the natural systems over a 100-year period for the entire watershed.7,8 Evaluation of net GHG emissions from hydro reservoirs is becoming more relevant to ensure that energy production methods are compared adequately and for assessing CO2 credits. This is the goal of the Eastmain 1 net GHG emissions project, carried out in collaboration with the University of Quebec at Montreal, McGill University and Environnement Illimite Inc.
Eastmain 1 is in the boreal ecoregion of Quebec, Canada (see Figure 1). The watershed is dominated by coniferous forest, shallow podzolic and peat soils over igneous bedrock, and quaternary sediments. Aquatic systems are described as oligotrophic with overall low primary production.
Hydro-Quebec’s 1,260-MW Eastmain 1 powerhouse was commissioned in 2006. The main dam and 33 dikes form Eastmain 1 Reservoir, with a surface area of 603 square kilometers. In 2012, construction will be complete on the 770-MW Eastmain 1A powerhouse, yielding a total energy output from this reservoir of 6.9 TWh per year.
The hydrology of the basin that supplies Eastmain 1 Reservoir (35,500 square kilometers) reflects the regional climate. Runoff is strongly seasonal, with high spring flows (typically peaking in May or June) and low flows in late winter. Eastmain 1 is part of the La Grande complex: water discharged from Eastmain 1 will flow into Opinaca Reservoir to be used at the 150-MW Sarcelle powerhouse (under construction) and again at the 5,328-MW Robert-Bourassa and 1,368-MW La Grande-1 stations.
Measuring and calculating aquatic flux
The natural aquatic ecosystem is divided into river, lakes and streams. Stretching over 138 kilometers within the reservoir area, this section of the Eastmain River covers an area of 82 square kilometers. Accounting for 55 percent of the total aquatic surface area, the Eastmain River represents the dominant areal component of the natural aquatic system in the region. Up to 827 lakes, with areas of 100 square meters to 10 square kilometers, account for 45 percent of the total aquatic surface. More than 800 streams from 10 meters up to 5.5 kilometers and totaling 1.3 square kilometers, represent the smallest areal component (less than 1 percent) of the natural system.
There are three main pathways by which a reservoir may emit GHGs:
- Diffusive emissions, measured at the water-air interface;
- Bubble emissions (mainly CH4), produced at the sediment-water interface and rising to the water-air interface; and
- Degassing or downstream emissions associated with turbulence at the turbines and spillway outflows.1
These pathways were investigated using different techniques, another unique aspect of this project. Diffusive fluxes were measured using four techniques: floating chamber, gas partial pressure, automated systems and an eddy covariance tower on Ile Marie-Eve (see Figure 2). More than 150 stations were spread over natural lakes, rivers, Eastmain 1 Reservoir (one to four years old), and Opinaca Reservoir (>30 years old) using the floating chamber and gas partial pressure techniques to determine the spatial and temporal variability of GHG emissions. One automated system was installed at Eastmain 1 powerhouse, and three were installed on rafts on Eastmain 1 Reservoir and on a reference lake.
Sampling was conducted mainly during the ice-free season (May to October) and to a lesser extent during winter (December to March) to calculate the GHG concentration increase under ice and an annual GHG flux (from ice melting to ice buildup).9 Diffusive fluxes were measured from 2003 to 2009 from reference lakes and rivers (outside the present reservoir) and from lakes and rivers that are now part of Eastmain 1 Reservoir (three years before flooding). Measurements on the reservoir were carried out from June 2006 to October 2009 (four years after impoundment).
To calculate fluxes, equations such as gas solubility in water, Henry’s law, the Thin Boundary Layer equation, gas transfer coefficient and a series of secondary equations were used.9,10,11 Annual emissions were estimated from summer fluxes, considering an average ice-free period of 215 days and assuming that the carbon pool accumulated under the ice, which quickly decreases within the first month after ice breakup, represents about 30 percent of annual emissions as suggested by CO2 partial pressure data.9
Degassing emissions were measured using a continuous gas monitor installed at the Eastmain 1 powerhouse from September 2006 to December 2009. This instrument provides, with a single sampling station, a robust time series data set representative of the whole reservoir.9
It was assumed that the concentration of CH4 and CO2 in the air was constant, that any gas concentration in the water exceeding that in the air was emitted into the air, and that the difference between both concentrations represents the degassing emissions that take place immediately downstream of the powerhouse.8,9 This is considered to be a conservative estimate of degassing because, under natural conditions, the concentration of CH4 and CO2 in water is often oversaturated compared with its concentration in air. Annual overall degassing emissions were estimated by multiplying the monthly mean concentration of CH4 or CO2 in the water by the monthly mean water flow and factoring in the monthly mean water temperature.
Bubble emissions of CH4 were measured using bubble traps 30 centimeters beneath the water surface. Fifty funnels were installed along eight transects of five to 10 funnels each, covering the four major preflooding cover types (forests, peatlands, lake and river) (see Figure 2). One transect was on Lac Mitsumis, a reference lake. Low CH4 production was anticipated in the oligotrophic boreal waters studied.
To estimate annual overall gross CH4 bubble emissions from Eastmain 1 Reservoir, as CH4 bubbles generally are produced in shallow warm water, the CH4 bubbling value was multiplied by the surface area of the reservoir, using 1 percent of 603 square kilometers for the lower limit of the extrapolated results, 5 percent for the mean value, and 10 percent for the upper limit. These percentages represent the relative contribution of shallow areas to the entire reservoir surface.
The carbon sink at the bottom of Eastmain 1 Reservoir was estimated using 14 sedimentation traps installed at different locations from June to the end of September during 2008. The natural variability in lakes was estimated from sediment cores and sedimentation trap data collected from 11 various-sized lakes in the immediate vicinity of the reservoir.12 The mean value of carbon storage in the reservoir and natural lakes corresponds to area-weighted averages. The lower limit corresponds to the lowest carbon accumulation in sedimentation traps and the upper limit to the highest rates.
Terrestrial flux measurements and calculation
The terrestrial ecosystem is divided into wetlands and forests. Forest can be divided into three types: coniferous, representing 167 square kilometers or 49 percent of the total terrestrial surface area, deciduous representing 16 square kilometers or 5 percent, and burned representing 114 square kilometers or 33 percent. Wetlands can be separated into three types: bogs representing 85 square kilometers or 14 percent of the total terrestrial surface area, fens representing 1 square kilometer or 0.2 percent, and wetlands-marsh-swamps representing 25 square kilometers or 4 percent. The remaining 8 percent is bare soils, from which no emissions were calculated.
Forest CO2 fluxes were estimated based on the net ecosystem exchange (NEE) measured from August 2006 in a mature, regionally representative black spruce forest using eddy covariance. Net CO2 fluxes were measured 10 times per second (10 Hz, 5 Hz during winter), and 30-minute averages were used in data processing. After quality control, data gaps were filled using different techniques, taking into account ecosystem respiration, photosynthetically active radiation, soil temperature and other parameters.13,14 These gap-filling methods are standard procedures used by the flux community in applying the eddy covariance technique.15 An overall annual CO2 budget was calculated by accumulating the NEE over each year of study.
Because the fire cycle is around 100 years, 1 percent of the landscape, on average, should burn yearly. Therefore, we assumed that about 3 square kilometers of the total burnable area (coniferous + deciduous + burned area) would burn every year and that 50 percent of the biomass burned would be emitted as CO2.
To compute the regional coniferous forest CO2 budget, NEE values measured at Eastmain 1 were combined with literature data from other boreal black spruce forests of different ages and jack pine forests. This accounts for the spatial and temporal variability in NEE for coniferous forest types. For deciduous forests, the CO2 budget was derived from literature data on eddy covariance NEE measurements made in boreal aspen forests. Literature data were used to estimate the CO2 budget of burned forests.
Forest CH4 fluxes were estimated from chamber measurements of soil fluxes taken in 2007 in regionally representative coniferous, deciduous and burned forest sites in the reservoir surroundings. Fluxes were determined based on linear change in gas concentration from samples collected over a 90-minute period and analyzed on a gas chromatograph.16
CH4 fluxes were measured six times between June and October 2007. Annual fluxes were estimated by multiplying daily average growing season values by the length of the season (determined by eddy covariance tower data) and assuming no winter fluxes. The forest CO2 and CH4 budget was calculated as the area-weighted sum of the CH4 budget for each forest type.
Peatlands chamber measurements of NEE CO2 and CH4 fluxes were performed in six regionally representative bogs between 2005 and 2009 and in fens from 2006 to 2008. Fluxes were measured from five microforms (high hummocks, low hummocks, hollows, lawns and pools) representative of the spatial heterogeneity of the peatlands. Sampling was done during the growing season (from May to October). Wintertime daily average fluxes were assumed to be 10 percent of growing season fluxes.17 Growing season fluxes, photosynthetically active radiation and other parameters were used to estimate overall annual fluxes.14,17,18
From June 2008 to December 2009, NEE CO2 was also measured with a portable eddy covariance tower in the Lac Le Caron peatlands. Data processing and annual CO2 budget calculation were performed similarly to those for the forest data. In summer 2009, a Los Gatos fast-methane analyzer was used to measure CH4 fluxes in the Lac Le Caron peatlands.19 Continuous CH4 concentrations measured at 1 Hz were obtained from two heights, and the resulting fluxes were averaged over 30 minutes.
The bog CO2 budget is an average of the fluxes measured for each year from 2006 to 2009, using the chamber data and the average annual CO2 budget derived from the flux tower data collected in 2008 and 2009. The fen CO2 budget was obtained by averaging the chamber fluxes measured in 2006 and 2008. The overall regional wetland CO2 budget consists of the area-weighted sum of the CO2 and CH4 budget for each peatland type (bogs, fens), with swamps/marshes given the average value for all peatlands.
Results and discussion
Net GHG emissions from a reservoir should be calculated over a 100-year period and for the watershed as a whole.7,8 However, to determine net emissions, we only consider the surface area flooded by creation of Eastmain 1 Reservoir. To calculate net GHG emissions at Eastmain 1, we considered the following elements:
- Bubbles, degassing and diffusive emissions are direct emissions from the reservoir and are related to reservoir creation;
- Sources of GHG emissions from natural ecosystems (lakes, rivers, streams, forest fire emissions, CH4 emissions from peatlands, and sedimentation in the reservoir) were subtracted from Eastmain 1 Reservoir emissions;
- GHG sinks in natural ecosystems (forest and peatland CO2 sinks) were added to Eastmain 1 Reservoir emissions; and
- Data from natural ecosystems and reservoir data from four years after flooding were used to predict Eastmain 1 net GHG emissions over 100 years.
- We may have overestimated the impact of the reservoir on carbon cycling, looking only at the reservoir surface and not at the catchment level. Estimation of the net GHG emissions at the catchment level will be done with a model being developed at McGill University.
Because it is difficult to predict precise values over a long period, we present the long-term trends using three scenarios: mean, lower limit and upper limit. The first uses mean net emissions from the reservoir and corresponds to the post-flooding gross carbon emissions minus the area-weighted pre-flooding carbon emissions from the terrestrial and aquatic ecosystems. If the pre-flooding ecosystem was a net sink, the resulting net reservoir effect will be larger than the gross emissions measured from the reservoir.
The lower limit scenario (least net change from pre- to post-flooding) uses the largest pre-flooding terrestrial and aquatic carbon emissions and lowest values for diffusive, degassing and bubble emissions from the reservoir. The upper limit scenario (most net change) uses the smallest pre-flooding terrestrial and aquatic carbon emissions and highest values for diffusive, degassing and bubble emissions from the reservoir. Negative values indicate a sink (absorption) and positive values indicate a source (emission) of GHG from the ecosystems.
Our results are based on more than 120,000 measurements taken over seven years. The data from aquatic and terrestrial ecosystems showed that the ecosystems to be flooded, including forest fires, were overall a low source of carbon, with a mean value of about 4,000 metric tonnes of C-CO2/year and about 1,700 metric tonnes of C-CH4/year (see Figure 3). The forests were CO2 sinks for the mean and lower limit scenarios, with values of -27,000 to -12,000 metric tonnes of C-CO2/year.
The upper limit scenario showed a low carbon sink at 1,200 metric tonnes of C-CO2/year. Peatlands/wetlands ecosystems showed the same trend with a lower sink or source of C-CO2, with values of -8,800, -4,200 and 600 metric tonnes of C-CO2/year for the lower limit, mean and upper limit scenarios, respectively. However, peatland/wetland were sources of CH4, ranging from 1,350 to 1,850 metric tonnes of C-CH4/year. On the other hand, all aquatic ecosystems were a source of CO2 and CH4, with low values of 13,200 metric tonnes of C-CO2/year and 47 metric tonnes of C-CH4/year and high values of 21,870 metric tonnes of C-CO2/year and 137 metric tonnes of C-CH4/year. The contribution of streams to the CO2 source in natural aquatic ecosystems is substantial relative to their small total surface area. However, lakes’ carbon sedimentation represents a low sink, with values of -250 to -1,255 metric tonnes of CO2/year.
Net Eastmain 1 emissions are changing over time, starting high in the first year (500,000 tonnes of C-CO2) and decreasing exponentially over the following four years (195,000 tonnes) for the mean scenario (see Figure 4). Eastmain 1 emissions are totally dominated by CO2 diffusive emissions, which represent more than 99 percent of total emissions. Sedimentation at the bottom of the reservoir is about twice as much as in natural lakes but represents a small fraction of the total flux. CH4 emissions represent less than 1 percent of total emissions.
To predict the evolution of net fluxes over the reservoir lifespan, we calculated the lower limit of gross CO2 fluxes from measurements taken on several older reservoirs in the same region1 and combined them with the initial Eastmain 1 data to determine the long-term pattern in minimum CO2 emissions. We further assumed that the difference between upper limit, mean and lower limit values of net CO2 fluxes estimated for the first four years for Eastmain 1 will be maintained over time. On this basis, we estimated the potential range in net CO2 emissions over a 100-year period (see Figure 4).
The pattern of decline in net carbon emissions with reservoir age is described as a first-order exponential decay, which indicates that after a sharp decrease in the first five years, net GHG emissions would tend to stabilize around 108,000 metric tonnes of C-CO2 equivalent/year (which also includes CH4). Accordingly, long-term average net emissions (over 100 years) correspond to about 117,000 metric tonnes of C-CO2 equivalent/year.
The model of net GHG emissions from Eastmain 1 can be used to estimate the temporal evolution of CO2 equivalent emissions per unit of energy generated (see Figure 5). For an initial output of 2.7 TWh/year, net CO2 equivalent emissions per unit of energy generated were initially relatively high, at about 670 metric tonnes of CO2 equivalent/GWh, but these annual emissions quickly decline and are below those from a combined-cycle natural-gas station after the initial three years (see Figure 5).1 However, it takes about five years for the CO2 equivalent emissions to fall below the combined-cycle natural-gas value (see Figure 6). Our model further predicts that these net emissions should stabilize at about 58 metric tonnes of CO2 equivalent/GWh after 10 years and stay roughly constant thereafter for the mean scenario if considering the addition of 4.2 TWh by 2012.
Flooded natural ecosystems are overall a low source of carbon. Net GHG emissions are substantial in the first years after flooding and decrease rapidly, stabilizing after about 10 years. This study also indicates that CH4 emissions, degassing and bubbling emissions are not significant in terms of net GHG emissions from Eastmain 1 and that they probably are not an issue in most boreal reservoirs.
With a relatively small surface area and short water residence time, Eastmain 1 Reservoir is a good example of a hydro project emitting small amounts of GHGs. The most efficient thermal plants, combined-cycle natural gas, emit about 380,000 tonnes of CO2 equivalent/TWh,1 whereas Eastmain 1 Reservoir emits 16 percent of this amount over 100 years. Based on McGill’s predictive model, we hypothesized that net Eastmain-1 reservoir emissions extrapolated to the watershed level would be within the range of natural ecosystem emission variations, therefore implying a very small GHG footprint of the reservoir creation.
These results clearly indicate that boreal hydroelectric reservoirs are low GHG emitters. Therefore, boreal plants should be considered part of the solution to reduce the impact on climate change.
This article was written in collaboration with Julie Bastien, MSc, Environnement Illimite Inc.; Marie-Claude Bonneville, MSc, McGill University; Dr. Paul del Giorgio, University du Quebec a Montreal; Dr. Maud Demarty, Environnement Illimite; Dr. Michelle Garneau, Dr. Jean-Francois Helie, Luc Pelletier, MSc, and Dr. Yves Prairie, Universite du Quebec a Montreal; Dr. Nigel Roulet and Dr. Ian Strachan, McGill University; and Dr. Cristian Teodoru, University du Quebec a Montreal. The Eastmain-1 study has involved more than 80 people over the seven years of the project. These are the major scientists behind the study.
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12Teodoru, C.R., P. del Giorgio, Y. Prairie, and A. St-Pierre. 2011. Particle deposition trends and carbon sources in a young boreal reservoir in northern Quebec relative to natural variability in lakes of the surrounding region. Submitted to Biogeochemistry.
13Baldocchi, D.D., “Assessing the Eddy Covariance Technique for Evaluating Carbon Dioxide Exchange Rates of Ecosystems: Past, Present and Future,” Global Change Biology, Volume 9, No. 4, April 2003, pages 479—492.
14Bonneville, M.-C., I.B. Strachan, E. Humphreys, and N.T. Roulet, “Net Ecosystem CO2 Exchange in a Temperate Cattail Marsh in Relation to Biophysical Properties,” Agricultural and Forest Meteorology, Volume 148, No. 1, January 2008, pages 69-81, Doi:10.1016/j.agrformet.20017.09.004.
15Barr, Alan G., et al, “Inter-annual Variability in the Leaf Area Index of a Boreal Aspen-Hazelnut Forest in Relation to Net Ecosystem Production,” Agricultural and Forest Meteorology, Volume 126, No. 3-4, November 2004, pages 237—255.
16Ullah, S., et al, “Potential Fluxes of N2O and CH4 from Soils of Three Forest Types in Eastern Canada,” Soil Biology and Biochemistry, Volume 40, No. 4, April 2008, pages 986-994.
17Pelletier, L., et al. “Methane Fluxes from Three Peatlands in the La Grande Rivià¨re Watershed, James Bay Lowland, Canada,” Journal of Geophysical Research, Volume 112, 2007, G01018, doi:10.1029/2006JG000216.
18Pelletier, L., M. Garneau, and A. Tremblay, “CO2 and CH4 Ecosystem Exchange from Peatlands: Eastmain-1 Hydro-Electric Project, Quebec, Canada,” Verhandlungen des Internationalen Verein Limnologie, Volume 30, No. 6, 2009, pages 862-865.
19Wagner-Riddle, C., et al., “Nitrous Oxide and Carbon Dioxide Fluxes from a Bare Soil using a Micrometeorological Approach,” Journal of Environmental Quality, Volume 25, 1996, pages 898-907.
Tremblay, Alain, Julie Bastien, Maud Demarty, and Claude Demers, “Measuring Greenhouse Gas Emissions from a Canadian Reservoir,” Hydro Review, Volume 29, No. 5, July 2010, pages 22-29.
Alain Tremblay, PhD, is a senior environment advisor with Hydro-Quebec Production (Generation) and manager of the greenhouse gas research program.