WWataer Ptaeperrs Papers
INTERIM TECHNICAL NOTE
Greenhouse Gases from Reservoirs Caused
by Biochemical Processes
Rikard Liden
April 2013
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Acknowledgments
This technical note was prepared by Rikard Liden, Senior Hydropower Specialist, TWIWA, World Bank. The note has benefited from a thorough review process, which refined its focus and content, and which included invaluable input from the wide-ranging knowledge of staff across the World Bank. The reviewers and contributors were Julia Bucknall, Sector Manager, TWIWA; Vivien Foster, Sector Manager, SEGEN; Harvey van Veldhuizen, Lead Environmental Specialist, OPCOR; Jean-Michel Devernay, Chief Technical Specialist for Hydropower, TWI; Alessandro Palmieri, Lead Dam Specialist, TWIWA; Marcelino Madrigal, Senior Energy Specialist, SEGEN; Sameer Akbar, Senior Environmental Specialist, ENV; and Rama Chandra Reddy, Senior Carbon Finance Specialist, ENVCF. Useful comments were received from a large number of experts, among them Daryl Fields, Senior Water Resources Specialist, ECSAR; Mohinder Gulati, Sector Leader, ECSSD; Wendy Hughes, Lead Energy Economist, SEGES; Mudassar Imran, Senior Energy Economist, SEGEN; Sunil Khosla, Lead Energy Specialist, ECSEG; Masami Kojima, Lead Energy Specialist, SEGEI; Stephen Lintner, Senior Advisor, OPSOR; Robert Montgomery, Lead Environmental Specialist, LCSEN; Jari Vayrynen, Senior Environmental Specialist, ECSEG; and Leiping Wang, Lead Energy Specialist, SASDE. Katherin Golitzen provided editorial support.
The International Hydropower Association provided guidance on reference literature and statistics on GHG emissions from reservoirs. The development of the technical note has been supported by the Water Partnership Program.
Acronyms v
Executive Summary vii
1. Purpose and Application of this Technical Note 1
2. Basic Overview of Greenhouse Gases from Reservoirs 3
2.1 The CO2 Cycle in a River Basin .3
2.2 Gross and Net Fluxes of Greenhouse Gases 5
2.3 Greenhouse Gas Emissions Vary with Time 6
3. Results from Conducted Research 7
3.1 Geographic and Temporal Distribution 7
3.2 Findings on Main Processes 7
3.3 Findings on Measurements of Gross Emissions 8
3.4 Results put into a Global Perspective 10
3.5 Future Results and Expected Outputs 11
4. Categorization of Reservoirs Based on GHG Emission Potential 13
5. Recommendations for Preparing Dam Projects 23
5.1 Qualitative Assessment of GHG Emission Potential 23
5.2 Preliminary Quantitative Assessment of Net GHG Emissions 26
6. Updating of this Technical Note 29
Glossary 31
Bibliography and References 35
Annex 1: Conversions of GHG Units and CO2 Equivalents 45
Annex 2: Illustration of Main Processes and Parameters 47
Annex 3: Detailed Steps for Tier 1 Estimation of GHG from Reservoirs 49
CONTENTS

AFOLU Agriculture, Forestry, and Other Land Use
CDM Clean Development Mechanism
CH4 Methane
CO2 Carbon Dioxide
DOC Dissolved Organic Carbon
EIA Environmental Impact Assessment
FAO Food and Agriculture Organization
GHG Greenhouse Gas
GWP Global Warming Potentials
ICOLD International Commission on Large Dams
IEA International Energy Agency
IHA International Hydropower Association
IPCC Intergovernmental Panel on Climate Change
kWh Kilowatt Hour
MW Megawatt
N2O Nitrous Oxide
UNESCO United Nations Educational, Scientific, and Cultural Organization
WWF World Wildlife Fund
Acronyms

A decade ago, the contribution of greenhouse gases (GHGs) from reservoirs
was estimated to be up to 7 percent of global GHG emissions from all
sources. Much research on GHG emissions from reservoirs has subsequently
been conducted and recent studies have indicated corresponding global
estimate to be less than 1 percent. However, these studies still have a limited
coverage of ecosystems and geographic areas, and, more critically, almost none
of them have measured the long-term change in GHG emissions over many years.
Therefore, the research conducted to date has shown disparity in GHG emission
magnitudes from reservoirs, which has caused a debate on methodologies and
reliability of results.
The purpose of this note is to provide interim guidance to World Bank staff on how to assess
GHGs from reservoirs in preparation for dam infrastructure projects. The note describes the major
biochemical processes that cause GHGs from reservoirs, provides the status of current knowledge
and research, and puts the issue into a global perspective. Based on the state-of-the-art, it makes
recommendation on how to assess GHG emissions and how to make preliminary rough estimates of
emissions caused by biochemical processes for planned reservoirs.
A fundamental concept for accurate description of GHGs from reservoirs created by biochemical
processes is the difference in gross and net fluxes. Rivers are major conveyances of carbon from
terrestrial areas to lakes and the sea. Terrestrial areas generally are net carbon sinks and aquatic
systems are net carbon emitters. Changes in GHG fluxes to the atmosphere because of an introduction
of reservoirs in a river system must therefore be viewed from a catchment perspective. Net GHG
emissions created by the reservoir are the difference between total fluxes for the whole river basin
before and after the reservoir is constructed.
The results from and understanding of gross GHG emission measurements during the last 15 years
have led to the following key conclusions in regard to assessment of reservoirs:
?? Reservoirs with significant GHG emissions are associated with high methane (CH4) emissions
because of the gas’s strength as GHGs.
Executive Summary
?? The likelihood of significant GHG emissions,
especially CH4, increases with
the number of variables contributing to
GHG emission that work in combination.
No single variable, for example, latitude
or reservoir size, should be used on its
own to estimate GHGs from a specific
reservoir.
?? The key for assessing GHG emissions lies
in understanding the availability of carbon
stock and the reservoir’s water quality
conditions, especially the temporal and
spatial extent of anoxic conditions.
For dam infrastructure projects with
significant inundation for which the World Bank
may provide financing, studying biochemically
generated GHGs from the reservoirs is recommended
as part of the Environmental Impact
Assessment. The purpose of including studies
of potential GHG emissions from reservoirs
as part of the EIA is to enable comparison of
alternative design options within the framework
of the investment project. Current science only
permits a crude comparison of alternatives.
The assessment of biochemically generated
GHGs from the reservoir is recommended to
follow a stepwise process to evaluate the supply
of carbon stock and the reservoir’s condition to
create and release GHGs:
1. Does the reservoir have the capacity to
create large carbon stock (amount of
flooded organic matter, inflowing organic
matter, and organic matter produced
in the reservoir)? If the carbon stock is
small, Global Warming Potential (GWP) is
likely negligible.
2. Does the reservoir have the capacity
to convert the organic matter to GHGs
and, if so, to what type? If the physical
conditions disfavor decomposition of
organic matter, and especially do not
favor creation of CH4 and N2O, GWP is
likely negligible.
3. Does the reservoir have the capacity
to release the created GHGs into the
atmosphere? If the pathways of CH4 and
N2O to the atmosphere are few and if the
physical conditions favor transformation
of these to CO2 before emission, GWP is
likely negligible.
For dam infrastructure projects with estimated
potentially significant GHG emissions,
streamlined post-implementation monitoring
of GHGs from the reservoir and immediate
river stretch downstream is recommended
to confirm the order of magnitude of the
estimated emission potential. The purpose of
such monitoring is not to provide a complete
estimation of net GHG emissions but to make a
rough comparative analysis of gross emissions
with previous results from reservoirs in similar
environments.
For dam infrastructure projects with estimated
potentially significant GHG emissions for
which the World Bank may provide financing, it
is further encouraged to investigate possibilities
to support from Trust Funds detailed measurements
to enhance research on biochemical GHG
emissions from reservoirs. In such cases, both
pre- and post-impoundment measurements
would enable estimation of net emissions in
accordance with the guidelines of UNESCO/IHA
and IEA Hydro Annex XII.
Biomass removal should be carefully
assessed before being used as a default mitigation
measure for preventing GHGs. The positive
effect on GHG emissions of biomass removal
depends on which vegetation is cleared and how
this biomass is treated after removal.
viii INTERIM TECHNICAL NOTE: GREENHOUSE GASES FROM RESERVOIRS CAUSED BY BIOCHEMICAL PROCESSES
Section 1
Purpose and Application
of this Technical Note
This note aims to provide interim guidance to World Bank staff on how to
assess greenhouse gases (GHGs)1 from reservoirs in preparation for dam
infrastructure projects.
The technical note is limited to the GHGs resulting from the biochemical processes that are initiated
when a river is dammed and the area upstream is flooded. Such GHG emissions are a vital part
of GHG accounting for projects involving reservoirs (such as hydropower storage dams) and the note
thus provides input to the World Bank’s methodology for estimating carbon footprints.2 The note does
not include guidance on direct or indirect emissions associated with the construction and operation of
dams or the counterfactual scenario for alternative development projects.
It must be understood that GHG emissions from reservoirs are a recent area of research. During
the last decade, research has significantly improved our knowledge and understanding. However,
these studies have been conducted in a rather narrow range of ecosystems and geographic areas,
and, more critically, almost none of them have measured the long-term change in emissions over
many years. Therefore, it should be no surprise that research conducted to date has shown disparity
in GHG emission magnitudes from reservoirs, which has caused a debate on methodologies and reliability
of results (see, for example, Cullenward and Victor 2006). World Bank staff therefore must take
care in applying this note within the recommended bounds of their use, and be very clear and frank in
discussing the uncertainties still underpinning the science.
This technical note describes the major biochemical processes causing GHGs from reservoirs,
provides the status of current knowledge and research, and puts the issue into a global perspective. It
makes recommendations based on the state-of-the-art on how to assess GHG emissions and how to
make preliminary rough estimates of emissions caused by biochemical processes for planned reservoirs.
The aim is to create a short and concise note written in easily understandable, not overly technical
language covering the most important and relevant facts relating to GHGs from reservoirs. Due to
the complexity of the dynamic physical, chemical, and biological processes, this means that not all
scientific and detailed information is described and some processes are simplified. For further details
and in depth description, the reader should study the key references found in the Bibliography and
References section.
1 See the Glossary on page 31.
2 Draft Guidelines for GHG Accounting for Hydropower (in progress).
This technical note has interim status. The
reason for this status is that, although there is an
urgent need for clear guidelines for practitioners,
there are ongoing research programs that will,
within a few years, further clarify this subject.
Preparation of the present text is a response
to requests from practitioners for guidelines
until the scientific process can deliver improved
methodologies that will produce results in which
decision makers can have greater confidence.
INTERIM TECHNICAL NOTE: GREENHOUSE GASES FROM RESERVOIRS CAUSED B 2 Y BIOCHEMICAL PROCESSES
Section 2
Basic Overview of Greenhouse
Gases and Reservoirs
2.1 The CO2 Cycle in a River Basin
Any change of land use and/or change of the natural cycles of water and energy will affect interaction
between the terrestrial, aquatic, and atmospheric environments and therefore have an effect on GHGs.
When a river is dammed, the flow dynamics are changed, riverine sediment and organic material are
trapped, and terrestrial ecosystems are flooded. This alters the previous cycle and fluxes of carbon
dioxide and other GHGs within the project footprint.
The main GHGs that may be emitted from a reservoir are carbon dioxide (CO2), methane (CH4),
and nitrous oxide (N2O). CH4 and N2O are stronger than CO2 and may be important even if emitted in
smaller amounts.
To account for the different Global Warming Potentials (GWP) of GHGs, the combined emissions
of CO2, CH4, and N2O are expressed as CO2 equivalents.3 Because the three different GHGs have
different lifespans in the atmosphere, a specific period needs to be set, normally 100 years. According
to IPCC,4 to obtain CO2 equivalents for a 100-year period, the quantities of CH4 produced should be
multiplied by 25 and N2O by 298.
To understand the effects of reservoirs on GHGs it is essential to understand the main processes
in the cycle of CO2 and other GHGs in a river basin.
?? Atmospheric CO2 is taken up by plants through photosynthesis but is in parallel lost through
respiration to the atmosphere either directly from vegetation or through decomposition of dead
organic matter. The balance of these CO2 fluxes creates the growth of biomass (live or dead) in
the terrestrial ecosystem, contributing to the biomass carbon pool. The live biomass, and thus
carbon, may be removed, for example, through harvesting or fires (manmade or natural), and
are in these cases (eventually) fed back to the atmosphere mainly as CO2.
?? The dead organic matter, which has not been directly decomposed or respired, is eventually
absorbed into the soil or transported to the river through rainfall and overland flow. Carbon
is thus either stored in the soil or transported out of the terrestrial ecosystem to the riverine
ecosystem as part of the erosion process. Carbon can also be leached from the dead organic
material or soil and enter the river systems directly.
3 See Annex 1 for conversion between units and CO2 equivalents.
4 See the 2007 IPCC Fourth Assessment Report (AR4).
?? In rivers and lakes (with or without
reservoirs) carbon can be leached from
the bed sediment to the water phase
and the dissolved CO2 can be lost to the
atmosphere at the surface. As part of the
aquatic ecosystem, CO2 from the atmosphere
or dissolved in the water can also
be consumed by algae, phytoplankton,
and zooplankton that will later decay and
create new dead organic material that
adds to the bed sediments.
?? CH4 is mainly created under anoxic conditions
(no oxygen available) in the soil or
the bed sediments of a water body. Such
conditions often occur at the bottom
of flooded areas, especially if the water
column is strongly stratified on a seasonal
basis. CH4 can be produced to varying
degree in all types of water bodies. If
released to the water column as dissolved
CH4, it is either oxidized (and thus transformed
to dissolved CO2) or, if there is lack
of oxygen in most of the water column, it
is lost directly as CH4 to the atmosphere.
CH4 may also be transported up through
the water column and into the atmosphere
in gas form through bubbling.
?? N2O is in some circumstances created
as a by-product of nitrification under
aerobic conditions, or denitrification
under anaerobic conditions. Creation of
N2O therefore mainly occurs in riparian
zones of water bodies, where saturation
varies with water levels.
The construction of a dam and impoundment
of a reservoir creates an alteration in the
GHG cycle. This means there will be a change in
flux of GHGs to the atmosphere compared with
the situation before the reservoir was created.
?? The reservoir area changes from the
previously terrestrial system into an
aquatic system, thereby changing the
conditions for interactions of GHGs with
the atmosphere for this area.
?? Following inundation, it creates conditions
for decomposing vegetation in the
flooded area, changing the amount of
GHGs released to the atmosphere from
the reservoir area or in downstream
rivers when the water is discharged.
?? The reservoir may provide for seasonal
growth and decomposition of vegetation
in the drawdown zone, resulting in
released GHGs to the atmosphere.
?? It may provide anoxic conditions for
creating CH4 instead of CO2, especially if
the water column is seasonally stratified.
?? It partially traps riverine organic material
and nutrients transported in the river
system, and may thereby change the
circumstances under which they are
transformed into GHGs compared to
where they would have otherwise been
carried further down the river system (in
a natural lake, wetland, or ocean).
?? Organic matter may be buried in the reservoir
bed sediment thus providing a carbon
sink.
In a reservoir, the flooded and inflowing
carbon will thus be exported to the atmosphere,
stored in the bed sediments, or transported
further down the river system. These three
processes occur in parallel in varying degrees,
depending on the topographical, geological,
and climatological conditions, and the biological
configuration of the water body.
4 INTERIM TECHNICAL NOTE: GREENHOUSE GASES FROM RESERVOIRS CAUSED BY BIOCHEMICAL PROCESSES
Section 2
2.2 Gross and Net Fluxes of
Greenhouse Gases
A fundamental concept for accurate description
of GHGs from reservoirs created by biochemical
processes is the difference in gross5 and net
fluxes. Rivers are major conveyances of carbon
from terrestrial areas to lakes and the sea.
Terrestrial areas generally are net carbon sinks
and aquatic systems are net carbon emitters.
Changes in GHG fluxes to the atmosphere
because of an introduction of reservoirs in a river
system must therefore be viewed from a catchment
perspective. Net GHG emissions created
by the reservoir are the difference between total
fluxes of CO2 equivalents for the whole river basin
before and after the reservoir is constructed.
Reservoirs are one of many anthropogenic
influences on the biochemical GHG cycle in a
river basin. Sources of carbon for a reservoir
are normally both natural (e.g., vegetation) and
anthropogenic (e.g., inflow of organic matter from
untreated sewage or flooded waste deposits).
Changes in GHG fluxes due to the introduction
of a reservoir must therefore also be considered
from the perspective of artificial influences
already in place in the river catchment.
In 2011, the IPCC defined biochemically
generated net emissions from reservoirs as
gross emissions minus pre-impoundment
emissions and minus unrelated anthropogenic
sources (IPCC 2011). Unrelated
anthropogenic sources would be, for example,
upstream industries or untreated sewage
systems. This definition is used in this interim
technical note.
The net emission of GHGs from the introduction
of a reservoir is limited to the processes
in which the changes create net increase in
CO2-equivalent fluxes to the atmosphere. The
main biochemical processes that may contribute
to increased net GHG emissions as a result of
building a reservoir are:
?? decomposition of a part of the flooded
biomass and organic matter in soil in
the reservoir area causing either CO2 or
CH4 emissions,
?? removal of forest areas (corresponding
to the flooded area) that would have been
absorbing carbon,
?? creation of an aquatic ecosystem that
traps inflowing carbon and transforms it
to atmospheric CO2 equivalent to a higher
degree than if it had been transported
further downstream, and
?? seasonal growth and decomposition of
shrubs and grasses in the drawdown
zone.
The main biochemical processes that may
contribute to decreased net GHG emissions to the
atmosphere as a result of creating a reservoir are:
?? creation of an aquatic ecosystem that
sequesters CO2 via photosynthesis by
algae and aquatic plants, and
?? creation of a sediment trap that buries
inorganic and organic carbon in the bed
sediment.
The net emissions described above refer
only to the processes involved in the change of
GHG fluxes due to the introduction of a reservoir
in a river system. In a complete lifecycle emission
assessment for a project involving a reservoir,
the baseline must be set according to alternative
future scenarios (such as when hydropower
replaces thermal power) and include emissions
related to the implementation of the entire project
(for example, including emissions for construction
projects).
5 Gross emissions for reservoirs are defined as the
emissions observed from the reservoir surface and the
immediate river stretch downstream of the reservoir.
Basic Overview of Greenhouse Gases and Reservoirs  5
2.3 Greenhouse Gas Emissions
Vary with Time
GHG fluxes vary over time from both terrestrial
and aquatic systems.
?? Gross GHG emissions from reservoirs
and all water bodies vary considerably
in the short term (seasonally) since
the physical, biological, and chemical
processes depend on external variables
such as temperature, inflow rate, water
depth, and wind.
?? GHG emissions from new aquatic
systems created by reservoirs will also
change in the long term as the flooded
organic material is decomposed and as
biochemical conditions change. Upon
inundation, easily decomposed organic
matter decays, causing initially high gross
emissions. As this matter is depleted, the
gross emission rates depend increasingly
on readily decayed material being transported
into the reservoir from inflowing
rivers.
?? The growth of biomass for the flooded
terrestrial system depends on the
age and type of vegetation. Typically,
young forests are efficient sinks of
CO2, whereas mature forests are
close to equilibrium, where absorption
through photosynthesis is close to the
losses through respiration and erosion.
Nonforest ecosystems have a roughly
constant carbon stock. The effect of
CO2 sinks lost by removing terrestrial
systems thus varies depending on
original forest growth rate and land use
practices that would have been applied
for the flooded areas if the reservoir was
not built.
All of these changes over time must be
considered when estimating GHG emissions
caused by the creation of a reservoir. IPCC
recommends expressing a GHG as average
emissions over a reservoir’s lifecycle, which is
normally set to 100 years.
6 INTERIM TECHNICAL NOTE: GREENHOUSE GASES FROM RESERVOIRS CAUSED BY BIOCHEMICAL PROCESSES
Section 3
Results from
Conducted Research
3.1 Geographic and Temporal Distribution
Research on GHGs from reservoirs is a relatively new science, and most studies have been conducted
in the last 15 years without standard methodology. The past two to three years have seen more
frequent publications, which have contributed to a better understanding.
Scientific publications can in principle be divided into two types.
?? measurements of GHGs in the sediments and water phase as well as gross emission from the
reservoir surface area and downstream river, and
?? interpretation and up-scaling of measured emissions to estimate GHGs and GWP caused by
reservoirs.
The geographical location of the studied reservoirs is not well distributed. Most studies are located
in tropical and subtropical areas of South America or in cold continental areas of North America and
Europe. Relatively few measurements at reservoirs in Africa and Asia have been made.
Very few studies so far have attempted to estimate a reservoir’s net GHG fluxes.
3.2 Findings on Main Processes
The main findings expressed in scientific publications6 on processes and GHG pathways are summarized
below.7
?? Flooded vegetation and litter in a reservoir is only partially decayed. Smaller vegetation
and labile (easily decayed) organic matter in the soil is easily decomposed and starts to
decay immediately as inundation occurs, whereas the tree trunks remain for many decades.
Research, although limited to a few reservoirs, indicates that even in old reservoirs, less
than 50 percent of the flooded vegetation has been decayed (Abril et al. 2005, Campo and
Sancholuz 1998).
6 See the Bibliography and References, beginning on page 35.
7 Illustrations of main GHG processes and parameters affecting GHG emissions are given in Annex 2.
?? The two most important parameters
affecting the type and amount of GHGs
produced in a reservoir are water temperature
and dissolved oxygen concentration.
Temperature directly influences the
decomposition rate of organic matter
and significant CH4 production only
occurs under anoxic conditions.
Strong seasonal stratification due to
temperature differences between surface and
deeper water and poor vertical mixing may quickly
produce anoxic conditions in the deeper, colder
water. The water depth and the stratification of
the water column into an anoxic zone (hypolimnion)
below the aerobic zone (epilimnion) have
large effects on GHG emissions. Dissolved CH4 is
produced in anoxic conditions and can be oxidized
to CO2 in aerobic conditions. If anoxic conditions
exist for the whole water column it allows for fluxes
of CH4 to the atmosphere. The greater the thickness
of the overlying epilimnion layer in the water
column, the less likelihood of diffuse CH4 emissions
since production area and volume become
less and the oxidizing area and volume (where
CH4 can transform to CO2) becomes larger.
Factors that indirectly affect GHG production
and diffuse emissions thus also include
temperature, stratification, water retention time
(determined by inflow rate and reservoir volume
and form), and quality of inflowing water:
?? The lower the temperature, the more
oxygen can be dissolved in water.
?? The longer the retention time, the less
mixing of oxygen-saturated water
occurs, thus favoring stratification and a
long period of anoxic conditions.
?? The poorer the quality of inflowing water
(for example, high content of nutrients
and organic matters), the more oxygen
demand is created, favoring anoxic
conditions.
?? Inflows with high inorganic sediment
concentration inhibit GHG production.
Transport of CH4 created from bed sediment
can also occur in gas form as ebullition (bubbling)
directly to the atmosphere. Ebullition is more
likely to occur in shallow waters since solubility
increases with pressure. At greater depths the
pressure is high and the CH4 is thus more likely
to be dissolved after creation. Because CO2 has
much higher solubility than CH4, bubbling of
CO2 is low even in shallow waters.
Wind speed at the surface affects the diffuse
fluxes of CO2 and CH4 from the water phase to
the atmosphere. More wind speed increases
fluxes.
Because of the sudden decrease in pressure
when water is released from a low-level (deep)
outlet of the dam, the solubility of gases will drastically
decrease and dissolved CH4 in particular
may be degassed just downstream of the reservoir.
Even if oxygen is available in this environment,
the depth and time available for oxidation
is short and CH4 is therefore transferred directly
to the atmosphere.
CO2 and CH4
discharged from the dam
and not immediately degassed may also be
released to the atmosphere in the river stretch
downstream. The ratio of CH4 to CO2 release will
gradually decrease with distance from the dam
because of oxidation.
CH4 emissions have been found to be higher
in the drawdown zone of the reservoirs as vegetation
is developed and flooded on a seasonal
basis.
3.3 Findings on Measurements
of Gross Emissons
The main findings expressed in scientific publications
on measurements of gross emissions
(Tables 1–2) are summarized below.
8 INTERIM TECHNICAL NOTE: GREENHOUSE GASES FROM RESERVOIRS CAUSED BY BIOCHEMICAL PROCESSES
Section 3
?? Measurements confirm the great
temporal variability of GHG emissions.
?? Observed gross emissions are highly
variable seasonally depending on
changes in temperature and inflow
rate.
?? Observed gross emissions show
a decreasing trend with age of the
reservoirs, with the highest values
during the first 5 to 15 years after
inundation.
?? Measurements confirm the great spatial
variability of GHG emissions.
?? Observed gross emissions are highly
variable spatially within the reservoir
area depending on depth and conditions
for stratification.
?? Observed gross emissions show large
differences between reservoirs both
in terms of total emissions, GHG type,
and pathways by which the GHG is
emitted.
?? Significant relationships between gross
GHG emissions and input of dissolved
organic material, age, and latitude have
been found when statistically analyzing
results at 89 reservoirs throughout the
world (Barros et al. 2011). Emissions
decrease with age and latitude and
increase with dissolved organic carbon
(DOC) input.
?? The highest observed gross GHG emissions
have been found for reservoirs
in dense tropical wet forests in South
America. All of these are associated with
a significant proportion of CH4 emissions.
?? The lowest observed gross GHG emissions
have been found in cold continental
Table 1: Results from Measurements of CO2 and CH4 Expressed as CO2 Equivalents
Range of average CO2 and CH4 emissions from freshwater reservoirs per year
Tropical climate
CO2 flux CH4 flux CO2 + CH4
(t CO2/km2) (t CH4/km2) (t CO2eq/km2) (t CO2eq/km2)
Min Value –340.5 0.6 42.6 –176.2
Max Value 3,806 565 13,867 15,494
Number of reservoirs 24
Temperate climate
CO2 flux CH4 flux CO2+ CH4
(t CO2/km2) (t CH4/km2) (t CO2eq/km2) (t CO2eq/km2)
Min Value –435.9 0.0 0.0 –34.7
Max Value 2,099 41 1,022 2,285
Number of reservoirs 64
Boreal climate
CO2 flux CH4 flux CO2+ CH4
(t CO2/km2) (t CH4/km2) (t CO2eq/km2) (t CO2eq/km2)
Min Value –376.5 1.3 32.8 –296.2
Max Value 2,000 145 3,632 3,652
Number of reservoirs 30
Note: Negative values mean the reservoir works as a sink.
Source: Barros et al. 2011 and Chanudet et al. 2011.
Results from Conducted Research   9
reservoirs in Canada and Scandinavia.
CH4 emissions in all of these are low.
?? Reservoirs located in tropical dry forests
in Brazil and Laos generally have lower
gross GHG emissions than reservoirs
located in tropical wet forests.
?? GHG emissions with a high proportion
of CH4 have been found to be higher
in reservoirs receiving large amounts
of nutrients and organic matter from
upstream urban areas than in similarly
aged reservoirs with natural inflow, irrespective
of latitude.
?? N2O emissions have so far only been
studied in tropical climates in South
America and contribute in all studied
cases to less than 10 percent of the gross
GHG emissions.
?? Some older reservoirs have been found
to work as carbon sinks, burying more
inflowing carbon in the bed sediment
than they release to the atmosphere or
downstream.
3.4 Results Put into Global
Perspective
Globally, natural lakes cover an estimated
area of 3.7 to 4.2 million km2 (Downing et al.
2006). Reservoirs cover an estimated area of
between 0. 3 and 1.5 million km2, depending on
reference (St. Louis et al. 2000, Cole et al. 2007,
Barros et al. 2011, Varis et al. 2012, Lehner et
al. 2011). Of these, large dams (above 15 m in
height) create inundation of 0.26 to 0.50 million
km2 according to ICOLD, whereas the estimated
area of small-scale dams is highly speculative.
The area of hydroelectric dams is estimated to
be 0.34 million km2.
Earlier estimates (St. Louis et al. 2000) of
GHG contribution from all reservoirs, using the
upper limit of global areas, was 7 percent of
global GHG emissions from all sources,8 whereas
recent studies have indicated estimates of less
than 1 percent (Varis et al. 2012). Both figures
refer to gross emissions.
Based on recent published studies, GHG
gross emissions from reservoirs are only a fraction
of all emissions from fresh waters (that is,
natural and manmade water bodies). Estimates
indicate that hydroelectric reservoirs contribute
globally to 4 percent of total GHG emissions from
fresh water (Barros et al. 2011).
Table 2: Results from Measurements of N2O Expressed as CO2 Equivalents
Range of Nitrous Oxide fluxes from tropical reservoirs per year
Reservoir Surfaces
Open Water Flooded Forest Downstream
(t N2O/
km2)
(t CO2eq/
km2)
(t N2O/
km2)
(t CO2eq/
km2)
(t N2O/
km2)
(t CO2eq/
km2)
Min Value –2.17 –646 1.04 311 0.21 62
Max Value 3.52 1,048 3.04 905 4.51 1,345
Number of reservoirs 6 3 2
Notes: Negative values mean the reservoir works as a sink
Source: Guerin et al. 2008.
8 Total global estimates of GHG emissions in 2010 were
30.6 Gt CO2-eqv/year according to IEA.
10 INTERIM TECHNICAL NOTE: GREENHOUSE GASES FROM RESERVOIRS CAUSED BY BIOCHEMICAL PROCESSES
Section 3
3.5 Future Research and
Expected Outputs
Research programs are ongoing, dealing mainly
with two focus areas.
?? Development of standardized methodology
for measuring GHG emissions both
for pre- and post- impoundment and
monitoring of more reservoirs. Emphasis
on the monitoring is to enable comparison
among studies and quantification
of net emissions, which today is basically
nonexistent.
?? Development and test of empirical and
process-based models to predict GHGs
from reservoirs.
Two major groups, UNESCO/IHA9 and
IEA Hydro’s Annex XII,10 lead the development
of standards for monitoring and conduct the
majority of reservoir studies. Regular meetings
and workshops are held with the wider scientific
community concerned with GHGs from reservoirs
to get as broad acceptance of the methods
and results as possible.
UNESCO/IHA has recently started development
of empirical modeling tools for predicting
GHGs. A beta version of a predictive model (GHG
Risk Assessment Tool) to calculate gross emissions
for a reservoir was released in August 2012.
The spreadsheet model is empirical and uses
data of gross emissions from previous assessments
on 169 reservoirs around the world. The
IEA Hydro Annex XII has initiated development
of physically based models for GHGs from reservoirs,
with results scheduled for 2014.
Monitoring of GHGs is very resource- and
time-demanding, especially for net emissions,
and increase in new measured data on GHGs
from reservoirs is expected to be limited in the
coming years. The results of previous measurements
of gross emissions (Tables 1 and 2) and
the development of empirical tools, such as
the UNESCO/IHA GHG Risk Assessment Tool,
to estimate gross emissions are essential for
improved understanding and awareness of
GHG emissions from reservoirs. Development
of generalized tools for estimation of net GHG
emissions from reservoirs, which is the key variable,
is however not expected in the near future
because of data scarcity.
9 www.hydropower.org/iha/development/ghg/
10 www.ieahydro.org
Results from Conducted Research   11

Section 4
Categorization of
Reservoirs Based on GHG
Emission Potential
The generally agreed-upon stepwise process to assess potentially significant
GHG emissions from reservoirs is to ask the following successive questions.
?? Does the reservoir have the capacity to create large carbon stock (amount of flooded organic
matter, inflowing organic matter, and organic matter produced in the reservoir)? If the carbon
stock is small, GWP is likely negligible.
?? Does the reservoir have the capacity to convert the organic matter to GHGs and to what type?
If the physical conditions disfavor decomposition of organic matter, and especially do not favor
creation of CH4 and N2O, GWP is likely negligible.
?? Does the reservoir have the capacity to release the created GHG to the atmosphere? If the
pathways of CH4 and N2O to the atmosphere are few and if the physical conditions favor transformation
of these to CO2 before emission, GWP is likely negligible.
The results and understanding from gross GHG emission measurements during the last 15 years
have led to the following key conclusions in regard to assessment of reservoirs:
?? Reservoirs with emissions causing significant GWP are associated with high CH4 emissions
because of the gas’s strength as a GHG.
?? The likelihood of significant GHG emissions, especially CH4, increases with the number of variables
contributing to GHG emission that work in combination. No single variable, for example,
latitude or reservoir size, should be used on its own to estimate GHGs from a specific reservoir.
?? The key for assessing GHG emissions lies in understanding the availability of carbon stock and
the reservoir’s water quality conditions, especially the temporal and spatial extent of anoxic
conditions.
Table 3 illustrates scenarios of different levels of GHG emissions depending on reservoir capacity
to supply carbon stock, and create and release GHGs. The observed gross emissions and estimated
net emissions for these scenarios indicate the following:
?? Each scenario still shows considerable
ranges of emission rates, which confirms
that GHG emissions are site specific and
caution should be used when applying
standard values.
?? Boreal and temperate reservoirs in pristine
environments generally give an order
of magnitude lower GHG emissions than
those in tropical pristine conditions.
?? Reservoirs affected by upstream
anthropogenic areas show in general
higher gross emissions than reservoirs in
pristine conditions. High rates are found
when combining tropical wet conditions
with large anthropogenic influence.
?? CH4/CO2 ratios are generally higher in
tropical areas than in temperate and
boreal conditions. Ratios show skewed
distributions with most reservoirs
showing low ratios but with a few very
high values. Data are not sufficient to
link these high ratios to a specific parameter,
for example, high anthropogenic
influence.
14 INTERIM TECHNICAL NOTE: GREENHOUSE GASES FROM RESERVOIRS CAUSED BY BIOCHEMICAL PROCESSES
Section 4
Table 3: Illustrated Scenarios of GWP from Reservoirs Depending on Capacity to Supply Carbon Stock and the Conditions
to Create and Release GHGsa
Amount of
carbon stock
Conditions to
create and release
GHGs Examples Typical characteristics of gross emissions
Anticipated net
emissionsb generated by
biochemical processes
Low density of
flooded material
Low concentration
of inflowing organic
matter
Relatively small
drawdown zone to
create new seasonal
vegetation
Low temperatures
most of the year
Mostly low-intensity
rainfall or snow melt
distributed over all
seasons
Low concentration
of inflowing
nutrients
Short retention
time, except in
extreme northern
part where long
winters create
periods of low
inflows
Reservoirs
located
in mostly
pristine
boreal areas
Low CO2 emissions
mainly during the
first 5–10 years and no
significant CH4 or N2O
emissions. After initial
years the emissions are
close to zero
Observed rates of
emissions
Median: 561 (–296–721)
t CO2-eqv per
km2 reservoir area and
year
Observed CH4/
CO2 ratio
Median: 1.6% (0%–5%)
Reason
Limited flooded biomass
providing easily decomposed
organic matter only during the
first years after inundation. Inflow
with low content of nutrients and
organic matter limiting supply of
new labile carbon stock as well as
preventing depletion of oxygen by
nitrification. Low temperatures
limiting high decomposition
rate and allowing high levels of
dissolved oxygen. Generally
short retention time allows for
regular oxygen supplies. CH4 that
is created in the limited anoxic
bottom zone has limited pathways
and will likely be oxidized before
reaching the surface of the
reservoir. Low temperature and
low nutrient load prevent creation
of N2O in drawdown zone.
Net emissions in the same
order of magnitude as gross.
Growth rate of biomass (sink)
for the pre-impoundment
conditions is small and
often balanced by natural
emissions from abundant
water bodies in the boreal
landscape. Main contribution
to net emissions is therefore
decomposable parts of
flooded soil and vegetation in
terrestrial zones.
Assuming flooding fully
forested areas the AFOLU
values for different boreal
ecological zones and soil
types give lifespan average
net emissions of 55–441 t
CO2-eqv/km2 and year.
A hydropower reservoir
with power density of 1 MW/
km2 and 0.5 plant factor
would (with the above
emissions) give 12–101 g
CO2-eqv/kWh (or 1%–11%
compared to a coal plant).
(continued on next page)
Categori zation of Reservoirs Based on Ghg Emission Potential   15
Table 3: Illustrated Scenarios of GWP from Reservoirs Depending on Capacity to Supply Carbon Stock and the Conditions
to Create and Release GHGsa
Amount of
carbon stock
Conditions to
create and release
GHGs Examples Typical characteristics of gross emissions
Anticipated net
emissionsb generated by
biochemical processes
Low density of
flooded material
Low concentration
of inflowing organic
matter
Relatively large
drawdown zone to
create new seasonal
vegetation
High temperatures
most of the year
Seasonally intensive
rainfall, long dry
season with very
low inflows
Long retention time
Low concentration
of inflowing
nutrients
Reservoirs
located
in mostly
pristine
tropical
savannah
or scrubland
areas
Moderate emissions
of CO2 and CH4,
mainly during the
first 5–10 years.
Generally low base level
after initial years
Observed rates of
emissions
Too little data to give
representative values
Observed CH4/
CO2 ratio (all tropical)
Median: 7.2%
(1%–30%)
Reason
Limited flooded biomass providing
easily decomposed organic
matter only during the first years
after inundation. Even if siltation
rates may be high, inflow has low
content of nutrients and organic
matter limiting supply of new
labile carbon stock as well as
preventing depletion of oxygen
by nitrification. Because of long
dry seasons, drawdown zone may
be large, creating new seasonal
vegetation that is flooded annually.
High temperatures and generally
long retention times create anoxic
zones during dry seasons, allowing
CH4 to be created. CH4 may be
released and degassed through
the intake canals or emitted to the
atmosphere during periods when
most of the water column is anoxic.
As labile organic matter is depleted
from original flooded material the
decomposing rate decreases,
which also decreases oxygen
depletion and thus the extent of
anoxic conditions. Low nutrient
load prevents significant creation
of N2O in drawdown zone.
Main contributions to net
emissions are decomposable
parts of flooded soil and
vegetation in terrestrial zones
plus removed sink from
biomass growth.
Assuming flooding fully
vegetated areas, the AFOLU
values for biomass volume
and growth for tropical
scrubland and different soil
types give lifespan average
net emissions of 210–297 t
CO2-eqv/km2 and year.
A hydropower reservoir
with power density of 1 MW/
km2 and 0.5 plant factor
would (with the above
emissions) give 48–68 g
CO2-eqv/kWh (or 5%–7%
compared to a coal plant).
(continued on next page)
(continued)
INTERIM TECHNICAL NOTE: GREENHOUSE GASES FROM RESERVOIRS CAUSED B 16 Y BIOCHEMICAL PROCESSES
Section 4
Table 3: Illustrated Scenarios of GWP from Reservoirs Depending on Capacity to Supply Carbon Stock and the Conditions
to Create and Release GHGsa
Amount of
carbon stock
Conditions to
create and release
GHGs Examples Typical characteristics of gross emissions
Anticipated net
emissionsb generated by
biochemical processes
Medium density of
flooded material
Low-medium
concentration of
inflowing organic
matter
Relatively small
drawdown zone to
create new seasonal
vegetation
Low temperatures
except for summer
periods
Mostly low-intensity
rainfall or snow melt
distributed over all
seasons
Low concentration
of inflowing
nutrients
Fairly short
retention time
Reservoirs
located
in mostly
pristine
temperate
forest areas
Emissions mainly during
the first 5–15 years,
after which the base
level is low. GHG
dominated by CO2 but
with some potential
CH4 emissions
Observed rates of
emissions
Median: 255
(–35–1,287)
t CO2-eqv/km2 reservoir
area and year
Observed CH4/
CO2 ratio (all
temperate)
Median: 1.4%
(0%–27%)
Reason
Significant carbon stock available
but limited conditions for creation
and release of GHG. Inflow with
fairly low content of nutrients and
organic matter, limiting supply of
new labile carbon stock as well as
preventing depletion of oxygen
by nitrification. Temperatures
limiting high decomposition
rate and low levels of dissolved
oxygen to mainly summer
periods. Generally short retention
time allows for regular oxygen
supplies. Except during short
periods when high temperatures
and low retention time coincide,
causing deep anoxic zones,
CH4 has limited pathways and will
likely be oxidized before reaching
the surface of the reservoir. Low
nutrient load and generally low
temperatures prevent creation of
N2O in drawdown zone.
Main contribution to
net emissions is from
the removal of carbon
sink since temperate
forests have relatively
high biomass growth. In
addition, the emissions from
decomposable parts of
flooded soil and vegetation
contribute to the net
emissions.
Assuming flooding fully
forested areas the AFOLU
values for different temperate
forests and soil types
give lifespan average net
emissions of 478–979 t
CO2-eqv/km2 and year.
A hydropower reservoir
with power density of 1 MW/
km2 and 0.5 plant factor
would (with the above
emissions) give 109–223 g
CO2-eqv/kWh (or 12%–24%
compared to a coal plant).
(continued on next page)
(continued)
Categori zation of Reservoirs Based on Ghg Emission Potential   17
Table 3: Illustrated Scenarios of GWP from Reservoirs Depending on Capacity to Supply Carbon Stock and the Conditions
to Create and Release GHGsa
Amount of
carbon stock
Conditions to
create and release
GHGs Examples Typical characteristics of gross emissions
Anticipated net
emissionsb generated by
biochemical processes
Medium density of
flooded material
Medium
concentration of
inflowing organic
matter
Relatively large
drawdown zone to
create new seasonal
vegetation
High temperatures
throughout the year
Seasonally intensive
rainfall and fairly
long dry season
with low inflows
Fairly long retention
time
Low concentration
of inflowing
nutrients
Reservoirs
located
in mostly
pristine
tropical dry
forest areas
Moderate emissions
of both CO2 and CH4.
Decreasing emissions
after 5–15 years to a
moderate base level
because of inflow of new
labile organic matters
Observed rates of
emissions
Median: 1,808
(439–5,449)
t CO2-eqv/km2 reservoir
area and year
Observed CH4/
CO2 ratio (all tropical)
Median: 7.2%
(1%–30%)
Reason
Significant but not abundant
carbon stock available. High
seasonal intensive rainfall gives
regular annual supply of new
labile organic matter through
riverine transport. Pristine areas
however prevent significant
nutrient load. Because of long dry
seasons, drawdown zone may
be large, creating new seasonal
vegetation that is flooded
annually. High temperatures
create deep anoxic zones with
long retention time, allowing
CH4 to be created. CH4 may be
released and degassed through
the intake canals or emitted to the
atmosphere mostly during the
dry season. Limited nutrient load
depleting oxygen and renewed
oxygen supply during high
intensive rainfall during the rainy
season limit large CH4 emissions.
Low nutrient load prevents
significant creation of N2O in
drawdown zone.
Net emissions of the lifecycle
generally lower than gross
because much carbon stock
is from riverine transport
that would be emitted further
downstream even without
the dam. Main contributions
to net emissions are
decomposable parts of
flooded soil and vegetation
in terrestrial zones plus
removed sink from biomass
growth.
Assuming flooding fully
forested areas the AFOLU
values for tropical dry
forests and different soil
types give lifespan average
net emissions of 481–587 t
CO2-eqv/km2 and year.
A hydropower reservoir
with power density of 1 MW/
km2 and 0.5 plant factor
would (with the above
emissions) give 110–134 g
CO2-eqv/kWh (or 12%–15%
compared to a coal plant).
(continued on next page)
(continued)
INTERIM TECHNICAL NOTE: GREENHOUSE GASES FROM RESERVOIRS CAUSED B 18 Y BIOCHEMICAL PROCESSES
Section 4
Table 3: Illustrated Scenarios of GWP from Reservoirs Depending on Capacity to Supply Carbon Stock and the Conditions
to Create and Release GHGsa
Amount of
carbon stock
Conditions to
create and release
GHGs Examples Typical characteristics of gross emissions
Anticipated net
emissionsb generated by
biochemical processes
High density of
flooded material
Medium
concentration of
inflowing organic
matter
Relatively small
drawdown zone to
create new seasonal
vegetation
High temperatures
throughout the year
High rainfall fairly
well distributed over
the year
Fairly short
retention time
Medium
concentration of
inflowing nutrients
Reservoirs
located
in mostly
pristine
tropical wet
forest areas
(rain forests)
Significant emissions
of both CO2 and
CH4, and potential
emissions of N2O.
Decreasing emissions
after 10–20 years to
a moderate base level
because of medium
inflow of new labile
organic matter
Observed rates of
emissions
Median: 1,520
(487–3,072)
t CO2-eqv/km2 reservoir
area and year
Observed CH4/
CO2 ratio (all tropical)
Median: 7.2%
(1%–30%)
Reason
Large carbon stock available
from flooded material. Inflow
that naturally contains organic
matters and nutrients. High water
temperatures throughout the year
favoring high decomposition rate
and anoxic conditions, allowing
for CH4 creation large part of the
year. High temperatures create
deep anoxic zones, allowing
CH4 to be released and degassed
through the intake canals or
emitted to the atmosphere
since most of the water column
is anoxic. Nutrient inputs
and high temperatures favor
denitrification allowing potential
N2O emissions in drawdown
zone. Short retention time and
pristine conditions, limiting very
high inflow of nutrient and organic
matter, prevents extreme high
emissions.
Net emissions of the lifecycle
generally lower than gross
because much carbon stock
is from riverine transport
that would be emitted further
downstream even without
the dam. Main contributions
to net emissions are
decomposable parts of
flooded soil and dense
vegetation in terrestrial zones
plus removed sink from high
biomass growth.
Assuming flooding fully
forested areas the AFOLU
values for different tropical
wet forest and soil types
give lifespan average net
emissions of 886–1,507 t
CO2-eqv/km2 and year.
A hydropower reservoir
with power density of 1 MW/
km2 and 0.5 plant factor
would (with the above
emissions) give 202–344 g
CO2-eqv/kWh (or 22%–37%
compared to a coal plant).
(continued on next page)
(continued)
Categori zation of Reservoirs Based on Ghg Emission Potential   19
Table 3: Illustrated Scenarios of GWP from Reservoirs Depending on Capacity to Supply Carbon Stock and the Conditions
to Create and Release GHGsa
Amount of
carbon stock
Conditions to
create and release
GHGs Examples Typical characteristics of gross emissions
Anticipated net
emissionsb generated by
biochemical processes
Medium density of
flooded material
High concentration
of inflowing organic
matter
Relatively small
drawdown zone to
create new seasonal
vegetation
Low temperatures
except for summer
periods
Mostly low intensity
rainfall or snow melt
distributed over all
seasons
Inflow of nutrients
from upstream
agriculture and
urban areas
Reservoirs
located in
temperate
forests
downstream
of densely
developed
areas
Moderate to
significant emissions
of both CO2 and
CH4 and some N2O.
Decreasing emissions
after 5–15 years to a
high base level because
of inflow of new labile
organic matter
Observed rates of
emissions
Median: 346
(170–2,285)
t CO2-eqv/km2 reservoir
area and year
Observed CH4/
CO2 ratio (all
temperate)
Median: 1.4%
(0%–27%)
Reason
Significant but not abundant
carbon stock available. Although
generally low temperatures,
the inflow of new labile organic
matter and nutrients from
anthropogenic sources cause
high oxygen depletion and deep
zones of anoxic conditions.
CH4 is thus created in a large
part of the water column and is
allowed to be released through
the intake canal or emitted to
the atmosphere at the reservoir
surface. Low temperatures during
a large part of the year, however,
prevent very high emissions. High
levels of nutrient inputs favor
denitrification during summer
periods, risking N2O emissions in
the drawdown zone.
Net emissions of the lifecycle
considerably lower than gross
because much carbon stock
and conditions to create GHG
are caused by upstream
artificial activities not linked to
the reservoir. The artificially
increased GHG emissions
would be emitted further
downstream in natural water
bodies even without the dam.
Main contributions to net
emissions are decomposable
parts of flooded soil and
vegetation in terrestrial zones
plus removed sink from high
biomass growth.
Net emissions are thus
the same as for pristine
temperate forests, see above.
(continued on next page)
(continued)
INTERIM TECHNICAL NOTE: GREENHOUSE GASES FROM RESERVOIRS CAUSED B 20 Y BIOCHEMICAL PROCESSES
Section 4
Table 3: Illustrated Scenarios of GWP from Reservoirs Depending on Capacity to Supply Carbon Stock and the Conditions
to Create and Release GHGsa
Amount of
carbon stock
Conditions to
create and release
GHGs Examples Typical characteristics of gross emissions
Anticipated net
emissionsb generated by
biochemical processes
Very high density of
flooded material
High concentration
of inflowing organic
matter
Relatively small
drawdown zone to
create new seasonal
vegetation
High temperatures
throughout the year
High rainfall fairly
well distributed over
the year
Fairly short
retention time
High concentration
of inflowing
nutrients because
of contribution
from upstream
agriculture and
urban areas
Reservoirs
located in
tropical
wet forests
downstream
of densely
developed
areas
Significant emissions
of both CO2 and CH4,
and potentially high
emissions of N2O.
Decreasing emissions
after 10–20 years to a
high base level because
of high inflow of new
labile organic matter
Observed rates of
emissions
Median: 2,950
(122–15,494)
t CO2-eqv/km2 reservoir
area and year
Observed CH4/
CO2 ratio (all tropical)
Median: 7.2%
(1%–30%)
Reason
Large carbon stock available
from flooded material. The
combination of high inflow of
nutrients and organic matter,
and high water temperatures
favoring high decomposing
rate and anoxic conditions in
the whole water column in a
large part of the reservoir and
during a large part of the year.
The limited oxygen availability
in the water column prevents
oxidation, allowing CH4 to be
emitted to the atmosphere.
High levels of nutrient inputs
and high temperatures favor
denitrification, allowing potentially
high N2O emissions.
Net emissions of the lifecycle
considerably lower than
gross because a large part
of the carbon stock and
conditions to create GHG are
caused by upstream artificial
activities not linked to the
reservoir. The artificially
increased GHG emission
would be emitted further
downstream in natural water
bodies even without the dam.
Main contributions to net
emissions are decomposable
parts of flooded soil and
dense vegetation in terrestrial
zones plus removed sink from
high biomass growth.
Net emissions are thus the
same as for pristine tropical
wet forests, see above.
a Observed gross emission rates based on presented results in Barros et al. 2011, which have been judged well representative for the respective category (values may thus
differ slightly from Table 1). If fewer than five reservoirs with data exist for the category, no values are presented. Calculated net emissions are based on the methodology (Tier
1) described in Section 5.2. Note that the net emissions are based on fully forested (vegetated) areas. If only part of the flooded area is forested (vegetated) the emissions will
be lower. Observed CH4/CO2 ratios are based on observed average emissions converted to atomic mass of carbon.
b Net emissions here refer only to biochemical processes and do not include emissions from dam construction or baseline emissions generated by alternatives built if the reservoir
would not be constructed, see Section 2.2. AFOLU – 2006 IPCC Guidelines for National Greenhouse Gas Inventories, Volume 4: Agriculture, Forestry and Other Land Use.
(continued)
Categori zation of Reservoirs Based on Ghg Emission Potential   21

Section 5
Recommendations for
Preparing Dam Projects
5.1 Qualitative Assessment of GHG Emission Potential
For dam infrastructure projects with significant inundation for which the World Bank may provide
financing, studying biochemically generated GHG from the reservoirs as part of the Environmental
Impact Assessment is recommended. The purpose of including studies of potential GHG emissions
from reservoirs as part of the EIA is to enable comparison of alternative design options within the
framework of the investment project. Current science only permits a crude comparison of alternatives.
The inclusion of GHG assessment in the EIA does not change the boundaries for a regular impact
assessment. GHG emissions can be anticipated to occur at the reservoir area and the immediate river
stretch downstream of the reservoir, which should guide implementation monitoring programs.
The qualitative assessment of biochemically generated GHG from the reservoir is recommended
to follow the stepwise process given in Section 4, evaluating the supply of carbon stock and the reservoir’s
condition to create and release GHG.
?? Based on available information, provide an overall description of the main factors affecting
future potential GHG emissions from the planned reservoir options (see Table 4).
?? Based on the compiled information, make a qualitative assessment of the reservoir’s capacity
to supply carbon stock and to create and release different types of GHGs (see Table 5).
?? Compare the reservoir alternatives with the examples given in Table 3 to assess the potential
GHG emissions from the planned reservoir alternatives. If emissions are not negligible, consider
a more detailed estimation of the magnitude of GHG according to Section 5.2.
For dam infrastructure projects with estimated potentially significant GHG emissions, streamlined
post-implementation monitoring of GHG from the reservoir and immediate river stretch downstream
is recommended to confirm the order of magnitude of the estimated emission potential. The purpose
of such monitoring is not to provide a complete estimation of net GHG emissions but to make a rough
comparative analysis of gross emissions with previous results. Comparison of observed emissions should
take into account local conditions and be made against relevant previous observations under similar
conditions (cf. Table 3). Such a monitoring program could have the following suggested characteristics:
?? focus on gross emissions of CO2 and CH4 measured by Surface Floating Chambers,11
11 See, for example, UNESCO/IHA 2010 for a description of monitoring methods.
?? covering spatially the reservoir along the
longitudinal axis as well as the immediate
river stretch downstream of the reservoir,
and
?? covering temporally a period of at least 3
years with seasonal measurements.
For dam infrastructure projects with
estimated potentially significant GHG emissions
for which the World Bank may provide
financing, it is further encouraged to investigate
possibilities to support from Trust Funds
detailed measurements to enhance research on
biochemical GHG emissions from reservoirs. In
such cases, both pre- and post-impoundment
measurements are recommended to enable
estimation of net emissions in accordance with
the guidelines of UNESCO/IHA and IEA Hydro
Annex XII.
Biomass removal should be carefully
assessed before being used as a default mitigation
measure for preventing GHG. A large part
of GHG emissions stems from easily available
carbon in soil and small vegetation, whereas
Table 4: Information to Retrieve to Assess Future Potential GHG from a Reservoir
Factor to retrieve Proposed methodology
Size and shape of
inundated area and
volume of reservoir
Use existing topographical maps or air photographs to delineate inundated
area up to planned Full Supply Level. When natural lakes are utilized as
reservoirs, estimate how much new inundated area is created by damming.
Calculate surface areas, volumes, and average depth. This information is
available from the technical feasibility study.
Size of drawdown area Combine information from planned operation rules with the obtained
information on inundated area at different reservoir levels to estimate
drawdown area. This information is available from the technical feasibility study.
Intake configuration Assess how far from the reservoir bed level the bottom outlet and intake
structure for downstream water use/hydropower production is located. This
information is available from the technical feasibility study.
Rainfall and
temperature in planned
reservoir area
Use historical records of rainfall and temperature in the vicinity of the
planned reservoir. This data and information is normally available from the
technical feasibility study.
River inflow to the
planned reservoir and
retention time
Use records of river flows rivers upstream from the reservoir to estimate
inflow amount and hydrograph shape. Divide average annual inflows with
the reservoir volume to retrieve an average retention time. This data and
information is available from the technical feasibility study.
Type and extent of
flooded vegetation
Use existing global maps of ecoregions (e.g., WWF’s Terrestrial Ecoregions
GIS Database or FAO Global Ecological Zones) combined with studies of air
photos and field surveys in planned reservoir area.
Type of flooded soil and
extent of soil carbon
Use existing national soil maps combined with tabulated carbon mass in
AFOLU (Table 2.3). If possible, conduct field surveys and soil sampling in
planned reservoir area to confirm soil types and carbon content.
Water quality of
inflowing rivers
Use historical records of suspended and dissolved organic material,
dissolved oxygen, nitrogen and phosphorus in the rivers upstream of the
reservoir (or if this is not available, in the nearest representative river). If
possible conduct water sampling of inflowing rivers.
Conduct an overall assessment of land cover, land use, and potential point
sources of organic matter and nutrients in upstream catchment area.
Reservoir’s position in
the river basin system
Conduct an overall assessment of downstream river to assess what likely
fate inflowing organic material would have had without the planned reservoir.
24 INTERIM TECHNICAL NOTE: GREENHOUSE GASES FROM RESERVOIRS CAUSED BY BIOCHEMICAL PROCESSES
Section 5
tree trunks remain in the reservoir for a long
time. The positive effect on GHG emissions
of biomass removal thus depends on which
vegetation is cleared and how this biomass is
treated after removal. Apart from the possible
GHG emission reduction, the biomass removal
assessment should take into account the
commercial value of the biomass as well as
possibly improved water quality and future use
of the reservoir for fishing and other ecosystem
services.
Table 5: Guide to Describe Future Potential GHG from a Reservoir
Proposed methodology
Describe the reservoir’s
capacity to supply carbon
stock for the design
alternatives
Key parameters are flooded surface area and coverage of forest/
vegetation. In addition, compare type of forest and soil, if different
between alternatives. Use standard values of carbon content in
AFOLU (Tables 2.2, 2.3, 4.12) for comparison. Estimate if the reservoir
alternatives will create significantly different storage conditions
for inflowing organic matter compared to the situation without the
alternatives. If so, take into account differences in inflowing organic
matter and trap efficiency of the reservoir alternatives. Estimate
possibility for growth of vegetation in drawdown zones based on the
estimated drawdown area in the reservoir design alternatives.
Describe the reservoir
alternatives’ ability to create
GHG from the carbon stock.
Key parameters are temperature, retention time, and quality of inflowing
water. Use information on climate (temperature and rainfall regime) to
assess if provisions for extensive anoxic conditions may occur. If so,
compare retention time and different degree of anthropogenic influence
to rank alternatives.
Describe the reservoir
alternatives’ ability to release
GHG to the atmosphere.
Key parameters are extent of shallow water and location of intake
structures. Compare form of reservoir and distance from bed level to
lowest intake channel for the options.
• Shallow reservoir
• Anoxic conditions in most of
the water column
• Intake for downstream releases
in anoxic zone
Ability to supply
carbon stock
Ability to create
GHG
Ability to release
GHG
E.g.
• High temperature
• Long retention time
• Stratification of reservoir
• Large inflow of nutrients
from upstream areas
E.g.
• Large reservoir area
• High percentage of forests
• High density of vegetation
• High carbon content in soil
E.g.
Negligible emissions
Negligible emissions
Negligible emissions
Potentially significant
GHG emissions
Yes
Yes
Yes
No
No
No
Recommendations for Preparing Dam Projects   25
5.2 Preliminary Quantitative
Assessment of Net GHG
Emissions
If potentially significant GHG emissions cannot
be discounted through the above qualitative
assessment, it can be useful to conduct a
rough analysis of emission quantities. Given the
present limited information and understanding
of GHG from reservoirs, only very preliminary
estimates can be made. Therefore, the use of
detailed emission predictions resulting from the
methodology below for decision-making is not
recommended and results should preferably be
published in broad intervals.
The assumptions for estimating GHG from
reservoirs using this methodology are based on
the available published literature and are given
below.
?? The estimate must be made based on
a life-cycle analysis, that is, the only
possible estimate is an average net emission
rate over the 100-year lifespan of a
reservoir. Existing data and knowledge
are not sufficient to describe the detailed
emission development over time.
?? Following IPCC’s definition (see
Section 2.2), net GHG emissions from
reservoirs are based on pristine conditions,
ignoring the effects of anthropogenic
influence from upstream areas
such as supply of carbon stock and
enhanced conditions for CH4 creation
from urban and agricultural activities.
?? The major contributions to net emissions
over a reservoir’s life span are
assumed to be flooded biomass, litter,
and soil carbon, as well as the removal
of carbon sinks in the inundated area.12
Contributions from seasonally produced
carbon stock in the drawdown zone
are not considered. Inflowing material
is further assumed not to contribute
significantly to the net emissions, since
the organic carbon in this material would
likely have been transported and eventually
emitted downstream even without
the reservoir. Sequestering of carbon
by plants in the new aquatic system and
burying of carbon in the bed sediments
are further not taken into account.
?? Only CO2 and CH4 emissions are considered
in the estimation. N2O emissions
are assumed to be insignificant for net
emissions.
?? Only part of the organic carbon flooded by
the reservoir is assumed to be available
for decomposition and emitted during
the reservoir’s life span. Limited research
(Abril et al. 2005, Campo and Sancholuz
1998) has indicated a decreasing rate of
decomposition with time and levels of
degradation at around 40 percent of the
biomass after 40 to 65 years. A figure
of 50 percent of flooded organic carbon
has thus been assumed to be contributing
to GHG over the 100-year life span
of a reservoir.
?? In the absence of clear understanding
and data, it is assumed that the part of
available flooded carbon stock, which is
emitted as CH4 under pristine conditions,
is dependent on latitude (as a coarse
screening tool). The assumed CH4/
CO2 ratios under boreal, temperate, and
tropical conditions are thus set as the
observed median values from 89 reservoirs
reported in recent studies (Barros
et al. 2011, Chanudet et al. 2011; see
Table 3 for values).
12 Note that net emissions here refer only to biochemical
processes and do not include emissions from dam
construction or baseline emissions generated by alternatives
built if the reservoir were not constructed, see
Section 2.2.
26 INTERIM TECHNICAL NOTE: GREENHOUSE GASES FROM RESERVOIRS CAUSED BY BIOCHEMICAL PROCESSES
Section 5
?? The calculation of removal of carbon sink
is based on the concept of net sink for
the inundated area, that is, taking into
account both the carbon sink of flooded
terrestrial areas and the emissions from
water bodies that existed prior to the
reservoir’s construction.
A minimum prerequisite for making an
estimate is having the reservoir’s geographical
coordinates, the spatial extent of flooded area,
the percentage coverage of forests/vegetation
and water bodies of this area, and regional
ecological and soil maps.
With the above crude assumptions, the estimation
of GHG emissions from reservoirs can be
applied to a varying degree of detail, depending
on available data and information. The following
tier levels can be used depending on available
information and resources.
?? Tier 1.13 Desk study based on AFOLU
stock estimate: Use available geographical
information on the reservoir from
feasibility and EIA studies together with
standard biomass, carbon content,
soil, and litter carbon in AFOLU to estimate
flooded organic carbon amount.
Combine with interim assumption given
above to estimate average GHG emissions
over the reservoir’s life span.
?? Tier 2. Desk study based on more
detailed stock estimates: Use available
geographical information on the reservoir
from feasibility and EIA studies together
with local and regional databases on
above-ground biomass, soil, and litter
carbon.
?? Tier 3a. Field studies: Use results from
Tier 1 or 2 augmented by field measurements
(plot tests) and laboratory
analyses to estimate carbon content in
biomass, litter, and soil. This is similar
to standard CDM methodology for
deforestation.
?? Tier 3b. Water quality modeling: Use
results from Tier 1, 2 or 3a augmented
by water quality modeling of the extent
in space and time of anoxic conditions in
the reservoir to inform the choice of CH4/
CO2 ratio.
The significant uncertainty associated
with the above methodology to quantify GHG
emissions from a reservoir should be noted
and transparent. The methodology is predominantly
to be used for comparison of alternatives
in the design of a project, in which the same
methodology and level of detail has been used.
Given the methodology’s scope and limitations,
it is not advisable to use this methodology to
compare sectorally different alternatives to the
project.
13 See Annex 3 for detailed steps.
Recommendations for Preparing Dam Projects   27

Section 6 The technical guidance above comprises an interim method to assess and
estimate order of magnitude of biochemically generated net GHG from
reservoirs based on findings published in research papers to date. This
simplified method is recommended as an interim tool until a more sophisticated
model is developed and accepted by the broad scientific community. When such a
major breakthrough is reached this technical note should be updated.
The following roadmap is envisaged for the World Bank to update and finalize this interim technical
note over the next two years.
?? Conduct regular meetings with EIA practitioners and Task Team Leaders in the World Bank to
obtain feedback on the applications of this note in preparation of World Bank projects involving
reservoirs.
?? Continuously update the research literature in the field of GHGs from reservoirs.
?? Regularly interact with UNESCO/IHA, IEA Hydro Annex XII, and other research programs to
update information and research on GHG from reservoirs.
?? Interact with other multilateral development banks (MDBs) with the aim of harmonization of
guidance on biochemically generated GHGs from reservoirs.
?? Based on the above interaction, the technical note will be updated and, if possible, finalized in
June 2015.
Updating of this Technical Note

Glossary
Aerobic Describes conditions or processes in water or sediments in which oxygen is
present.
AFOLU Agriculture, Forestry, and Other Land Use, common name for the 2006 IPCC
Guidelines for National Greenhouse Gas Inventories
Anaerobic Describes conditions or processes in water and sediments in which oxygen
is absent.
Anoxic Same as anaerobic. Describes conditions in water and sediments in which
oxygen is absent.
Anthropogenic Resulting from or produced by human beings.
Biomass The total mass of living organisms in a given area, volume, or ecosystem at
a given time; recently dead plant material is included as dead biomass. The
quantity of biomass can be expressed as a dry weight or as the energy or
carbon content.
Carbon cycle The process of carbon flow through the atmosphere, ocean, terrestrial
biosphere (including freshwater systems), and sediments, as well as its
transformation processes (chemical alteration, photosynthesis, respiration,
decomposition, air-sea exchange, etc.).
Carbon dioxide (CO2) A naturally occurring GHG fixed by photosynthesis into organic matter and
released during respiration. It is a by-product of fossil fuel combustion, biomass
burning, land use changes, and other industrial processes.
CO2 equivalents The amount of CO2 emission that would have the same GWP over a given
time horizon, as an emitted amount of a GHG or a mixture of GHGs. The
CO2 equivalent emission is obtained by multiplying the emissions of a GHG
by its GWP for the given time horizon. For a mix of GHGs it is obtained by
summing the CO2 equivalent emission of each gas.
Carbon footprint A form of carbon calculation that considers the net emissions of GHG throughout
the life cycle of a project or investment.
Carbon mass flow Carbon in a water body can be in particulate or dissolved form and can be
organic or inorganic. The forms to be measured are: Total Organic Carbon
(TOC), Dissolved Organic Carbon (DOC), Dissolved Inorganic Carbon (DIC),
and Particulate Organic Carbon (POC). Carbon inputs and outputs to be
considered are: carbon brought in by macrophytes, carbon exchanges with
groundwater, carbon lost permanently to sediment, carbon exchanged with
atmosphere in the form of CO2 and CH4, and humic substance income and
output.
Carbon sequestration Build-up of GHG concentration in vegetation, water, or sediments.
Carbon stock The quantity of carbon in a water body and its sediments.
Decomposition Chemical processes by which organic matter in a water body is transformed
into gaseous end products. Major processes are oxidative decomposition,
methanogenesis, and denitrification and their end products are CO2, CH4 and
N2O.
Degassing GHG flux induced by dramatic pressure change immediately after water
discharge from reservoir outlets.
Denitrification Denitrification describes the conversion of nitrate into nitrite, then to N2O,
and finally to nitrogen gas. This process happens in the slightly anoxic upper
layer of sediment.
Diffusive flux Discharge of GHG from the air-water interface of a water body.
Dissolved Oxygen (DO) The oxygen in a water body in its dissolved form. Dissolved oxygen influences
organic matter decomposition processes and serves fish and other aquatic
organisms for respiration.
Ebullition (Bubbling) Discharge in form of bubbles of gaseous substances from a water body,
which results from carbonation, evaporation, or fermentation.
Ecosystem The interactive system formed from all living organisms and their abiotic
(physical and chemical) environment within a given area. In the context of
this note a difference is made between aquatic and terrestrial ecosystems.
Epilimnion The dense top-most layer of water in a thermally stratified water body.
Global Warming Potential An index, based on the radiative properties of GHGs, measuring the radiative
forcing of a unit mass of a given GHG in today’s atmosphere integrated over
a chosen time horizon, relative to that of CO2. The GWP represents the combined
effect of the differing lengths of time that these gases remain in the
atmosphere and their relative effectiveness in absorbing outgoing infrared
radiation. The IPCC considers the GWP of GHG within a 100-year time frame.
Greenhouse gas GHGs are those gaseous constituents of the atmosphere, both natural and
anthropogenic, that absorb and emit radiation at specific wavelengths within
the spectrum of thermal infrared radiation emitted by the Earth’s surface, the
atmosphere itself, and by clouds. In the context of this note, the evaluation
of net emissions from water bodies includes the three GHG species, CO2, CH4
and N2O.
Hypolimnion The dense bottom layer of water in a thermally stratified water body.
Macrophyte Rooted plant that grows in or near water.
Methane (CH4) A naturally occurring GHG, a main component of natural gas, and an end
product of animal husbandry and agriculture.
Methane oxidation Process by which CH4 is oxidized to CO2 and which occurs in aerobic conditions.
Methanogenesis Production of CH4 by anaerobic bacteria and microbes present in the anoxic
layers of a water body, which feed on the detritus of organic matter and
respire CH4.
Nitrification An aerobic process in which bacteria change the ammonia and organic
nitrogen in water and decomposed matter into oxidized nitrogen (nitrate).
Nitrous Oxide (N2O) A naturally occurring GHG produced through bacterial nitrification and denitrification
processes.
Oxic Describes conditions in water and its sediments in which oxygen is present.
Photosynthesis Process driven by solar energy by which atmospheric CO2 is fixed by plants
and algae for the primary production of organic matter and oxygen as a byproduct.
INTERIM TECHNICAL NOTE: GREENHOUSE GASES FROM RESERVOIRS CAUSED B 32 Y BIOCHEMICAL PROCESSES
Residence time Average time a water molecule spends in a reservoir. Used to describe
the flow rate of the water through the reservoir. Value can vary inside one
reservoir.
Respiration Heterotrophic respiration is the process whereby microorganisms grow by
converting organic matter to sugars; autotrophic (or maintenance) respiration
is the process through which plants and animals burn sugars to give
energy. Both reactions produce CO2.
Stratification A water body can be stratified in layers of temperature, salinity, or chemical
compositions, which can act as barriers to water mixing.
Glossar y 33

Bibliography and References
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(continued on next page)
(continued)
INTERIM TECHNICAL NOTE: GREENHOUSE GASES FROM RESERVOIRS CAUSED B 36 Y BIOCHEMICAL PROCESSES
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(continued on next page)
(continued)
Bibliograph y and References  37
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cerrados from a source to a sink of CO2, but with
significant net emissions of methane, and turned
nitrous oxide sinks into sources. The carbon
sinking rate is bigger than the GHG emissions.
2008 Fearnside, P. “A Framework for Estimating
Greenhouse Gas Emissions from Brazil’s
Amazonian Hydroelectric Dams.”
English translation of Fearnside, P. M.
2008. “Hidrelétricas Como ‘Fábricas
de Metano’: O Papel Dos Reservatórios
em Áreas de Floresta Tropical na
Emissão de Gases de Efeito Estufa.”
Oecologia Brasiliensis 12, no. 1 (2008):
100–115. ISSN: 1980-6442. doi: 10.4257/
oeco.2008.1201.11.
A framework for estimating GHG emissions from
hydroelectric dams is proposed based on scaling
up measured values from a small number of
reservoirs in the Amazonian region.
(continued on next page)
(continued)
INTERIM TECHNICAL NOTE: GREENHOUSE GASES FROM RESERVOIRS CAUSED B 38 Y BIOCHEMICAL PROCESSES
Bibliography
Year Paper Description
Guérin, F., G. Abril, A. Tremblay, and
R. Delmas. “Nitrous Oxide Emissions
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Geophysical Research Letters 35 (2008).
doi:10.1029/2007GL033057.
N2O fluxes from two tropical reservoirs (Petit
Saut, French Guiana, and Fortuna, Panama), rivers
downstream, natural aquatic ecosystems, and
rainforest soils were measured. Results showed
that N2O emissions from tropical reservoirs were
mainly at the reservoir surface, fluxes downstream
of dams being minor; net N2O emissions were less
than 50 to 70 percent of gross N2O emissions;
and the contribution of N2O to GWP is less than
10 percent for net emissions, disregarding N2O
degassing emissions.
Lima, I., F. M. Ramos, L. A. W.
Bambace, and R. R. Rosa. 2008.
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as Renewable Energy Resources:
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Mitigation and Adaptation Strategies for
Global Change 13 (2008): 193–206.
Engineering technologies are proposed to avoid
CH4 emissions to the atmosphere through
reservoir surfaces, turbines and spillways, and to
recover the nonemitted CH4 for power generation.
2007 Cole, J. J., Y. T. Prairie, N. F. Caraco, W. H.
McDowell, L. J. Tranvik, R. R. Striegl, C. M.
Duarte, P. Kortelainen, J. A. Downing, J.
Middleburg, and J. M. Melack. “Plumbing
the Global Carbon Cycle: Integrating
Inland Waters into the Terrestrial Carbon
Budget.” Ecosystems 10 (2007): 172–185.
doi:10.1007/s10021-006-9013-8.
A budget for the role of inland water ecosystems
in the global carbon cycle is presented, providing
useful insight about the storage, oxidation, and
transport of terrestrial carbon.
Kemenes, A., B. R. Forsberg, and J.
M. Melack. “Methane Release Below a
Tropical Hydroelectric Dam.” Geophysical
Research Letters 34 (2007): L12809.
doi:10.1029/2007 GL029479.
Emissions of CH4 downstream of Balbina (Brazil)
were calculated from regular measurements
of degassing in the outflow of the turbines and
downstream diffusive losses. The downstream
emission alone represented 3 percent of all
methane released from the central Amazon
floodplain.
2006 Guérin, F., G. Abril, S. Richard, B. Burban,
C. Reynouard, P. Seyler, and R. Delmas.
“Methane and Carbon Dioxide Emissions
from Tropical Reservoirs: Significance
of Downstream Rivers.” Geophysical
Research Letters 33 (2006): L21407.
doi:10.1029/2006GL027929.
CH4 and CO2 were measured in three tropical
reservoirs and downstream of the dams, in
French Guiana (Petit Saut) and Brazil (Balbina
and Samuel). Significant downstream emissions
were observed, and CH4 degassing appeared to be
significant, although it could not be quantified.
Santos, M. A., L. P. Rosa, B. Matvienko,
E. Sikar, and E. D. Santos. “Gross
Greenhouse Gas Emissions from
Hydro-Power Reservoir Compared to
Thermo-Power Plants.” Energy Policy 34
(2006): 481–488.
Data from measurements at Miranda, Barra
Bonita, Segredo, Tres Marias, Xingó, Samuel,
Tucuruí, Itaipu, and Serra da Mesa used in
comparisons between emissions from hydropower
plants and their thermo-based equivalents. The
estimated values for hydropower plants posted
lower emissions than their equivalent thermobased
counterparts.
(continued on next page)
(continued)
Bibliograph y and References  39
Bibliography
Year Paper Description
2005 Abril, G., F. Guérin, S. Richard, R. Delmas,
C. Galy-Lacaux, P. Gosse, A. Tremblay,
L. Varfalvy, M. A. Santos, and B.
Matvienko. “Carbon Dioxide and Methane
Emissions and the Carbon Budget
of a 10-Year-Old Tropical Reservoir
(Petit-Saut, French Guiana).” Global
Biogeochemical Cycles 19 (2005): GB
4007. doi:10.1029/2005GB002457.
CO2 and CH4 emissions during the first 10 years
of a special tropical reservoir were analyzed.
Emissions reduce with age, degassing is a major
CH4 pathway, with terrestrial carbon being the
main source.
Delmas, R., S. Richard, F. Guérin, G.
Abril, C. Galy-Lacaux, and A. Grégoire.
“Long-Term Greenhouse Gas Emissions
from the Hydroelectric Reservoir of
Petit Saut (French Guiana) and Potential
Impacts.” In Tremblay, A., L. Varfalvy, C.
Roehm and M. Garneau, eds. Greenhouse
Gas Emissions: Fluxes and Processes,
Hydroelectric Reservoirs and Natural
Environments. Environmental Science
Series, 293–312. Berlin: Springer-Verlag,
2005.
Net GHG emissions for a 100-year period
are estimated and compared with fossil fuel
alternatives, finding values similar to emissions for
a coal power plant after 25 years, 35 years for oil,
and 57 years for gas plants.
Fearnside, P. M. “Brazil’s Samuel Dam:
Lessons for Hydroelectric Development
Policy and the Environment in Amazonia.”
Environmental Management 35 (2005):
1–19.
Impacts of Brazil’s Samuel Dam are estimated,
including environmental costs, GHG emissions,
social costs and mitigation measures.
Fearnside, P. M. “Do Hydroelectric Dams
Mitigate Global Warming? The Case of
Brazil’s Curuá-Una Dam.” Mitigation and
Adaptation Strategies for Global Change
10 (2005): 675–691.
GHG emissions at Curuá-Una are assessed for the
year 1990, and estimated to be 3.6 times greater
than an equivalent oil power plant.
Tremblay, A., L. Varfalvy, C. Roehm,
and M. Garneau, eds. Greenhouse Gas
Emissions—Fluxes and Processes,
Hydroelectric Reservoirs and Natural
Environments. Berlin: Springer-Verlag,
2005. ISBN 3-540-23455-1.
Comprehensive state-of-the art on GHG in 2005.
2002 Duchemin, E., M. Lucotte, V. St. Louis,
and R. Canuel. 2002. “Hydroelectric
Reservoirs as an Anthropogenic Source
of Greenhouse Gases.” World Resource
Review 14, no. 3 (2002): 334–353.
Synthesis of CO2, CH4, and N2O emissions from
reservoirs in boreal, temperate and tropical
regions are measured by different research teams.
Diffusive fluxes from tropical reservoirs are found
to be much higher than those from northern
reservoirs.
Fearnside, P. M. “Greenhouse Gas
Emissions from a Hydroelectric Reservoir
(Brazil’s Tucuruí Dam) and the Energy
Policy Implication.” Water Air Soil Pollution
133 (2002): 69–96.
Tucurui’s GHG emissions (from Fearnside’s
previous studies) are adapted to assess the
impacts of hydro compared to fossil fuels.
Although many dams in Amazonia are expected to
have positive balance as compared to fossil fuels,
substantial emissions indicated by the present
study reduce the benefits often attributed to
hydro.
(continued on next page)
(continued)
INTERIM TECHNICAL NOTE: GREENHOUSE GASES FROM RESERVOIRS CAUSED B 40 Y BIOCHEMICAL PROCESSES
Bibliography
Year Paper Description
Rosa, L. P., B. Matvienko, M. A. Santos,
and E. Sikar. First Brazilian Inventory
of Anthropogenic Greenhouse Gas
Emissions. Background Reports: Carbon
Dioxide and Methane Emissions from
Brazilian Hydroelectric Reservoirs, Annex
B. Brasilia: Ministry of Science and
Technology, 2002.
CO2 and methane emissions from Brazilian
hydroelectric reservoirs are estimated for the
Brazilian Inventory of Anthropogenic Greenhouse
Gas Emissions.
2001 Delmas, R., C. Galy-Lacaux, and S.
Richard. “Emissions of Greenhouse
Gases from the Tropical Hydroelectric
Reservoir of Petit Saut (French Guiana)
Compared with Emissions from Thermal
Alternatives.” Global Biogeochemical
Cycles 15 (2001): 993–1003.
Net GHG emissions for a 100-year period
are estimated and compared with thermal
alternatives, finding values similar to emissions for
a gas power plant producing the same energy.
2000 St. Louis, V. L., C. A. Kelly, E. Duchemin,
J. W. M. Rudd, and D. M. Rosenberg.
“Reservoir Surfaces as Sources of
Greenhouse Gases to the Atmosphere:
A Global Estimate.” Bioscience 50, no. 9
(2000): 766–775.
Gives an overview of GHG from reservoirs based
on knowledge in 2001 and makes a first estimate of
global emissions from reservoirs.
1999 Galy-Lacaux, C., R. Delmas, G. Kouadio,
S. Richard, and P. Gosse. “Long-Term
Greenhouse Gas Emission from a
Hydroelectric Reservoir in Tropical Forest
Regions.” Global Biogeochemical Cycles
13 (1999): 503–517.
Estimates of CO2 and CH4 emissions over a
20-year period are made, based on 3.5 years
of data from Petit Saut and an experimental
campaign on four older reservoirs in the Ivory
Coast.
1997 Fearnside, P. M. “Greenhouse-Gas
Emissions from Amazonian Hydroelectric
Reservoirs: The Example of Brazil’s
Tucuruí Dam as Compared to Fossil Fuel
Alternatives.” Environmental Conservation
24, no. 1 (1997): 64–75.
Emissions from Tucurui dam are calculated for 100
years. At low annual discount rates (1–2 percent)
Tucurui is still 3–4 times better than fossil fuel
generation; at discount rates of 15 percent, fossil
fuel generation becomes more attractive from a
global warming perspective.
Galy-Lacaux, C., R. Delmas, C. Jambert,
J. F. Dumestre, L. Labroue, S. Richard,
and P. Gosse. “Gaseous Emissions and
Oxygen Consumption in Hydroelectric
Dams: A Case Study in French Guiana.”
Global Biogeochemical Cycles 11, no. 4
(1997): 471–483.
Results from two years of measurement of CO2,
CH4 and H2S from Petit Saut reservoir.
1995 Duchemin, E., M. Lucotte, R. Camuel,
and A. Chamberland, A. “Production of
the Greenhouse Gases CH4 and CO2 by
Hydroelectric Reservoirs in the Boreal
Region.” Global Biogeochemical Cycles 9,
no. 4 (1995): 529–540.
CH4 and CO2 in two reservoirs of northern Quebec.
Fearnside, P. M. “Hydroelectric Dams
in the Brazilian Amazon as Sources of
‘Greenhouse’ Gases.” Environmental
Conservation 22 (1995): 7–19.
GHG emissions from Amazonian dams are
calculated for the year of 1990. Global warming
impact of Amazonian Hydro is much higher than
fossil-fueled power plants.
(continued)
Bibliograph y and References  41
IPCC References
Reports available at http://www.ipcc.ch
Intergovernmental Panel on Climate
Change (IPCC). 2000. IPCC Good
Practice Guidance and Uncertainty
Management in National Greenhouse
Gas Inventories. Edited by J. Penman,
D. Kruger, I. Galbally, T. Hiraishi, B.
Nyenzi, S. Emmanul, L. Buendia, R.
Hoppaus, T. Martinsen, J. Meijer, K.
Miwa and K. Tanabe. Published for
the IPCC by the Institute for Global
Environmental Strategies, Japan. ISBN
4-88788-000-6
These documents describe recommended procedures for
estimating GHG emissions for National GHG Inventories.
Reservoirs are contemplated under the general category
“wetlands” (defined as “land that is covered or saturated
by water for all or part of the year and that does not fall into
the forest, cropland, grassland or settlements categories”),
and more specifically, under “Land converted to flooded
land.” The “managed land proxy”: Emissions and removals
from “managed land” are used as a proxy for estimating
anthropogenic emissions and removals.
The scope of the IPCC (2006) assessment includes guidance
on estimating and reporting CO2 emissions, applying a
carbon stock change method, and assuming that all the
carbon in biomass that existed prior to flooding is emitted.
An alternative approach assumes that CO2 emissions can
be estimated using default emission factors and aggregated
area data.
Intergovernmental Panel on Climate
Change (IPCC). 2003. Good Practice
Guidance for Land Use, Land Use
Change and Forestry. Edited by J.
Punman, M. Geytarsky, T. Hiraishi, T.
Krug, D. Kruger, R. Pipatti, L. Buendia,
K. Miwa, T. Ngara, K. Tanabe, and F.
Wagner. Published by the Institute for
Global Environmental Strategies (IGES)
for the IPCC. ISBN 4-88788-003-0
Intergovernmental Panel on Climate
Change (IPCC). 2006. 2006 IPCC
Guidelines for National Greenhouse Gas
Inventories. Edited by H. S. Eggleston,
L. Buendia, K. Miwa, T. Ngara, and K.
Tanabe. Published by the Institute for
Global Environmental Strategies (IGES)
for the IPCC. ISBN 4-88788-032-4
Intergovernmental Panel on Climate
Change (IPCC). 2011. Special Report on
Renewable Energy Sources and Climate
Change Mitigation (SRREN). Edited
by O. Edenhofer, R. Pichs-Madruga, Y.
Sokona, K. Seyboth, P. Matschoss, S.
Kadner, T. Zwickel, P. Eickemeier, G.
Hansen, S. Schloemer, C. von Stechow.
Cambridge: Cambridge University
Press.
The SRREN provides an assessment and thorough analysis
of renewable energy technologies and their current and
potential role in the mitigation of GHG emissions.
• Important chapters:
• Hydropower
• Summary for Policy Makers
• Technical Summary
• Renewable Energy in the Context of Sustainable
Development
INTERIM TECHNICAL NOTE: GREENHOUSE GASES FROM RESERVOIRS CAUSED B 42 Y BIOCHEMICAL PROCESSES
Additional References
Campo, J., and L. Sancholuz. 1998. “Biogeochemical Impacts of Submerging Forests through Large
Dams in the Río Negro, Uruguay.” Journal of Environmental Management 54: 59–66.
Cullenward, D., and D. Victor. 2006. “The Dam Debate and Its Discontents.” Climatic Change 75 (1–2):
81–86.
Downing, J.A., Y. T. Prairie, J. J. Cole, C. M. Duarte, L. J. Tranvik, R. G. Striegl, W. H. McDowell, P.
Kortelainen, N. F. Caraco, J. M. Melack, and J. J. Middelburg. 2006. “The Global Abundance and Size
Distribution of Lakes, Ponds, and Impoundments.” Limnology and Oceanography 51 (5): 2388–2397.
Intergovernmental Panel on Climate Change (IPCC). 2007. Climate Change 2007: Synthesis Report.
Contribution of Working Groups I, II and III to the Fourth Assessment Report of the Intergovernmental
Panel on Climate Change. Edited by Core Writing Team, R. K. Pachauri, and A. Reisinger. Geneva: IPCC.
Lehner, B., C. Reidy Liermann, C. Revenga, C. Vörösmarty, B. Fekete, P. Crouzet, P. Döll, M. Endejan,
K. Frenken, J. Magome, C. Nilsson, J. C. Robertson, R. Rödel, N. Sindorf, and D. Wisser. 2011. “High-
Resolution Mapping of the World’s Reservoirs and Dams for Sustainable River-Flow Management.”
Frontiers in Ecology and the Environment 9 (9): 494–502.
Bibliograph y and References  43

Annex 1
Conversion from moles to grams
In chemistry, a mole is considered to be Avogadro’s number (6.02 × 1023) of molecules (or anything) of
a substance. So, depending on the density of the substance, the mass of that amount of the substance
could vary widely. To convert from moles to grams, you must first find the molar mass of the element
or compound. Use the periodic table to read off the atomic mass from an element. If it is a compound,
you must know the molecular formula, and then you find the total molar mass of the compound by
adding up the atomic masses of each atom in the compound. The unit of the molar mass will be in
grams per moles (g/mole). Once you have the molar mass, you can easily convert from grams to
moles, and also from moles to grams.
Number of moles = (# of grams) ÷ (molar mass)
Number of grams = (# of moles) × (molar mass)
For the most common GHGs in reservoirs:
Element Atomic mass (g/mole) GHG Molar mass (g/mole)
N 14.0067 CO2 44.0095
C 12.0107 CH4 16.0107
O 15.9994 N2O 44.0128
H 1.00794
CO2 equivalents (CO2eq or CO2e)
The international practice is to express GHG in CO2 equivalents (CO2eq or CO2e). Emissions of gases
other than CO2 are translated into CO2eq by multiplying by the respective GWP. From the 2007 IPCC
report:
Annex 1 – Conversions of Units and
CO2 Equivalents
Conversion from “g of GHG” to
“g of Carbon”
The conversion between “g of GHG” and “g of
carbon” is directly related to the ratio of the
atomic mass of a GHG molecule to the atomic
mass of a carbon atom. Essentially, this practice
accounts for the carbon in the GHG molecule, as
opposed to counting the entire molecule.
For carbon dioxide, the ratio of the atomic
mass of a CO2 molecule to the atomic mass of a
carbon atom is 44:12.
?? To convert from “g of C” to “g of CO2”,
multiply by 44/12
?? To convert from “g of CO2” to “g of C”,
multiply by 12/44
?? Sometimes you find this noted as
gC-CO2 or tC-CO2 (to make clear that
these “g of C” refer to carbon in a
CO2 molecule).
For methane, the ratio of the atomic mass of
a CH4 molecule to the atomic mass of a carbon
atom is 16:12.
?? To convert from g of C to g of CH4, multiply
by 16/12
?? To convert from g of CH4 to g of C, multiply
by 12/16
?? It is important to make clear that these
g of C refer to carbon in a CH4 molecule
(i.e., NOT CO2eq—not taking into account
GWP). It is common to use gC-CH4 or
tC-CH4
Carbon dioxide equivalents vs.
carbon equivalents
Although the international standard is to express
emissions in CO2 equivalents (CO2eq), many
US sources have expressed emissions data in
terms of carbon equivalents (CE) in the past.
In particular, the US Environmental Protection
Agency has used the carbon equivalent metric in
the past for budget documents.
For the purposes of national greenhouse
gas inventories, emissions are expressed as
teragrams of CO2 equivalent (Tg CO2eq). One
teragram is equal to 1012 grams, or one million
metric tons.
?? To convert from CE to CO2eq, multiply
by 44/12
?? To convert from CO2eq to CE, multiply
by 12/44.
Gas name
Chemical
formula
GWP for given time horizon
20 years 100 years 500 years
Carbon dioxide CO2 1 1 1
Methane CH4 72 25 7.6
Nitrous oxide N2O 289 298 153
Source: 2007 IPCC Fourth Assessment Report (AR4).
GWP relative to CO2 at different time horizon for the most common GHG species in reservoirs:
46 INTERIM TECHNICAL NOTE: GREENHOUSE GASES FROM RESERVOIRS CAUSED BY BIOCHEMICAL PROCESSES
Annex 2
Annex 2 – Illustration of Main Processes
and Parameters
Parameters important for creating GHG stock
Parameters that modulate the rates of biological processes such as organic matter production, respiration,
methanogenesis and CH4 oxidation:
?? concentrations of dissolved oxygen;
?? water temperature;
?? organic matter storage, concentrations and C/N, C/P and N/P ratios in water and in sediments;
?? supply of nutrients;
?? light (absence of turbidity);
?? biomass of plants, algae, bacteria and animals in the reservoir and in drawdown zone;
?? sediment load; and
?? stratification of the reservoir body.
Figure 2.3: Carbon dioxide and methane pathways in a freshwater reservoir with an anoxic hypolimnion.
For reservoirs with a well-oxygenated water column, methane emissions through pathways (2), (4) and (5)
are reduced (adapted from UNESCO/IHA, 2008)
Source: UNESCO/IHA Greenhouse Gas Emissions from Freshwater Reservoirs Research Project, GHG Measurement
Guidelines for Freshwater Reservoirs (2010).
Parameters important for
releasing GHG stock
Parameters that modulate gas exchange
between the atmosphere and the reservoir or
downstream river:
?? wind speed and direction;
?? reservoir shape;
?? rainfall;
?? water current speeds;
?? water temperatures;
?? water depth and changes in water depth;
?? reductions in hydrostatic pressure as
water is released through low level outlets;
?? increased turbulence downstream of the
dam associated with ancillary structures,
e.g., spillways and weirs.
48 INTERIM TECHNICAL NOTE: GREENHOUSE GASES FROM RESERVOIRS CAUSED BY BIOCHEMICAL PROCESSES
Annex 3 Below are the detailed steps to apply the Tier 1 methodology to estimate GHG emissions from reservoirs
(see Section 5.2).
Flooded material
1. Identify the reservoir’s exact location (coordinates). Define where it is located—in a tropical,
temperate, or boreal zone. Best source would be feasibility study or similar.
2. Identify the size and exact location of the reservoir area at Full Supply Level (FSL). Best source
would be feasibility study or similar.
3. Identify type of forests/vegetation in the area of the reservoir. Best source would be EIA study
or similar. If not available use local, regional, or global ecological maps.
4. Identify the land use of the reservoir area at FSL. Note specifically percentage of forests,
nonvegetated land (agriculture or grasslands), and water bodies. Best source would be EIA
study or similar. If not available in reports, use easily available thematic maps, air photos,
or satellite images. If no information is available at all assume 90 percent forested areas,
10 percent water bodies for dry ecological zones, and 80 percent forested areas, 20 percent
water bodies for wet ecological zones.
5. Identify the type of soils in the area of the reservoir. Best source would be EIA study or similar.
If not available, use local or regional soil maps.
6. Use AFOLU Table 4.8 (or 4.12) to estimate above-ground biomass for identified type of forests.
Calculate total biomass for reservoir area by applying percentage of forested/vegetated area.
7. Use AFOLU Table 4.3 to calculate the fraction of carbon in above-ground biomass. Calculate
total above-ground biomass carbon for the reservoir area.
8. Use AFOLU Table 2.2 to estimate amount of carbon in litter for the reservoir area. Calculate
total litter carbon in reservoir area by applying percentage of forested/vegetated area.
9. Use AFOLU Table 2.3 to estimate amount of carbon in soil for the reservoir area. Calculate
total soil carbon in reservoir area by applying total reservoir area.
10. Add the total amount of carbon in the reservoir area from biomass, litter, and soil and apply
the assumption of 50 percent available for decomposition. This is the total amount of carbon
Detailed Steps for
Tier 1 Estimation of
GHG from Reservoirs
stock to be emitted from the reservoir
during its life span.
11. Depending on latitude of reservoir,
estimate how much of the total amount
of available carbon stock will be emitted
as CO2 and how much will be emitted
as CH4. For tropical use CH4/CO2 =
7 percent, and for temperate and boreal
use CH4/CO2 = 1.5 percent
12. Convert the total amount of available
carbon stock that will be emitted as
CO2 by multiplying by 44/12 (see
Annex 1).
13. Convert the total amount of available
carbon stock that will be emitted
as CH4 by multiplying by 16/12 (see
Annex 1).
14. Calculate the total emitted CO2 equivalents
by multiplying the amount of
CH4 by 25 (see Annex 1) and add to the
total emitted CO2.
15. Divide the total amount of emitted
CO2 equivalents by 100 years to get an
average annual rate of emission caused
by flooded biomass and soil carbon.
Removal of sink
16. Use AFOLU Table 4.9 (or 4.12) to estimate
above-ground biomass growth
per km2 for identified type of forests.
Calculate total biomass growth for
reservoir area by applying percentage
of forested/vegetated area.
17. Apply fraction of carbon (AFOLU
Table 4.3) to calculate annual sink of
carbon for terrestrial areas. Convert to
CO2 by applying a factor of 44/12.
18. Apply a standard emission from natural
waters (463 t CO2/km2 and year [Barros
et al. 2011]) for the existing area of water
bodies to estimate the total natural
emissions from the reservoir area.
19. Calculate the net sink per year for the
reservoir by subtracting the natural
emissions per year (water bodies) from
the natural sink (terrestrial forests).
Net emissions14
Add the annual emission caused by flooded
biomass and soil carbon in CO2 equivalents
to the annual net sink to get the total net GHG
emissions
14 Note that net emissions here refer only to biochemical
processes and do not include emissions from dam
construction or baseline emissions generated by alternatives
built if the reservoir were not constructed, see
Section 2.2.
INTERIM TECHNICAL NOTE: GREENHOUSE GASES FROM RESERVOIRS CAUSED B 50 Y BIOCHEMICAL PROCESSES

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