Eos, Vol. 95, No. 1, 7 January 2014
VOLUME 95
NUMBER 1
7 January 2014
EOS, TRANSACTIONS, AMERICAN GEOPHYSICAL UNION
PAGES 1–12
Measuring Greenhouse Gas
Emissions From China’s Reservoirs
For example, Yang and Flower [2012] considered that potentially massive amounts of
GHGs would be emitted from nine of the proposed Mekong River dams because of their
low altitudes (10°N and 20°N) and large inundated areas (2120 square kilometers). GHG
emissions from four proposed giant world-class
dams on the Jinshajiang River in southwest
China are also a serious concern. Researchers
at the Zhejiang Forestry Academy estimate
GHG emissions to be 0.1–0.4 teragrams of CO2
equivalents per year from these four reservoirs, based on the available data (Table 1).
PAGES 1–2
Hydroelectricity has typically been regarded
as a green energy source, but reservoirs created for its generation emit greenhouse gases
(GHGs) just as natural lakes and rivers do.
The role of reservoirs in GHG emissions has
been overlooked. Substantial amounts of
methane (CH4 ) are emitted from reservoir
surfaces every year, which account for about
20% of the total CH4 emission from inland
waters. GHG emissions (transferred into carbon dioxide (CO2 ) equivalents) from some
tropical reservoirs even exceed CO2 emissions
from thermal power plants if the same amount
of electricity is generated.
Although approximately half of the world’s
large dams are located in China, GHG emissions have been monitored for only a few large
reservoirs, such as Three Gorges Reservoir on
the Yangtze River (Figure 1), which results in
difficulties when trying to estimate the total
GHG emissions from reservoirs accurately.
Studies of Emissions From Reservoirs
Around the World
The study of GHG emissions from reservoirs began in the 1990s. Rudd et al. [1993] first
pointed out that GHG emissions from reservoirs are not zero per unit of energy produced
(Table 1) and may be significant compared to
GHG emissions from fossil-fueled electricity
generation. Thereafter, GHG emissions were
gradually monitored at reservoir surfaces and
studied in the northern temperate climate zone
(e.g., Canada and Finland) as well as humid,
tropical areas in South America (e.g., Brazil
and French Guiana). CO2 and CH4 emissions
from the tropical regions are about 5–20 times
larger per unit area than those from the northern areas because of temperature differences.
Monitoring GHG emissions at reservoir surfaces was encouraged by many countries,
and about 100 reservoirs worldwide have now
BY L. YANG, F. LU, AND X. WANG
been studied. Based on the available results,
Barros et al. [2011] estimated that 48 teragrams
of carbon as CO2 and 3 teragrams of carbon
as CH4 are emitted globally from the reservoirs, which accounts for about 0.5% of CO2
and 1% of CH4 emissions from anthropogenic
sources. These available data are from reservoirs distributed in North America, South
America, and Europe; no Asian reservoirs were
included in the estimation, although 60% of
large dams are located in Asia.
Limited Studies in Asia
In the past 5 years, some investigations
have focused on GHG emissions from Chinese
reservoirs such as Three Gorges Reservoir,
Ertan Reservoir, Miyun Reservoir, and several
cascade reservoirs along the Wujiang River.
The results indicate that the GHG fluxes were
not as large as expected and show a significant spatial and temporal variability.
However, field measurements of GHGs have
been limited to a few Chinese and Laotian
reservoirs. GHG emissions from Asian reservoirs might actually be quite large because of
the reservoirs’ huge areas, numbers, installed
capacity, and wide geographical distribution.
Factors Affecting GHG Emissions
and Measurements
Production, transport, and emission are the
three main processes in GHG emissions from
reservoirs, similar to those from natural rivers
or lakes. First and foremost, identifying the
carbon sources is important because they
reveal the formation mechanism of CO2 and
CH4 in reservoirs. Carbon sources in reservoirs can be divided into two types: autochthonous and allochthonous. The former is
the initial flooded carbon in the inundated
areas and the phytoplankton in the reservoir
surfaces; the latter includes the organic carbon input from upstream rivers and the terrestrial ecosystem. To analyze the carbon
sources of the GHGs, the range of the fraction
of carbon-13 (δ13C) in the GHGs can be compared with that in the autochthonous and
allochthonous carbon sources.
Fig. 1.Water running through the turbines of China’s Three Gorges Dam produced 98,107 gigawatthours of energy in 2012. How much greenhouse gas is emitted by the water in the reservoir
behind the dam? Photo by F. Lu.
© 2014. American Geophysical Union. All Rights Reserved.
Eos, Vol. 95, No. 1, 7 January 2014
CH4 and CO2 can be transported from
sediments to the atmosphere by ebullition
(bubbling) or diffusion. During the transport
process, CH4 ebullitive emissions are influenced by many environmental variables. Thus,
bubbles are episodic and not representatively
captured by the usual short-term measurements. Moreover, diffusive gas is determined
by the gas gradient between the surface water
column and the ambient gas (∆C), and the
gas piston velocity at the air-water interface
(k600), which are influenced by wind speed,
precipitation, and other meteorological factors.
However, studies of GHG emissions from Chinese reservoirs have focused on the diffusive
fluxes at the air-water interface, while the
ebullitive fluxes were seldom measured. Thus,
it is vital to measure the bubble fluxes by the
traditional method of inverted funnels or by
other newly advanced methods (e.g., echosounder and automated flux chambers) in
the future.
CH4 is easily oxidized by methane-utilizing
bacteria (methanotrophs) during the transport process. In a stratified water column, the
CH4 oxidation rate reaches a maximum in
the upper hypolimnion (the bottom layer of
water in a lake) but maintains lower values in
the epilimnion (the top layer of water in a lake).
CO2 is easily absorbed or fixed through photosynthesis by vegetation or photosynthetic
bacteria in the water column. Thus, the distribution patterns of CO2, CH4, and oxygen (O2)
concentrations in the vertical profiles can reflect the consumption processes of CO2 and
CH4 during transport. Except for Guérin and
Abril [2007], methodological limitations
have meant that few studies have been conducted to quantify the CH4 oxidization rate
in reservoirs.
In addition, turbines, which are unique to
dams, in recent decades were found to be a
hot spot of GHG emissions. The dissolved gas
in the hypolimnion is easily degassed to the
atmosphere when water passes through turbines. CH4 emissions are significantly larger
than CO2 emissions under the strongly disturbed conditions at turbines. Degassing
emissions have not been measured in China
because of the country’s military supervision
of dams. Cooperation should be set up with
the hydroelectric power plants to study the
effect of turbines on the CO2 and CH4 emissions from Chinese reservoirs.
The operation of power stations also has
an effect on CO2 and CH4 emissions from downstream rivers below dams. Hypolimnion water,
rich in dissolved CO2 and CH4, is discharged
into the surfaces of downstream rivers by turbines and spillways, and so more GHGs are
contained in the surface waters than in those
in upstream or natural rivers. These GHGs
diffuse into the atmosphere faster because
of the enhanced gas gradient (∆C) and the
strong disturbance in the downstream rivers.
Adding the original diffusion fluxes in the
downstream river, the total GHG emissions
are comparable to those upstream. In China,
CO2 and CH4 emission fluxes have not been
studied systematically up to now, but a preliminary attempt was carried out on the
Yangtze River downstream of Three Gorges
Reservoir [Yang et al., 2013a; Yang et al.,
2013b].
Furthermore, GHG emissions from reservoirs are a complex biogeochemistry process,
influenced by many environmental variables
such as reservoir conditions (flooded organic
carbon and reservoir age), hydrology and
water quality (water level, retention time,
acidity, and dissolved oxygen), climate conditions (wind speed, water temperature, light,
and rainfall), and biological conditions (type
and biomass of aquatic vegetation). Because
of this, a remarkable spatial variation has been
observed in GHG emissions from Chinese
reservoirs, which results in difficulty in accurately estimating the total GHG emissions from
reservoirs.
Based on the available literature, four aspects of GHG emissions from Chinese reservoirs should be studied further. First, most
studies have concentrated on GHG emissions
from temperate and tropical reservoirs, but
subtropical Chinese reservoirs have been
ignored. Second, GHG emissions are the comprehensive results of GHG production, transport, and oxidation; thus, the relationships
between emission, production, transport, and
oxidation need to be clarified. Third, there
are many inputs to a dendritic reservoir, and
no study has been conducted on the influence of different sources of organic carbon
on GHG emissions from reservoirs. The total
amount of carbon input and the mineralization rate from different upstream rivers should
be quantified so that the carbon contribution
to reservoir surface GHG emissions from different input tributaries can be judged. Fourth,
the carbon sources of GHG emissions from reservoirs are not clear and need to be identified.
Planned Studies
of the Xin’anjiang Reservoir
With the rapid development of hydroelectricity in China, the installed capacity of
Chinese hydroelectricity is the largest in the
world, having surpassed 200 million kilowatts
in 2010. Xin’anjiang Reservoir, the largest reservoir in the eastern China region, is representative; it is also called “the Thousand Islands
Lake” because 1078 islands are distributed in
it. The reservoir, located in typical subtropical
monsoon climate conditions, is dendritic in
shape and can be divided into five sublakes.
Xin’anjiang River is the largest of the 25 rivers
flowing into the reservoir and provides about
60% of the total surface runoff. The dominant
water source is located in the northwest lake
of Xin’anjiang. Water quality and organic carbon also can be distinct in different sublakes
because of the different water sources. The
input of organic carbon at the entrance of a
reservoir also has an effect on GHG emissions
at the reservoir surfaces.
In January 2014, a new project called Spatial
Heterogeneity of Greenhouse Gas Emissions
© 2014. American Geophysical Union. All Rights Reserved.
From Xin’anjiang Reservoir and Analysis of
Their Carbon Sources, conducted by researchers at the Zhejiang Forestry Academy,
will start to study GHG emissions from
Xin’anjiang Reservoir. The experiments will
monitor CO2 and CH4 emission fluxes at fixed
sampling points distributed in the upstream
rivers, open water areas, drawdown areas,
turbines, and downstream rivers below the
dams. In addition, the carbon sources of
GHG emissions from the reservoirs will be
analyzed using the stable carbon isotopic
method, which could provide basic data on
the biogeochemistry cycle of carbon in reservoirs. This study and other future studies
could help to identify carbon sources of
GHGs in reservoirs and help scientists and
resource managers to determine appropriate
measures to control GHG emissions from reservoir surfaces.
Acknowledgments
We thank the National Natural Science
Foundation of China (grant 41303065) and
the Project of Zhejiang Scientific and Technological Plan (grant 2011F20025) for support
of the study on GHG emissions from Xin’anjiang
Reservoir.
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Author Information
LE YANG, Zhejiang Forestry Academy, Hangzhou,
China; email: [email protected]; and FEI LU and
XIAOKE WANG, State Key Laboratory of Urban
and Regional Ecology, Research Center for EcoEnvironmental Sciences, Chinese Academy of Sciences, Beijing, China
Table 1. GHG Emission per Energy Unit Produced at Representative Reservoirs in Different Latitudes
Nation
Reservoirs
Latitude
Age
( years)
Total GHG
Emission
( Tg CO2eq
yr–1)a
Brazil
Balbina
01°55'S
18
Brazil
Tucuruí
03°45'S
6
French Guiana
Annual
Energy
Generation
(GWh yr–1)b
Plant GHG
Emission
Factor
( g CO2eq
kWh–1)c
Reference
4.7–44.0
1,095
4,277.3–40,203.8
Kemenes et al. [2007, 2011]
7.0–10.1
21,000
333.3–481.0
Fearnside [2002]
Petit Saut Reservoir
05°04'N
1–10
0.7–3.6
560
1,307.6–6,451.9
Abril et al. [2005]
Brazil
Samuel
08°45'S
4–5
1.5–2.6
946
1,599.7–2,734.2
dos Santos et al. [2006]
Laos
Nam Leuk
18°27'N
10
0.01–0.03
263
49.2–107.8
Chanudet et al. [2011]
Brazil
Itaipu
25°26'S
7–8
0–0.86
90,000
0–9.6
dos Santos et al. [2006]
China
Wudongde
26°19'N
0
0.2–0.3
39,460
6.2–8.4
Estimationd
China
Baihetan
26°26'N
0
0.3–0.4
60,240
4.6–6.3
Estimationd
China
Ertan
26–28°N
10
0.03–1.0
17,000
1.82–58.8
Zheng et al. [2011]
China
Xiluodu
28°15'N
0
0.1–0.2
51,720
3.3–4.5
Estimationd
China
Xiangjiaba
28°38'N
0
0.1–0.2
30,747
4.0–5.4
Estimationd
China
Xin’anjiang
29°28'N
54
0.7–1.0
1,861
397.7–539.7
Estimationd
China
Three Gorges
30°51'N
7
1.4–1.9
84,700
16.4–22.6
Yang et al. [2013b]
Eastmain-1
51–52°N
1–4
0.3–0.5
2,700
238.3–671.0
Teodoru et al. [2012]
Canada
a
Tg CO2eq yr –1 is teragrams of CO2 equivalents per year.
GWh yr –1 is gigawatt hours per year.
c
g CO2eq kWh–1 is grams of CO2 equivalents per kilowatt-hour.
d
Based on flux values in the Three Gorges Reservoir, we estimated the total annual GHG emission and plant GHG emission factor in the four giant world-class
reservoirs in Jinshajiang River and Xin’anjiang Reservoir because of their similarity in location latitude, valley topography, and flooded vegetation.
b
© 2014. American Geophysical Union. All Rights Reserved.