Environ Sci Pollut Res DOI 10.1007/s11356-014-3165-4


Carbon emission from global hydroelectric reservoirs revisited Siyue Li & Quanfa Zhang

Received: 17 March 2014 / Accepted: 5 June 2014 # Springer-Verlag Berlin Heidelberg 2014

Abstract Substantial greenhouse gas (GHG) emissions from hydropower reservoirs have been of great concerns recently, yet the significant carbon emitters of drawdown area and reservoir downstream (including spillways and turbines as well as river reaches below dams) have not been included in global carbon budget. Here, we revisit GHG emission from hydropower reservoirs by considering reservoir surface area, drawdown zone and reservoir downstream. Our estimates demonstrate around 301.3 Tg carbon dioxide (CO2)/year and 18.7 Tg methane (CH4)/year from global hydroelectric reservoirs, which are much higher than recent observations. The sum of drawdown and downstream emission, which is generally overlooked, represents 42 % CO2 and 67 % CH4 of the total emissions from hydropower reservoirs. Accordingly, the global average emissions from hydropower are estimated to be 92 g CO2/kWh and 5.7 g CH4/kWh. Nonetheless, global hydroelectricity could currently reduce approximate 2,351 Tg CO2eq/year with respect to fuel fossil plant alternative. The new findings show a substantial revision of carbon emission from the global hydropower reservoirs. Keywords Greenhouse gases (GHGs) . CO2 . Methane . Hydropower reservoir . Carbon cycling

Introduction Greenhouse gas (GHG) emissions including carbon dioxide (CO2) and methane (CH4) from hydropower reservoirs have been increasingly concerned recently (Tranvik et al. 2009; Responsible editor: Constantini Samara S. Li (*) : Q. Zhang Key Laboratory of Aquatic Botany and Watershed Ecology, Wuhan Botanical Garden, The Chinese Academy of Sciences, Wuhan 430074, People’s Republic of China e-mail: [email protected]

Barros et al. 2011), and some research has questioned green credentials of hydropower reservoirs (cf. Giles 2006). Highly oversaturated aqueous GHG concentrations in reservoirs relative to atmospheric levels result from riverine CO2 input and net heterotrophy. The net heterotrophic ecosystem is attributable to supplements of labile organic matter from upper catchment that fuel in situ bacterial respiration and decay of the flooded biomass (Li et al. 2013; Hertwich 2013). Lightlimited phytoplankton growth and degradation of organic carbon particularly anaerobic digestion of sediment organic matter further increase GHG concentrations in the deep water column (Guérin et al. 2006; Kemenes et al. 2007; 2011; Baulch et al. 2011). The former causes diffusion of CO2 and CH4, while the latter potentially causes bubbling emission of CH4 and substantial amounts of downstream emissions via water leaving turbines. However, downstream emissions are often neglected or underestimated (Barros et al. 2011; Fearnside and Pueyo 2012; Chen et al. 2013). The human-induced carbon emission from hydropower reservoirs represents a crucial component in global carbon cycling and becomes increasingly interested (cf. Kemenes et al. 2007; 2011; Barros et al. 2011; Bastviken et al. 2011). Barros et al. (2011) estimated the carbon emissions of 176 Tg CO2/year and 4.4 Tg CH4/year from global hydroelectric reservoirs, which was clearly downgraded from the earlier estimate with a consideration of the matter of surface area (709 Tg C as CO2 equivalents per year for artificial reservoirs; St. Louis et al. 2000). A more recent work revised the GHG emissions from hydropower reservoirs to be 278.7 Tg CO2/ year and 9.7 Tg CH4/year (cf. Hertwich 2013). The large range of estimates reflects both data paucity and inherent differences in the various methods for extrapolation (St. Louis et al. 2000; Lima et al. 2008). For example, more datasets (i.e., 150 global measurements) were utilized to estimate average emission rates in different climate zones by Barros et al (2011), while very limited data by St. Louis et al. (2000), and a weighted mean was adopted by Hertwich

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(2013). Generally, diffusive flux of CH4 is measured using surface floating chambers, while CO2 by floating chambers and theoretic diffusion model from pH and alkalinity in particular (Hertwich; 2013; Li et al. 2012; 2013). It is difficult to capture the ebullitive CH4 release as bubbles which is usually determined using eddy covariance (Schubert et al. 2012), funnels (Tremblay et al. 2005) and ecosounders (Delsontro et al. 2011). Besides temporal heterogeneity and poor spatial coverage of GHG fluxes that contribute to large uncertainty of global GHG emission, GHGs from drawdown and reservoir downstream (including spillways and turbines as well as river reaches below dams), which are proved to be significant carbon emitters, have not been included in prior global and regional scenarios (St. Louis et al. 2000; Cole et al. 2007; Barros et al. 2011; Bastviken et al. 2011). Also, the reservoirs used for global GHGs budget are in Americas and North Europe, and no data on GHG flux from hydropower reservoirs are from China, where there are already more than 80,000 reservoirs. Moreover, the previously neglected drawdown and downstream carbon emissions definitely result in a clearly underestimate of GHG emission from the associated hydropower reservoirs (Guérin et al. 2006; Kemenes et al. 2007; 2011; Chen et al. 2009, 2013; Yang et al. 2012). Here, we present a new estimate of carbon emissions from the global hydropower reservoirs with the combination of reservoir surface area, drawdown zones and reservoir downstream (spillways, turbines and river downstream the dams) through literature review, data collection and model development (see Table 1).

Table 1 Source proportions of CH4 and CO2 via different pathways for hydropower reservoirs (tropical source proportions of CH4 and CO2 are based on the well-studied tropical hydropower reservoirs (Petit Saut, Reservoir surfacea

Tropical CH4

Reservoir surface/downstream=1/1b

Methodology Little insight is known about GHG flux from drawdown area and dam downstream (including river reaches downstream of dams as well as spillways and turbines) that are associated with hydropower reservoirs; thus, we developed estimate models based on well-studied cases (see Table 1). GHGs from classically reservoir surface Carbon emission fluxes from reservoir surface were based on the data collection by Barros et al. (2011) and Hertwich (2013). GHGs from drawdown area Renewable carbon sources such as anthropogenic inputs and periodic flooding of vegetation in particular contribute to exceptional high GHG fluxes in the drawdown zone (Chen et al. 2009; Yang et al. 2012). Source apportionments of CO2 and CH4 emissions in relation to drawdown area for tropical reservoirs were from Rosa et al. (2002, 2004) who reported a drawdown evasion rate of 13,000 mg CO2/m2/day and 235 mg CH4/m2/day. The submerged area is highly variable due to water fluctuations; nonetheless, an averaged 10 % of the reservoir surface for drawdown area and a half year was designated for simplified calculations of GHG emission. The source apportionments in the temperate reservoirs were from the well-documented Three Gorged Reservoir (TGR) in China (Table 1). Li et al. (unpublished) compiled the data on GHG emissions for China’s reservoirs from literatures, and consequently, carbon emission by considering

Balbina and Samuel), while the source proportions of temperate and boreal reservoirs are based on China’s studies)

Downstream Spillways and turbines

Rivers downstream

Drawdown area

0.8-fold reservoir surfacec

0.2-fold reservoir surfacec


See section methodology c

CO2 Reservoir surface/downstream=10/3 1-fold rivers downstream 0.15-fold reservoir surface Temperate and boreale CH4 Reservoir surface/downstream=1/8.25 6.5-fold reservoir surface 1.75-fold reservoir surface CO2 Reservoir surface/downstream=1/0.98 0.83-fold reservoir surface 0.15-fold reservoir surface a

See section methodology 2.75-fold reservoir surface 0.45-fold reservoir surface

Drawdown area is excluded from surface area


Kemenes et al. (2007), while the ratio is revised to 1/18 for tropical-Amazonian with consideration of methodological factors (Fearnside and Pueyo 2012) c

CH4 emission from spillways and turbines is fourfold that from rivers downstream (Guérin et al. 2006)


The estimate of CO2 emission from turbines is conducted on the basis of half of the downstream emission contributed by degassing at the turbine outflow (Kemenes et al. 2011) e

From China’s hydropower reservoirs (Chen et al. 2009; Yang et al. 2012; Li et al. unpublished)

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reservoirs surface, drawdown area and reservoirs downstream was quantified. The source proportions of CO2 and CH4 were developed as follows: (1) land use in the drawdown zone was categorized to two significant land use types (rice paddy and dryland) based on the CH4 evasion rate, (2) inundated days and area for the two land-use drawdown zones were designated, and (3) the total CH4 emission (FE) from drawdown area was ultimately quantified using the following model (Yang et al. 2012):


i¼b X FE ¼ ½Pi  f in þ ð365−Pi Þ  f dr Š


i¼a 0

Ai þ ½P  f




þ ð365−P Þ  f


dr Š

is based on studies of cascade reservoirs in the Yangtze basin (Yang et al. 2009). Source proportion of CO2 from rivers downstream the dam was proximate to tropical reservoirs. In terms of boreal hydropower reservoirs, source apportionments of GHGs emissions were extrapolated using the models in the temperate reservoirs (Table 1).



Where, i is the elevation (m) from a to b, Pi is inundated days for dryland at respective elevation and fin and fdr are the GHGs fluxes of dryland in the inundated and drained durations, respectively. Ai is the submerged area at respective elevation. P′ is the inundated days of the rice paddy; f′in and f′dr are the GHG fluxes of the rice paddy during the inundated and drained seasons, respectively. A′ is the total area of the rice paddy. Then, CH4 flux normalized to drawdown area is consequently obtained. For the drawdown CO2 flux, lower fluxes for marshes in China are adopted. Here, we assume that (1) drawdown area is around 30 % of the reservoirs surface area because of similar geomorphological characteristics (Chen et al. 2009) and (2) the area ratio of rice paddy to dryland in the drawdown zone is 1:2. As regards boreal reservoirs, similar source proportions with temperate hydropower reservoirs are designated.

The estimate of total CO2 emission (301.3 Tg CO2/year) from hydropower reservoirs is 1.7 times as high as the estimate provided by Barros et al. (2011), but is proximate to that (ca. 278.7 Tg CO2/year) of Hertwich (2013). Global CO2 emission is dominantly contributed by tropical reservoirs, while boreal and temperate reservoirs respectively contribute to 17.7 and 14.8 % of the total emissions (Table 2). The sum of drawdown and downstream emissions (including spillways, turbines and river reaches below dam), which is usually overlooked by prior studies (cf. Barros et al. 2011; Hertwich 2013), accounts for 42 % of the total CO2 emission (Table 2). The estimate of total CH4 emission (13.3 Tg CH4/year) from reservoirs is three times as high as the estimate provided by Barros et al. (2011). The tropical reservoirs contribute 64 % of total CH4 emission, and boreal and temperate reservoirs, respectively, contribute 27 and 9 % of the total emission. The generally neglected drawdown emission could contribute up to 12 % CH4 emission, and both of drawdown and downstream emission contribute 67 % of the total emission (Table 2).

GHGs from reservoir downstream Discussion The source apportionments of CO2 and CH4 emissions in tropical reservoirs were developed from three welldocumented hydroelectric reservoirs (Petit Saut, Balbina and Samuel) (Table 1) (Guérin et al. 2006; Kemenes et al. 2007, 2011), while the ratio of CH4 from surface to dam downstream was revised to 1/18 in the tropical-Amazonian reservoirs (Fearnside and Pueyo 2012). With regard to temperate reservoirs, emission estimate models were developed based on the observations in the reservoirs of China. The immediate GHG degassing as water emerges from turbines was calculated using the total volume of water passing through turbines and the differences in CH4/ CO2 concentrations in the reservoirs water at the turbine level and in the water below the dam ([CO2 above /CH4above − CO2below /CH4below]×water outflow) (Fearnside and Pueyo 2012). Two cases (i.e. Danjiangkou and Hongjiadu) with data on CO2 concentration in water column and in the river below dam were collected and then extrapolated to the whole climate zone (Li et al. unpublished). The immediate degassing

Reservoirs have hypoxic or anoxic environment in the deep water, and lower water temperature particular thermocline in the water column drastically increase the aquatic CH4 solubility (cf. Yang et al. 2009). These provide a highly efficient pathway for CH4 emission as the water emerges from turbines (normally near the bottom of the reservoir). However, this CH4 emission from water passing through the spillways and turbines depends on methane concentration in the water at the turbine level and thus, it is depth dependent (cf. Fearnside and Pueyo 2012), and the critical turbine level could result in large uncertainty of GHG emission. For example, the ratio of CH4 from reservoir to reservoir downstream is revised to 1/18 for tropical-Amazonian reservoirs (Fearnside and Pueyo 2012), which could contribute to an additional 2.4 Tg CH4/year. Further, the ebullitive CH4 emission has been ignored in the prior reports (Barros et al. 2011), whereas Hertwich (2013) demonstrated that hydropower reservoirs could contribute an extra ca. 3 Tg CH4/year to the global CH4 budget via

Environ Sci Pollut Res Table 2 Revised carbon emissions from hydroelectric reservoirs

Carbon efflux Hydropower reservoir



Barros et al. (2011)


Chen et al. (2009), Yang et al. (2012), Li et al. (unpublished)

c d

Rosa et al. (2002, 2004)

Additional CH4 contribution includes the following: (1) bubbling CH4 emission is 3 Tg CH4/year (Hertwich 2013) and (2) CH4 proportion (reservoir surface/downstream) is revised to 1/18 for tropi cal -A maz o ni an re se rvoi rs (Fearnside and Pueyo 2012); another extra 2.4 Tg CH4/year is accounted for reservoirs

Reservoir surface Boreal Temperate Tropical Drawdown area Boreal Temperate Tropical Downstream of dam Turbines and spillway Boreal Temperate Tropical River below dam




105 km2 3.4 0.8 1.3 1.2



753 386 3,097

9.1 2.8 91.3

2,110b 13,000c

110b 235 (7.5–967)c

Boreal Temperate Tropical Total

correcting bubbling emission. The resulting total emission of CH4 will be 18.7 Tg CH4/year, which is four times as high as the estimate provided by Barros et al. (2011), while two times that estimated by Hertwich (2013). This new estimate is defensible considering the uncertainty of a multiplicative factor of 2 provided by Hertwich (2013). The most recent estimate of CO2 from lakes and reservoirs is lower than the previous studies, while a flux of 1.8 Pg C/year as CO2 from river and streams is much higher (Barros et al. 2011; Bastviken et al. 2011; Raymond et al. 2013) (see Table 3). If the previously ignored GHG emission from drawdown and downstream areas is included, global inland waters including reservoirs, lakes and rivers emit a carbon flux of 3.5 Pg C (CO2eq) per year (Table 2), accounting for ~28 % of the total anthropogenic carbon emission. If the maximal emission of 104 Tg CH4/year is considered (Lima et al. 2008), inland waters show a carbon flux of 3.9 Pg C (CO2eq)/year, much higher than the current terrestrial carbon sink of 2.4 Pg C (CO2eq)/year. This implies that freshwaters play a fundamental role in the continental GHG balance. Based on ca. 150 available measurements of GHGs from hydroelectric reservoirs (Barros et al. 2011), the global averaged CO2 and CH4 fluxes from temperate hydropower reservoirs are respectively 387 mg CO2/m2/day and 2.8 mg CH4/ m2/day. However, further increase in GHG emissions may emerge once China’s reservoirs are considered owing to their much higher GHG emissions (the largest hydroelectric

4,296 (2,092–9,511)c

Carbon emission CO2


Tg/year 176 22 18.3 135.7 45.1 9.9 8.2 27 80.204 53.804 18.26 15.189 20.355 26.4

Tg/year 4.4 0.3 0.1 4 1.6 0.825 0.275 0.5 7.3 5.8 1.95 0.65 3.2 1.5

3.3 2.745 20.355 301.304

0.525 0.175 0.8 13.3 (18.7d)

generator; cf. Li et al. (unpublished) and references therein) than the global temperate average. Our recent studies compile the field data of CO2 and CH4 emissions in the China’s hydropower reservoirs and demonstrate that the averaged CO2 and CH4 fluxes are respectively three and twofolds of reports by Barros et al. (2011). Therefore, GHG emission from temperate reservoirs will drastically increase (i.e., 133.3 Tg CO2/year and 2.4 Tg CH4/year) (source proportions of CH4 and CO2 are considered constant), and consequently, the total emission of GHGs would be maximal 390.2 Tg CO2/year and 21.1 Tg CH4/year from global hydropower reservoirs. In addition, extremely high GHG fluxes especially CH4 flux were estimated for some temperate reservoirs (Maeck et al. 2013), i.e. 315 mg CH4/m2/day that exceeds the global average (ca. 2.8 mg CH4/m2/day) by more than ten orders of magnitude and even exceeds the flux previously reported for tropical-Amazonian reservoirs (ca. 137 mg CH4/m2/day; Barros et al. 2011). This implied possible increasing estimates of CH4 emission from hydropower reservoirs. CH4 has been revised upward to currently accepted level of 34 times more powerful as a GHG than CO2 on an equivalent mass basis over 100-year period horizon (Shindell et al. 2009). Thus, expressed as CO2 equivalents (eq), 937 Tg CO2eq/year is accounted for hydropower reservoirs. When the total hydropower production of 3,288 TWh in 2009 is considered, the global average emissions from hydropower are estimated to be 92 g CO2/kWh and 5.7 g CH4/kWh, and this is equivalent to 285 g CO2eq/kWh for hydroelectricity. Comparative

Environ Sci Pollut Res Table 3 Comparison with other global estimates

Hydropower reservoirs Global inland waters Hydropower reservoirs Hydropower reservoirs





This study

146.5 (102.6–186.5)b 9.7 4.4

12,880 608.5 288

This study Hertwich 2013 Barros et al. 2011

Total emissions (Tg/year)

105 km2





3.4 3.4

7,900a 278.7 176






CO2eq (Tg/year)




Cole et al. 2007 Tranvik et al. 2009 64 (20c–104d)


Barros et al. 2011

The most recent work (Raymond et al. 2013) revised the CO2 emission from global inland waters to be 7,773 Tg CO2/year. Here, additional CO2 emission from drawdown area and downstream of the dam is added

Natural lakes


Cole et al. 2007

Natural lakes


Tranvik et al. 2009





Rivers and streams



Lakes and reservoirs


We revised the previous data with a consideration of the drawdown and downstream emissions c d

Bastviken et al. (2011)

Natural lakes


Rivers and estuaries





Rivers and streams

Cole et al. 2007


Anthropogenic sources

Tranvik et al. 2009 1.5


Bastviken et al. 2011 Raymond et al. 2013

1,173 35,000

Barros et al. 2011

Raymond et al. 2013 450


Lima et al. 2008

Lima et al. (2008)

analysis between GHGs emissions by hydropower reservoirs and fossil alternatives indicates that carbon emission by thermo-power plants is 1.9 (natural gas) to 3.5 (modern coal plant) times that is released by hydroelectricity over a 100-year period. This indicates that hydroelectric reservoirs reduce a carbon emission of 2,351 Fig. 1 Comparison of carbon emission between hydropower and fossil fuel alternatives (3,288 TWh for hydroelectricity is considered) (IGCC integrated gasification combined cycle)

Tg CO2eq/year in comparison to equivalent coal-fired plant alternative (Fig. 1). Similar to prior studies at national and global scales, there is a large degree of uncertainty associated with our estimates shown in Table 2. Firstly, our dataset come from published data that were measured in the field on very few occasions and

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sampling sites, and most measurements do not include bubbling CH4 release. Also, drawdown area depends on the slope of the flooded land and the hydrology of the impounded river and, thus, shows huge spatial and temporal heterogeneity. Considering the large uncertainty of surface area (ca. ±100 %) and areal flux (ca. ±50 %), an uncertainty of a multiplicative factor of 2.5 is understandable (Barros et al. 2011; Hertwich 2013). However, our study provides an important advancement on systematic quantification of hydropower-induced GHG emission and much needed data from Asia are included. Albeit uncertainties are still large (Li and Lu 2012), GHG emissions are revised to some 301.3 Tg CO2/year and 18.7 Tg CH4/year released by hydroelectricity (Table 2). Nevertheless, global hydroelectric reservoirs could reduce 2,351 Tg CO2eq/ year (937 vs 3,288 Tg CO2eq/year for, respectively, hydroelectricity and fossil fuel plants). The capacity of the world reservoirs is projected to 7,000 TWh in 2050; hopefully, this will amount to a GHG emission reduction of 4,700 Tg CO2eq/ year when compared to the equivalent coal-fired plants. However, more datasets particularly downstream emissions are urgently needed for accurate quantification of GHGs from global reservoirs. Acknowledgments The research is funded by the National Natural Science Foundation of China (No. 31100347; 31130010) and Youth Innovation Promotion Association, the Chinese Academy of Sciences, China (Y129431C06). Competing financial interests The author declares no competing financial interests.

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Carbon emission from global hydroelectric reservoirs revisited.

Substantial greenhouse gas (GHG) emissions from hydropower reservoirs have been of great concerns recently, yet the significant carbon emitters of dra...
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