RESERVOIRS AND CLIMATE CHANGE
November 2008
Authors:Antoni Palau1 and Miguel Alonso2
1 Environment and Sustainable Development Department ENDESA.2 URS (United Research Services España, S.L.)
CONTENTS
FOREWORD 5
I. INTRODUCTION
1. GENERAL CLIMATE FEATURES 8
2. CONTEMPORARY CLIMATE CHANGE 10
3. THE GREENHOUSE GASES N2O, CO2 AND CH4 11
4. GLOBAL WARMING AND GLOBAL DIMMING 13
II. HYDROELECTRICITY IN THE GLOBAL ENERGY BALANCE
1. INTRODUCTION 15
2. ENERGY PRODUCTION AND CONSUMPTION IN SPAIN 15
3. THE ROLE OF HYDROELECTRICITY IN MEETING THE DEMAND FOR ENERGY 16
4. HYDROELECTRICITY AND OTHER ENERGY SOURCES IN TERMS OF CO2 BALANCE 18
III. AQUATIC ECOSYSTEMS AND GREENHOUSE GASES
1. CARBON 21
2. NITROGEN 22
3. RESERVOIRS AND GREENHOUSE GASES 22
IV. A CASE STUDY: THE SUSQUEDA RESERVOIR (TER RIVER, GIRONA)
1. INTRODUCTION 32
2. STUDY APPROACH 33
3. FIELD METHODS 34
4. ANALYTICAL METHODS 36
5. RESULTS 36
6. FINAL BALANCE 41
7. DISCUSSION OF RESULTS 42
V. SOME IMPORTANT FINAL QUESTIONS 44
BIBLIOGRAPHY 47
FOREWORD
The purpose of this publication is to analyze the role of reservoirs in the dynamics
of greenhouse gases in order to highlight how they are involved in climate change. The
analysis is carried out by describing their metabolism as aquatic systems. We illustrated
this description by a case study of the Susqueda Reservoir (Ter River), which focuses on
its carbon balance.
Previously, we review the situation of climate change, starting with the discussion of the
concept of climate and its natural variability. We provide a brief account of the factors and
processes that have contributed, and still contribute, to the global and regional evolution of
the Earth’s climate on both a geological and contemporary time scale.
In one of the chapters, we address the role of hydroelectricity in the overall energy balance.
We analyze the relationship between available energy, the production of regulable energy
by reservoirs, and the energy usage and demand by Spanish today`s society. We also
make a brief comparison of hydroelectric energy with other electricity-generating technolo-
gies in relation to climate change.
1. GENERAL CLIMATE FEATURES
Climate is perhaps the most important factor in biological evolution
and in the history of mankind. It has always been a decisive factor
for the survival and economy of the people. We know that climate is
related to sunlight, air temperature, winds, atmospheric humidity and
rainfall. However, to avoid confusion about the meaning of “climate
change”, we should perhaps define the term ‘climate’, as it is often
used incorrectly to refer to “weather” or “atmospheric conditions”. A
very precise and accurate version is given by Inocencio Font Tullot1,
one of the most prestigious Spanish meteorologists: “Climate is the
synthesis of the fluctuating set of weather conditions in a given area
recorded during a long period enough to be geographically represent-
ative”. This definition includes key concepts in the current perception
of climate change by society:
• The parameters used to define climate have large variations.
• Climate is spatially dependent.
• Very long periods of time are necessary to guarantee that a climate
analysis is representative or significant.
The identification of regular features of climate in different areas of
the earth’s surface has led to classifications based on combinations
of elements that have been expressed in geographical terms. These
classifications are mainly based on atmospheric and marine circula-
tion, latitude and altitude. In other words, we have climatic maps. How-
ever, these maps show major inaccuracies at the boundaries between
climate zones and only help to understand climate-dependent proc-
esses, such as the organization of vegetation, at a particular spatial
and temporal scale. In the short term, the farmer still relies on sky
observation.
In addition to the great difficulty of accurately predicting climate to
be able to organize our economy suitably, it also changes over time.
Throughout the history of the world there have occurred very great
variations in climate associated with continental drift and its effects
on the configuration and circulation of the hydrosphere. The lands
that today make up the Iberian Peninsula travelled over the South
Pole about 500 million years ago and over the equator about 300
million years ago. In the Cenozoic Era, the Earth’s geography was
established more or less as we know it today. This was a period of
cooling, when the formation of the Isthmus of Panama led to the def-
inition of the polar ice caps and the onset of glaciations—a climatic
phenomenon that had only been previously recorded in the Permian.
During the Pleistocene there have been four glacial periods, when
ice covered a third of the continents and turned temperate climates
into Arctic ones. The last glacial period ended only about 10,000
years ago. However, the traditional concept of the “four glacials”
conceals a more complex situation. In fact, during these major
glacials thirty or more intermediate pulses of glacial advance and
retreat have been identified, each of them lasting about 43,000
years. Furthermore, within these intermediate pulses there were
smaller ones lasting between 19,000 and 24,000 years. Super-
imposed on these pulses, even smaller changes in climate are re-
corded, like those occurred between the 10th and 14th centuries,
known as the Medieval Warm Period. This period was warmer than
the present, but it was followed in the 16th and 17th centuries by
the Little Ice Age, which despite its name was of great importance
in History (Figure 1).
Because of the periodicity of the Pleistocene glaciations, many
studies have attempted to situate the present time within another
interglacial cycle. We wish to know whether we are going towards
a fifth glacial period, and if so whether we are still in the warm-
ing phase or moving towards the cooling phase. Several soundly-
based scientific theories have been developed to explain the past,
but their predictive accuracy is not demonstrable. According to
the series of Milankovitch cycles, we are at the end of the current
interglacial warm period, so the Earth should start cooling. The Mi-
lankovitch theory states that the Earth’s orbit varies from almost
circular to clearly elliptical every 90,000 or 100,000 years, so the
intensity of sunlight received by Earth varies. Superimposed on this
variation are those of precession and the changes in the tilt of the
Earth’s rotational axis, which have a cycle of 43,000 years: when
the tilt is greater, the magnitude of the seasonal change from win-
ter to summer is accentuated. This interval fits with the duration
of the intermediate pulses of glacier advance and retreat, but the
timing of the fluctuations is irregular, so other factors must be in-
volved. W.M. Ewing2 proposed a mechanistic model of cyclical phe-
nomena based on the relationships between the atmosphere and
INTRODUCTION8
1 Inocencio Font Tullot, a highly active and prolific Spanish meteorologist who started his career at the Izaña Observatory, have managed several relevant international meteorology projects. In 1974 he was appointed Director of the National Meteorology Service. He wrote Climatología de España y Portugal (Climatology of Spain and Portugal, 1983) and Historia del Clima en España (A History of Climate in Spain, 1988).
2 William Maurice Ewing, an American geologist and professor of Columbia University, who in the 1950s proposed a theory on the possible causes of glaciations, and the hypothesis that the glacial periods were only a recent stage in the geological and climatic history of the Earth.
the hydrosphere. In an interglacial period with an ice-free Arctic
Ocean, the temperature would be low though slightly higher than
at the present time. However, evaporation would be large, provok-
ing heavy snowfall that would accumulate into continental glaciers
which increases the albedo of Earth’s surface. The drop in sea level
associated with this process would isolate the polar basin of the
Atlantic, which would lower water temperature, keep ice in solid
state and reduce precipitation. This would make the glaciers re-
treat again, and climate would return to interglacial conditions.
On a different scale of time and importance, other factors such as
solar activity and volcanoes also act on climate. Solar activity may
have increased in the last 200-300 years, in turn causing an in-
crease in the thermal radiation that reaches the Earth. Sunspots,
which are colder parts of the Sun’s surface linked to greater mag-
INTRODUCTION9
Figure 1. Evolution of atmospheric temperature across different temporal scales. The increase in human population, coinciding to the temperature increase during the last two centuries, is also depicted.
INTRODUCTION10
netic activity, increase and decrease with a periodicity of about
11 years, with primary cycles of 9 to 14 years and secondary cycles
of about 80 years. However, the reduction in the energy emitted
by sunspots is compensated by the intensification of the surround-
ing bright areas, or faculae, so the larger the area occupied by
sunspots, the more intense the solar activity is. In January 2008
the European/US SOHO satellite detected the appearance of a
new sunspot. This phenomenon has been interpreted as indicating
the start of a new solar cycle that will cause a gradual increase in
sunspots and solar flares, reaching a peak of activity between 2011
and 2013.
The relationship between sunspots and climate is subject to
debate. Some scientists attribute climate change to cycles of
solar activity rather than to the concentration of CO2 in the at-
mosphere. Without going into whether or not there is a causal
relationship, the fact is that total solar radiation has increased
in recent centuries (Figure 2). The Maunder Minimum—the name
given to a period of almost 50 years with hardly any sunspots
(1645-1715)—coincided with the coldest time of what is known
as the Little Ice Age, with very cold winters at least in the north-
ern hemisphere. The particles expelled by volcanoes can reach
the stratosphere in sufficient amounts to intercept the radiant
energy and cause cooling.
slowly, leading to heat increase in the air inside. The atmosphere
can be compared to this system: the gases usually found in it,, such
as water vapor, carbon dioxide (CO2), methane (CH4) and ozone—
together with pollutants such as nitrous oxide (N2O) and CFCs—trap
the infrared radiation that is radiated back into space by the hydro-
sphere and the lithosphere when heated by solar radiation. In the
absence of these gases and the predominant nitrogen and oxygen,
Earth would not be protected from violent thermal changes and
from extreme cold, and life as we know would be impossible.
But what are the effects of climate change in the past and in the fu-
ture? They are decisive in the history of the biosphere and particular-
ly in the history of humanity. The climate has governed the complex
and dynamic puzzle of the evolution of species and their distribution
on the planet. Many large extinctions can be traced to some sudden
climatic changes, even as far back as the end of the Mesozoic, when
a sudden cooling of the climate caused by the impact of a huge me-
teorite wiped out the dinosaurs and many plants but was favourable
to mammals and seed-bearing plants. The Pleistocene glaciations
had a particularly devastating effect on the continents with east-west
mountain ranges, which prevented the movement of populations to
the south as the cold advanced. The sea could have fallen down to
120 m below the current level. Global warming in the Boreal period
was decisive for the birth of the Neolithic Age, particularly in Spain,
where rainfall decrease decimated the herds of wild ruminants, so
humans had to adapt by enhancing their ability to intervene in the
environment. Later, great civilizations such as those in North Africa,
the Indus Valley and Mesopotamia rose and fell under the influence
of climate changes. The Little Ice Age in the 16th and 17th centuries
ruined the wool trade and the textile industry in both Castiles and
fostered the resurgence of Catalonia. After the experience of the
last few decades, we are very aware of the effect of cycles of wet and
dry years and cool and warm periods on the Spanish economy and
on the health and welfare of the population.
2. CONTEMPORARY CLIMATE CHANGE
Though humanity is aware of the natural tendency of climate changes,
the current concern is whether economic activities will make these
changes so sudden that could catch us unawares and without sufficient
time and means to adapt. And within climate change, the most tangi-
ble aspect is global warming. This phenomenon presumably arises as
a direct consequence of economic development based on combus-
Figure 2. Evolution of solar radiation intercepted by Earth, which have in-creased in the last centuries (data from Lean, 2004).
Greenhouse gases, which are the main subject of this publication,
are another factor that affects climate. They are named after the
glasshouses that maintain a warm atmosphere inside them using
only sunlight. The short-wave electromagnetic radiation from the
sun enters the greenhouse through the glass or plastic surfaces,
but the longewave infrared radiation leaves the structure more
INTRODUCTION11
tion of coal, oil and natural gas, which has been growing continuously
since the early twentieth century. The main substance released in the
combustion reaction of these products is carbon dioxide (CO2), one
of greenhouse gases. Based on isotope discrimination techniques,
it has been concluded that this combustion has led to the increase
in atmospheric CO2; the atmospheric gas is lacking in C14, which is
lower in fossil fuels, due to its reduced half-life of between 5,500 and
6,000 years. Furthermore, the measuring station of atmospheric
CO2, which has been collecting data at Mauna Loa (on the Hawaiian
Tropic of Cancer) since 1958, shows a steady increase up to the
present that runs parallel with the global consumption of fossil fuels.
Agricultural activities such as deforestation and draining of wetlands
also release CO2. Overall, it is considered that the CO2 produced
through all the above processes is twice the retained in the atmos-
phere. The carbon sink of the other half could be the ocean and per-
haps the vegetation. However, although in the long term this might
be possible, the speed at which the CO2 is being produced is clearly
leading to its accumulation in the atmosphere. CH4 is also showing
considerable increases (around 1% per year) attributed to increased
livestock production and creation of new anaerobic aquatic environ-
ments such as rice fields.
Between 1856 and 2000 the global temperature increased be-
tween 0.4 and 0.8ºC. However, since 1975 the correlation be-
tween the increases in temperature and CO2 has been greater. The
Intergovernmental Panel on Climate Change (IPCC) has undertaken
the task of modelling the future evolution of temperature in differ-
ent scenarios depending on whether society comes out in favour
of the economy or the environment, and in each case whether the
actions are taken at a global or regional level. Estimates of the cli-
matic consequences of the different scenarios show increases in
temperature of 2ºC to 4.5ºC and sea level rise between 31 and
49 cm by 2100.
Without questioning the hard data of increased atmospheric CO2 and
rising temperature, some experts do question the predictive value
of the IPCC’s models. Their doubts are based mainly on whether we
know the true effect of CO2 on temperature, whether there may be
other factors involved in the increase in temperature that have not
been considered, and whether we know the true qualitative and quan-
titative consequences of the increase. These questions are very dif-
ficult to consider scientifically because of the enormous complexity of
the physical, chemical and mechanical processes taking place in the
Earth’s fluid envelopes. Though the temperature and the evolution
of human population follow similar temporal patterns (see Figure 1),
the relationships between the physical and biological world are not
always causal or easy to establish.
There is an amusing classical example to criticize the use of correla-
tions to establish simplistic cause-effect relationships. A study con-
ducted between 1970 and 1980 could be used to attribute the fall-
ing birth-rate in Poland to the reduction in stork populations, whereas
in reality the processes involved in this coincidence were of far great-
er socioeconomic and environmental importance: the industrializa-
tion of the country resulted on the one hand in the incorporation of
women into employment, and on the other hand in the degradation
of the habitat of storks.
3. THE GREENHOUSE GASES N2O, CO2 AND CH4
Nitrous oxide (N2O) is present in the atmosphere in a concentration
of about 300 ppm and has been increasing recently by 2% per year.
This gas makes little contribution to the greenhouse effect due to its
Figure 3. Sustained increase in atmospheric CO2 concentration over the last fifty years. Annual fluctuation due to the seasonality of photosynthetic activity in the northern hemisphere can be clearly observed.
very slight concentration in the atmosphere, though it has a ther-
mal potential 310 times greater than that of CO2. Nitrous oxide is
produced as an intermediate in the denitrification process that oc-
curs in anaerobic environments with sufficient availability of oxidizable
organic matter and nitrate, and also as a result of burning biomass
and fossil fuels. The two main nitrous oxide sinks are the fixation by
nitrifying organisms, which can use nitrous oxide as an alternative to
nitrogen, and the reaction with atomic oxygen in the stratosphere,
which is the most important mechanism of removal of this gas from
the atmosphere.
Carbon dioxide (CO2) and methane (CH4) are the most stable car-
bon gases. CO2 is the most abundant of these gases, but currently
only represents 0.038% of the atmosphere. CH4 is only considered
a trace gas but has a greenhouse effect 20 times greater than CO2.
Atmospheric CO2 is in equilibrium with the bicarbonate of the hydro-
sphere, which is the most important carbon store on the planet. The
direction of the shift from one to another system is governed by the
CO2 pressure in the atmosphere and the bicarbonate concentration
in the water. However, apart from these purely physical aspects, the
biogeochemical carbon cycle includes complex redox processes in-
volving living organisms. Both bicarbonate and CO2 are reduced to
organic molecules by photosynthetic organisms using solar energy,
which is the way carbon enters the biosphere. Subsequently, the or-
ganic molecules are oxidized by oxygen or other terminal electron
acceptors. The return to the initial oxidized form, i.e. CO2, may follow
long and tortuous routes depending on the complexity of the ecosys-
tems and the involvement of other mechanisms, which may include
geological processes. Some of the CO2 is fixed by plants and rapidly
returned to the atmosphere or to the water through respiration or
decomposition of organic matter in the ecosystems; some was fixed
in ancient times and is stored as wood, coal, oil, natural gas or even
limestone. Consequently, when referring to CO2 emissions we should
differentiate between those from fossil origin and those emitted in
the current carbon cycle (Figure 4).
Methane (CH4) is produced in organic deposits by anaerobic bacteria
that couple the oxidation of reduced compounds such aslike hydro-
gen and acetate with the reduction of carbon dioxide to methane. It
disappears from the atmosphere by reaction with the hydroxyl radical
and may be involved in the formation of carbon monoxide and tropo-
spheric ozone..
The concentration of CO2 and CH
4 in the atmosphere has undergone
large variations over time. The initial atmosphere was very different
to the current one: it was a reducing, i.e. anaerobic, atmosphere
containing CH4, NH
3, H
2S and water vapor. Evidence of this predomi-
nantly reducing nature is provided by the ferruginous rocks, with
iron in ferrous form, dating from the Precambrian, over three billion
years ago. The oxygen that appeared later as a result of autotrophic
biological activity was first used to oxidize the reduced molecules,
and then passed into the atmosphere as a waste gas until it reached
the current concentration a billion years ago. As oxygen increased
to the detriment of CO2, the planet cooled down. The atmospheric
CO2 of natural origin comes from the respiration of the biosphere
and emissions from volcanoes and geysers. There is no precise in-
formation on how the concentration of CO2 evolved in ancient times;
two billion years ago the proportion was probably about 75%, which
is well above the current 0.038%. However, records suggest that
INTRODUCTION12
Figure 4. Simplified scheme of carbon cycle on Earth.
during the interglacial periods the temperature increase was ac-
companied by a significant increase in CO2, which is logical if one
considers in accordance withthat the higher water temperature of
the hydrosphere was warmer and therefore had aits lower capacity
to dissolve gases.
Aquatic systems have played a decisive role in the changes in com-
position of the atmosphere. Oxygen comes from photosynthesis, and
the first organisms to use this way of carbon incorporation were simi-
lar to the current cyanobacteria, inhabiting shallow and probably very
eutrophic seas three and a half billion years ago.
4. GLOBAL WARMING AND GLOBAL DIMMING
In the last few decades, policies, investments and business have been
driven by major global issues, particularly those that can have cata-
strophic effects. Some of them are still great causes of concern (the
hole in the ozone layer, global warming), while others have been left
behind (the nuclear threat during the Cold War, meteorite impact)
but may reappear at any time.
The major issue at present is global warming, probably for three
reasons: First, it is very difficult to quantify and predict, so an infinite
number of hypotheses and theories have emerged to satisfy all pos-
sible opinions. Second, related to the first but with a greater impact
on society, any meteorological event can be claimed to be the result
of global warming: now it rains more, now it rains less, it is hotter than
before or colder than before, the winters are not so cold, etc. Third,
the effects are predicted in the medium and long term (50 years or
more), so none of the prophets can be taken to task.
However, an increasing number of experts are sceptical on the
effects of climate change, and even on its relationship to human
activities. Another major global issue is therefore needed, and “global
dimming” is well placed to gain favour.
Indeed, pollutant emissions from human activities of combustion
not only introduce greenhouse gases into the atmosphere, as dis-
cussed above. They also emit many particles like ash and soot, which
intercept part of the solar radiation reaching the earth’s surface. It
seems that in certain parts of the world this radiation and the rate
of evaporation have decreased measurably, in spite of global warm-
ing. The hypothesis to explain this is that the suspended particulate
INTRODUCTION13
contaminants block the solar radiation and convert the clouds into
giant mirrors. The greater the number of particles in suspension,
the greater the number of condensation nuclei for water droplets
to form, the smaller these droplets are, the higher the density of
the clouds, and the greater the screening (mirror) effect they have
against solar radiation. Some scientists suggest that solar radiation
reaching the Earth’s surface has been reduced by between 6% and
30%, depending on the considered zone. According to those who sup-
port this theory, these “mirror” clouds alter the distribution of rainfall
on the planet by cooling the oceans, as—they claim—occurred in the
northern hemisphere in the 1970s and 1980s. Greater evaporation
intensify drought.
Global dimming thus acts in the opposite direction to global warming:
the former cools and the latter heats. We thus have the following
paradox: if we avoid particulate pollution in the atmosphere, the global
dimming effect will decrease, thus reinforcing global warming. Put
another way, thanks to global dimming the effects of global warming
have so far been reduced, but if we reduce the causes of global dim-
ming (which is being done by using particle filters in power stations
and internal combustion engines), global warming will have no limit.
Its effects would be even more devastating than expected because
the current prediction models do not take this process into account.
Average temperature increases of 10ºC are being predicted for the
end of the 21st century.
One option is to immediately stop eliminating the particles emitted
into the atmosphere by combustion processes in order to keep the
effects of global dimming active and thus delay or mitigate the apoca-
lyptic effects of global warming. Howver, it is surprising that the pro-
ponents of the global dimming theory fail to appreciate the contribu-
tion of suspended particles to global warming, which is undoubtedly
not negligible. It can also be assumed that a high concentration of
“protective” suspended particles in the atmosphere would affect the
respiratory processes of many life forms, among other effects.
The fact that assumptions and theories lead to such simple and ob-
vious paradoxes is indicative of enormous gaps in our knowledge,
particularly on the regulatory processes involved in the nature and
functioning of natural ecosystems. Therefore, it might be argued that
if global dimming is important, part of the current climate change
may be due to the sharp reduction in the concentration of suspended
particles in the atmosphere in comparison with the late 19th century,
when the use of coal as a fuel was widespread.
HYDROELECTRICITY IN THE GLOBAL ENERGY BALANCE15
1. INTRODUCTION
Reservoirs store water that can be used for a variety of purposes
and host complex aquatic ecosystems. Furthermore, they play a very
important role in the production of energy through hydroelectricity.
Before analyzing how—and to what extent—reservoirs are involved in
the greenhouse effect, we will explain the important role of hydroelec-
tric power in the global energy balance and the key role that it plays
in the energy organization of countries that are highly dependent on
both water and energy like Spain.
2. ENERGY PRODUCTION AND CONSUMPTION IN SPAIN
Spain is an energy-poor country that imports more than 80% of the
consumed energy. This is probably a little known fact, although it is
the key to understand current energy management in Spain and to
raise public awareness about the importance of saving energy. To
be exact, 139.5 x 106 tonnes of oil equivalent (toe) were imported
in 2005, showing a 7.7% rise over 2004 and representing 85.1%
of the total energy consumption. Figure 5 shows the distribution of
installed capacity and electricity production by different technologies
in 2007, according to the annual report on the electricity balance of
Spanish Electricity Network.
The installed power for energy production has doubled in the past
10 years (Table 1) thanks to wind farms and especially the combined
cycle plants that began to operate in 2003. By contrast, the installed
capacity in hydroelectric plants operating in the ordinary regime
(above 10 Mw) has barely changed in the past 10 years because
no large new facilities have been created. For instance, In the last
Figure 5. Installed power (MW) and electric energy production (GWh) of different technological methods in Spain in 2007. The term “Other renewable” includes mini hydroelectric, solar and biomass energy.
Table 1. Annual evolution of the total installed power (Gw) for electric energy production under ordinary regime in Spain during the last decade.
Installed Power (Gw)
1998 43,522
1999 43,662
2000 44,079
2001 44,181
2002 46,255
2003 61,223
2004 68,425
2005 73,970
2006 78,754
2007 85,959
HYDROELECTRICITY IN THE GLOBAL ENERGY BALANCE16
5 years, it has remained stable at 16,658 MW. This has reduced
largely the percentage share of hydroelectricity in total electric power
production: from 37.8% in 2000 to 19.4% in 2007. In the coming
years new hydroelectric pumping projects will involve a power in-
crease of about 3000 MW, MW corresponding to projects that are
already in place.
The net production of electrical energy under the ordinary and special
regimes has followed the trend shown in Figure 6. The special re-
gime includes the production of renewable energy (mainly wind, solar,
mini-hydro and biomass), whereas the ordinary regime includes the
remaining energy production systems, including large-scale hydro-
electric systems.
The trend of total production is obviously marked by the demand
curve. The demand is growing every year, reaching an average an-
nual increase of 4.5% in the past 12 years. In 2006 and 2007 it was
about 4.0% after adjustments for the effects of working days and
temperature. In the same two years, domestic demand rose from
38.63% of the total in 2006 to 41.53% of the total in 2007. We
must have in mind that part of the energy consumption and its in-
crease over time is inherent to economic growth.
In Spain the consumption per capita per year is still lower than in other
countries. The total energy consumption (electricity, fossil fuels, etc.) is
about 3.2 toe per inhabitant per year. This compares with 7.8 toe for
the US, 4.1 toe for Japan 3.6 toe for the whole European Union.
Figure 6. Evolution of electric energy production in Spain, both under ordinary regime (R.O.) and special regime (R.S.).
3. THE ROLE OF HYDROELECTRICITY IN MEETING THE DEMAND FOR ENERGY
Hydroelectricity plays a key role in meeting energy demand due to
its versatility. Indeed, not all sources of energy have a comparable
capacity for modulation. Due to their production process and con-
nection to the transport and distribution network, some sources
have reduced capacity to vary the electric supply over time. This is
the case of thermal and nuclear power plants, which operate con-
tinuously with little possibility of variation in production because of
their industrial process. Other energy sources, on the other hand,
have a highly variable production because they depend on uncon-
trollable natural processes. This is the case of wind energy and
of small, run-of-the-river hydroelectric plants, which depend on the
availability of wind and water, respectively.
In any case, the timing of power demand on an annual or daily
scale is completely independent from the processes of energy
production and the availability of natural energy resources. The
trend of demand is marked by consumption. Therefore, to meet
demand and production, we need controllable processes that can
increase or decrease the energy production as needed by the
most precise (efficient) adjustment between demand and produc-
tion possible.
Figure 7 shows a typical daily demand curve of electrical energy.
There is a period with low demand during the central hours of the
Figure 7. Distribution of different energy sources supplying the medium power demand curve in a typical day.
HYDROELECTRICITY IN THE GLOBAL ENERGY BALANCE
night, from 2 to 6 a.m., followed by an initial peak demand between
11 and 13 a.m. and a higher and longer peak between 4 and 9 p.m.
These time differences in energy requirements are obviously related
to social habits (work schedules, domestic and industrial activities,
etc.). The daily demand curve is established from one day to the
next by a forecasting model based on historical data and variation
trends.
To meet the daily demand curve, the managing body of the elec-
tricity market gives priority first to the production processes that
are most difficult to modulate (nuclear) or regulate (wind and run-
of-the river hydroelectric power). There follows thermal produc-
tion (coal, fuel-oil, gas, combined cycle), which has a very limited
capacity for regulation. The only energy source that can fine-tune
production to demand is conventional hydroelectric power (with
regulation).
The use of conventional hydroelectric plants associated with large
reservoirs and the support of reversible hydroelectric plants (pump
turbines), allows electric production to be adjusted precisely to the
daily demand curve. For example, in a situation of failure in a thermal
or nuclear plant, the only energy source that can compensate for the
fall in production quickly enough is conventional regulated hydroelec-
tricity (conventional or pumped).
Regulated hydroelectric production is also the only one that can
compensate for the fluctuations in production that can occur in the
course of a day in renewable energy sources that depend on natural
processes such as wind (very important in Spain), solar energy or
run-of-the-river hydroelectric power. When a wind farm stops pro-
duction due to lack of wind or feeds electricity into the grid due to
availability of wind, this is compensated precisely by hydroelectric
power stations feeding electricity in or drawing it out, respectively.
This regulatory role of hydroelectric plants also adds quality to the
energy supply because it allows a constant frequency (hertz) to be
maintained by compensating for the fluctuations in the grid according
to consumption.
Hydroelectric pumping stations add additional capabilities in the
management of electricity. These facilities consist of two reservoirs
located at different altitudes and connected by a hydroelectric plant
that can act as a pump or turbine as needed. When the consump-
tion of energy is minimal (at night), the surplus production can be
used to pump water to the upper reservoir, increasing the poten-
tial energy storage. When demand increases in the middle of the
day, the water accumulated during the night can be run through the
turbines in the hydroelectric plants, thus optimizing the production
and providing a good adjustment to the demand curve. These plants
17
HYDROELECTRICITY IN THE GLOBAL ENERGY BALANCEHYDROELECTRICITY IN THE GLOBAL ENERGY BALANCE18 19
are also an essential complementary system for optimizing energy
resources when the installed capacity of renewable sources is high,
as is the case with wind energy in Spain. In periods of high wind and
low demand for electricity, the wind power production can also be
used for pumping.
4. HYDROELECTRICITY AND OTHER ENERGY SOURCES IN TERMS OF CO2 BALANCE
Hydroelectric energy—like wind energy, photovoltaic energy and nu-
clear energy—is one of the sources that emits the least CO2 into the
atmosphere. However, we must consider in the emission assess-
ment of emissions not only the production process but also the full
life cycle of industrial production facilities (manufacture, construc-
tion, maintenance, demolition, etc.). In the case of hydroelectric-
ity, the type of facility (run-of-the-river, large reservoir, etc.) and the
hydro-morphological and biological characteristics of the water body
involved are of particular importance. In hydroelectric plants that
use large reservoirs, one must also take into account the types of
natural communities that they replace and their potential capacity
as carbon sinks.
Yield is an important factor in the energy production by different tech-
nologies. The yield is 33% for a nuclear power plant, 38.5% for a
thermal power plant and 90% for a hydroelectric power station. In
comparative terms, for each GWh of hydroelectric energy, we avoid
burning 223 t of oil, 248 hm3 of natural gas, 319 t of coal or 25 kg
of natural uranium.
Table 2 lists the emission of greenhouse gases related to the life
cycle of different electricity production technologies. The emissions
Table 2. Greenhouse gases emission during life cycle for different technologies of electric energy production.
Type of power plant Range (g CO2 eq/kWh) Mean value
Combined cycle (sinthetic coal gas)
Combined cycle (natural gas)
Hydroelectric (boreal reservoirs)
763-833
469-622
8-60
798
545
36
Coal (modern plant)
Diesel
Photovoltaic
959-1.042
555-880
12,5-104
1.000
717
58
Wind 7-22 14
from hydroelectric production are slightly higher than those from
wind power, similar to photovoltaic energy and far below those of
natural gas combined cycles, which are the options with least emis-
sions among fossil fuel technologies. In comparison with fossil fuel
power plants that use fuel-oil and carbon, each GWh produced by
a hydroelectric plant avoids emitting into the atmosphere between
450 and 1000 tonnes of CO2.
With regard to inland aquatic ecosystems and the production of
renewable energy, two European Directives must be respected in
conjunction. First, the Water Framework Directive (2000/60/
EC) aims to achieve by 2015 a “good ecological status” (or good
ecological potential for heavily modified water bodies) in all con-
tinental aquatic systems. Second, the Renewables Directive
(2001/77/EC) on the promotion of electricity from renewable
HYDROELECTRICITY IN THE GLOBAL ENERGY BALANCEHYDROELECTRICITY IN THE GLOBAL ENERGY BALANCE18 19
energy sources establishes the aims that, by 2010, renewable en-
ergy sources will provide 12% of the unprocessed primary energy
consumption (gross national consumption) in the EU and 29.4%
of total electricity generation in Spain. In 2007 renewable energy
achieved a 7% share of primary consumption, with an average an-
nual increase of 0.5%. At this increasing rate, in 2010 the figure
will be 8.5%—far from the 12% target. Obviously, in compliance
with the Renewables Directive, a significant role is played by the
development of hydroelectric power, which often affects the eco-
logical quality of the water bodies involved. It will be necessary to
apply the best strategies and technologies available to achieve the
objectives proposed in the two Directives.
AQUATIC ECOSYSTEMS AND GREENHOUSE GASES21
1. CARBON
Carbon enters aquatic systems through the dissolution of atmos-
pheric CO2. It is also a component of the particulate and dissolved
organic and inorganic matter that comes from their basins (Figure
8).The amount of atmospheric CO2 dissolved in water depends on
water temperature and acidity and the difference between atmos-
pheric CO2 partial pressure and concentration in the water (mainly
as CO2 and bicarbonate), which must end in equilibrium. As water
pH increases, the dissolved CO2 transforms first into gas, followed
by carbonic acid, bicarbonate and carbonate, in that order. If calcium
is present, it tends to form Ca CO3, which has low solubility and pre-
cipitates chemically, creating a carbon sink. Through photosynthesis,
CO3 and the HCO3- form part of the aquatic vegetation in the form
of phytoplankton, macrophytes or phytobenthos. From then on the
complexity of the cycle may vary.
Part of the assimilated carbon is rapidly released as CO2 by res-
piration of the vegetation. The rest is circulated through the food
web in zooplankton, zoobenthos, birds, fish and bacteria. It is also
returned to the water through the respiration of each compart-
ment and can be reused by autotrophs. If the CO2 partial pressure
in the water exceeds that of the atmosphere, it returns to the at-
mosphere.
Part of the assimilated carbon can be deposited in mineral form or
be chemically reduced in anaerobic sediments. A fraction may also be
transformed into methane through the reduction of carbon dioxide
when the conditions are suitable. Methane can be further oxidized to
carbon dioxide in the water and follow its pathway or be released into
the atmosphere. The biogeochemical carbon cycle varies according
to the dominant metabolic pathways. Autotrophy-dominated systems
are those in which the main carbon source is inorganic (CO2 or par-
ticulate or dissolved inorganic matter).
In general the dominant pathway followed by carbon in aquatic eco-
systems is that of autotrophy, in which the atmospheric CO2 enter-
ing the water is fixed by photosynthetic organisms. If the system is
oligotrophic, carbon cycling in the water body is closed and there is
little exchange with the atmosphere and sediments. However, if the
system is eutrophic, its role as a scavenger of atmospheric CO2 is
enhanced because the carbon cycle is opened to the sediment (Figu-
re 9). Finally, if the system is heterotrophic, carbon is mostly taken up
in organic form. Respiration of animals or bacteria is dominant in these
systems, so they release CO2.
Figure 8. Carbon cycle in aquatic ecosystems.
Figure 9. Carbon cycle in oligotrophic (closed) and eutrophic (opened) aquatic ecosystems.
2. NITROGEN
TThe nitrogen cycle in aquatic ecosystems is very complex and is regu-
lated by the redox potential in the different compartments of the water
body. In the presence of light, autotrophs in general are able to use all
forms of nitrogen, from molecular nitrogen to nitrate, incorporating it
into the food chain. In the decomposition stage, the nitrogen in the or-
ganisms is oxidized by bacterial action. If oxygen is present, correspond-
ing to a high redox potential, nitrate is produced through the action of
nitrifying bacteria, which are aerobic. However, if the oxygen is depleted,
the decomposition process of organic matter continues with fermenta-
tion, leading to the appearance of denitrifying bacteria (among others)
that use oxidized nitrogen molecules as terminal electron acceptors.
In other words, instead of oxygen they “breathe” nitrite and nitrate, and
this metabolism produces N2 and N2O, which can be released into the
atmosphere or incorporated again by nitrogen-fixing organisms such as
cyanobacteria in the water or bacteria from the root nodules of many
plants (e.g. legumes and alder) in terrestrial soils.
3. RESERVOIRS AND GREENHOUSE GASES
The change from river to reservoirRivers incorporate dissolved and particulate organic carbon from ter-
restrial systems that they drain. These particles are processed along
the river course.
Reservoirs are modified rivers in which the hydraulic section and the
water residence time are artificially increased. The change from a
horizontal to a vertical organization leads to substantial changes in
their functioning as ecosystems. Carbon metabolism is different in
reservoirs, but it is often more similar to rivers than to lakes. Hetero-
trophic component is very strong in rivers, so they tend to release
greenhouse gases rather than to sequester them. The response
of reservoirs will depend on whether they behave more as rivers or
as lakes. The discharge of hypolimnetic water, which is colder and
subject to greater hydrostatic pressure, is a unique feature of reser-
voirs respect to rivers and lakes. This water has a greater capacity
to maintain dissolved gases (N2O, CO2 and CH4), which emerge down-
stream and are released into the atmosphere.
Moreover, reservoirs flood terrestrial ecosystems. Therefore, at least in
the initial stages of their history, they contain a large amount of organic
matter that is metabolized through detrital processing. In general this
first phase, which could be called heterotrophic, lasts about a decade
and is distinguished by higher CO2 and CH4 emissions. After this period
the reservoir reaches its equilibrium and the emission of greenhouse
gases falls to rates typical of equivalent natural aquatic systems.
However, regarding to greenhouse gases balance in reservoirs, we
must also consider the the replacement of the terrestrial ecosystem
by the aquatic one. On the one hand, regardless of the metabolism of
the organic matter that has been flooded, the change from river to
reservoir involves shifts in the hydromorphology, physico-chemestry
and biology of the aquatic system. It is also interesting to compare
AQUATIC ECOSYSTEMS AND GREENHOUSE GASESAQUATIC ECOSYSTEMS AND GREENHOUSE GASES22 23
Cavallers Reservoir. Noguera de Tor River (Lleida province, Spain).
AQUATIC ECOSYSTEMS AND GREENHOUSE GASESAQUATIC ECOSYSTEMS AND GREENHOUSE GASES22 23
the new aquatic with the former terrestrial ecosystems that were
flooded.
When the river is dammed it changes from being a turbulent sys-
tem, which is generally well oxygenated and mixed, to a far more
static system with a tendency to stratify. Whereas the biological
communities in running waters adhere to submerged substrates, in
reservoirs they are composed almost exclusively of plankton.
Reservoirs are more favourable to the sequestration of CO2 than riv-
ers because atomized phytoplankton has a high reactive capacity: it
fixes carbon and transports it to the bottoms as it sediments. If the
bottoms are anoxic, the carbon may be removed from the cycle. Al-
lochthonous carbon could be also fixed this way. On the other hand,
rivers have large availability of oxygen, so the entering organic matter
is more easily oxidized to CO2, which is returned to the atmosphere.
The balance of greenhouse gases in lakes and reservoirsThe data presented in Table 3 was obtained from specialized lit-
erature. They illustrate the order of magnitude of CO2 emitted from
lakes and reservoirs. Despite the large variability between water
bodies, this order of magnitude ranges between 100 and 10000
mg CO2 m-2 day-1, which would be equivalent to 10 and 100 g C m-2
year-1. Tremblay et al. (2005) report far more modest emissions of
CH4: 0.6 ± 13 mg CH4 m-2 day-1 from lakes and 8.8 ± 12 mg CH4
m-2 day-1 from reservoirs, both situated in a boreal climate. For the
same water bodies, these authors measured emissions of –1.3 to
3.1 mg N2O m-2 day-1, which are consistent in order of magnitude
with those found in lakes in temperate regions (0.01 to 0.9 mg N2O
m-2 day-1; Mengis et al., 1997), and those found in the Great Lakes
of the United States (3.5 mg N2O m-2 day-1; Leman & Leman, 1981).
It seems that the emission of CO2 and N2O in reservoirs is lower
than or equal to lakes, whereas the emission of CH4 is greater in
reservoirs than in lakes.
Furthermore, the creation of reservoirs reduces sink capacity, be-
cause in all cases the communities that are lost were CO2 fixers (see
Table 4). In summary, when examining the effect on greenhouse gas-
es of a reservoir, we must consider its net CO2 emission rates as a
heterotrophic aquatic system, and the differences in fixation rate with
the replaced terrestrial system. We should also take into account all
the detrital carbon from its tributary basin that it is able to retain in
its sediment, which was once atmospheric CO2. Therefore, the major
factors that are involved in the balance are:
CEI = f(En,DFec, CDTs)
where:
• CEI is the contribution of the reservoir to the greenhouse effect,
in carbon.
• Ne is the net emission of the water body (the balance between res-
piration and production.
• Fec is the difference in fixation capacity with the replaced terrestrial
ecosystem (positive or negative).
• CDTs is the total organic and mineral detrital fraction of the car-
bon provided by the basin, which is retained in the sediment of the
reservoir.
Cala Reservoir. Ribera de Cala River (Seville province, Spain).
Table 3. CO2 emission from reservoirs and lakes in different countries.
Type of water bodyMean CO2 emission rate
Hydroelectric reservoir (Duchemin et. al., 1995)
Reservoir in Brazil (Rosa et al., 1999)
Reservoirs and lakes ( Southeastern USA, Therrien et al., 2005)
Reservoirs and lakes (Canada, Tremblay et al., 2005)
Hydroelectric reservoirs in Amazonas (Brazil, Rosa et al., 1997)
Canada
Curuá-Una
Manic ReservoirsManic Reference LakeGouin ReservoirGouin Reference Lake
Reservoirs (n=259)Lakes (n= 31)
Embalses (n=56)Lagos (n=43)
Tucuruí ReservoirSamuel ReservoirXingó ReservoirMiranda ReservoirSegredo ReservoirSerra de Mesa ReservoirTrês Marias ReservoirItaipú Reservoir
500-1.000
134,3
1.170 (6470)1.010 (6405)1.165 (6685)1.700 (6950)
664 (61.091)874 (62.214)
1.508 (61.471)1.013 (61.095)
8.4756.7196.0484.3883.8912.6952.6541.138
49,8-99,5
13,4
116,5100,5116,0169,2
66,13 (6108,7)87 (6220,5)
150,2 (6108,7)100,9 (6220,5)
843,6668,6602,1436,8387,3268,3264,2113,3
(mg CO2 wm-2 day -1) (g C m-2 year -1)
Reservoir and lakes (Canada, Duchemin et al., 1999)
AQUATIC ECOSYSTEMS AND GREENHOUSE GASESAQUATIC ECOSYSTEMS AND GREENHOUSE GASES24 25
Baserca Reservoir. Noguera Ribagorzana River (Lleida province, Spain).
Greenhouse gases according to the type of reservoir
Regarding to greenhouse gases balance, nota all reservoirs behave
have different processes in the same way with regard to the balance
of greenhouse gases. The bahaviour dependings on factors such as
climate, location in the basin, hydromorphological characteristics
and trophic state. Their water management should also be taken into
account.
Processes in reservoirs located in boreal zones and semi-arid zones
are similar (Figures 10 and 11). They emit high levels of CO2 and CH4
during the first decade after flooding, then the emission rates are
very low. CO2 emission is always higher than CH4 emission. Tropical
reservoirs have a greater input of particulate organic matter, which
increases the reducing power of the sediments and favours the emis-
sion of CH4 rather than CO2. Additionally, in these reservoirs the net
emissions of greenhouse gases are higher than in boreal and semi-
arid zones. The heterotrophic stage after filling lasts for 10 years
or may even be maintained for ever due to the high loads of organic
matter per unit area covered by water (Figures 12 and 13). All the
carbon involved in these balances belongs to the current carbon cy-
cle, so it does not have the same impact on the environment as the
carbon from fossil fuels. This is must be kept in mind.
Table 4. Mean CO2 fixation rates on both a daily and an annual basis for different land vegetal communities.
Type of community
Mean CO2 fixation rate
(mg CO2 wm-2 day -1) (g C m-2 year -1)
Arid steppe
Unirrigated agricultural land
Irrigated agricultural land
Mediterranean shrubland
Humid shrubland
Mediterranean woodland
Humid woodland
502
804-1.205
1.004-1.406
804-1.004
1.506
1.406
2.511
50
80-120
100-140
80-100
150
140
250
AQUATIC ECOSYSTEMS AND GREENHOUSE GASESAQUATIC ECOSYSTEMS AND GREENHOUSE GASES24 25
Chocón Reservoir. Limay River (Neuquén, Argentina).It supplies the largest hydroelectric plant in Argentina. It has 20,200 Hm3 in capacity and 816 km2 in area.
AQUATIC ECOSYSTEMS AND GREENHOUSE GASESAQUATIC ECOSYSTEMS AND GREENHOUSE GASES26 27
Figure 10. Most relevant processes related to carbon cycle occurring in boreal reservoirs immediately after filling.
Figure 11. Most relevant processes related to carbon cycle occurring in boreal reservoirs several years after filling.
AQUATIC ECOSYSTEMS AND GREENHOUSE GASESAQUATIC ECOSYSTEMS AND GREENHOUSE GASES26 27
Figure 12. Most relevant processes related to carbon cycle occurring in tropical reservoirs immediately after filling.
Figure 13. Most relevant processes related to carbon cycle occurring in tropical reservoirs several years after filling.
AQUATIC ECOSYSTEMS AND GREENHOUSE GASES28
The location of the reservoir in the basin determines the load of al-
lochthonous organic matter that can enter it. Reservoirs located in
the headwaters of rivers also have small catchment areas and in
theory have less carbon to oxidize. However, in highly forested areas
the large biomass input can create peat bogs which emits more CH4
than CO2, i.e. they have a greater potential for greenhouse effect. As
reservoirs occupy lower reaches, there is increasing likelihood of re-
ceiving external inputs of dissolved and particulate organic matter,
not only from natural communities but also from cultural activities.
These inputs increase the heterotrophic metabolism of the reservoir
and its potential for emission of greenhouse gases.
The water regime combined with the volume of the reservoir deter-
mines the rate of water renewal. As the rate decreases it favours
phytoplankton growth and consequently the formation of an au-
totrophic system, which increases CO2 fixation by the stored water.
Regarding morphometry, two extreme scenarios can be defined. On
one hand, the embedded reservoirs with steep slopes and therefore
a low relative area subject to water level fluctuations and on the other
hand, the reservoirs with gentle slopes in which small decreases of
water level expose large areas of land. In the first case, the waves
beating the shore transport the sedimented organic matter to the
water column and then to the bottom, which is a carbon trap if re-
ducing conditions are predominant. In the second case the organic
matter in the shallow areas is recycled in situ, with the consequent
emission of greenhouse gases. However, reservoirs with this kind of
morphology, are more prone to eutrophication.
The trophic status is equivalent to the biogenic capacity of the aquatic
ecosystem and is regulated by the nutrient load (particularly phos-
phorus) received by the water body, which is considered to be the
limiting factor in biological production. Limnologists classify lakes and
reservoirs on a scale ranging from oligotrophic to eutrophic waters,
i.e. from unproductive to highly productive waters. In oligotrophic wa-
ters there is little life, and therefore few biogeochemical changes take
place. Eutrophic waters (i.e. well fed) waters can become extremely
reactive and very dynamic in their physicochemical characteristics,
particularly the parameters related to biological phenomena. In
lakes, the biological community shows varying degrees of diversity.
The maximum degree of complexity includes a number of elements
such as littoral and submerged aquatic vegetation, zoobenthos, fish,
zooplankton and phytoplankton. The trophic level is depends on the
features of the water body, which favours the development of certain
elementstherefore expressed in a greater development of the ele-
ments that are most favoured by the characteristics of the wa- ter
body. Generally, in shallow lakes eutrophication leads to a greater-
Accumulation of different scoured materials in the dam of Sabiñánigo Reservoir (Gállego River, Huesca province, Spain).
AQUATIC ECOSYSTEMS AND GREENHOUSE GASES29
favours the development of submerged macrophytes. In reservoirs,
due to their permanent state of immaturity, the matter and energy
flow mainly through plankton, particularly especially phytoplankton,
and through the heterotrophic bacteria that are found in the plank-
tonit and in the benthos.
Eutrophic reservoirs have high turbidity due to suspended algae, green
water and their chemical features tend to be stratified: high oxygen,
high pH and low phosphorus at the surface; low oxygen, low pH and
higher phosphorus in deep waters. The higher the trophic status of
lakes and reservoirs, the greater the amount of atmospheric CO2 they
fix. However, the final balance depends on other characteristics that
determine the potential to return the fixed carbon to the atmosphere.
Among them, the most important are the oxidation of the sediments
and the alkaline reserve. If the carbon-enriched sediments remain
anoxic, the carbon is immobilized or transformed into CH4, though
this process is efficient only in absence of SH2, which inhibits metha-
Cárdena Reservoir. Cárdena River (Zamora province, Spain).
Negratín Reservoir. Guadiana Menor River (Granada province, Spain).
nogenic activity. On the other hand, if there is enough calcium in the
water, the increases in pH associated with photosynthesis favour the
precipitation of CaCO3, which has a very low solubility. Consequently,
the reservoir model with the greatest capacity to sequester carbon
is the one with deep eutrophic waters that are highly mineralized by
both calcium and sulfates, which are precursors of SH2. Furthermore,
shallow eutrophic reservoirs—particularly if the waters are weakly
mineralized—return the fixed carbon to the atmosphere in the form
of CO2 and/or CH4, and the net balance of the production-respiration-
decomposition cycle tends to be null.
The water management also affects the rate of water removal in the
reservoir, the fluctuations in level and the selection of the depth from
which the outgoing water is taken. The less time the water is retained
in the reservoir, the lower the autotrophic activity, the higher the ten-
dency to emit CO2, and the lower the possibility of retention of carbon
by sedimentation of organic and inorganic matter. Changes in water
level imply changes in depth. When the depth falls, anoxic zones re-
ceive dissolved oxygen more easily and oxidizing conditions can be
established. Some flooded areas may even dry out, with the resulting
oxidation of the stored organic carbon to CO2.
Finally, water is usually released from the deep levels of reservoirs,
where water is rich in dissolved and particulate organic carbon of
detrital origin. This carbon is released into the atmosphere under
AQUATIC ECOSYSTEMS AND GREENHOUSE GASES30
the more oxidizing conditions downstream. CO2, CH4 and N2O may
also be emitted by these deep waters through degasification, as
stated above. Finally, unlike lakes located in basins that are in a
state of hydromorphological balance, reservoirs are condemned to
be filled with sediment. The sediment load transported by the tribu-
tary river is deposited at the bottom of the reservoir, where it buries
both the allochthonous organic sediments and those that are syn-
thesized in the reservoir. These sediments are rarely remobilized,
so the net result is carbon retention. Therefore, the rate of siltation
is the last important factor to take into account in the carbon bal-
ance of stored water bodies.
Mediano Reservoir Cinca River (Huesca province, Spain). Sedimentary de-posit uncovered by the almost complete emptying of the reservoir.
Linsoles reservoir. Ésera River (Huesca province, Spain).
A CASE STUDY: THE SUSQUEDA RESERVOIR (TER RIVER, GIRONA)32
1. INTRODUCTION
The overall role of reservoirs in the balance of greenhouse gases is
of little importance in Spain because their surface water layer repre-
sents only 0.61% of the total land area. However, as Spain is the Eu-
ropean country with the greatest number and diversity of reservoirs,
the study of the carbon balance in the Susqueda reservoir that is
presented in this publication is of great interest.
Between 2002 and 2003, ENDESA conducted a study of the carbon
balance in the Susqueda Reservoir over an annual cycle. This res-
ervoir is located in the middle stretch of the Ter River between the
Sau and El Pasteral Reservoirs in the province of Girona (Figure 14).
The Susqueda Reservoir is medium-sized, eutrophic and moderately
mineralized. Table 5 summarizes its main characteristics during the
study period.
Figure 14. Location map of the reservoir network including Sau, Susqueda and Pastoral Reservoirs (Ter River, Girona province, Spain).
View of the dam and the two intake towers.
A CASE STUDY: THE SUSQUEDA RESERVOIR (TER RIVER, GIRONA)33
2. THE APPROACH TO THE STUDY
Gas emissions from aquatic systems can be estimated by vari-
ous methods. Canadian scientists have been measuring emis-
sions of greenhouse gases in aquatic systems since 1993,
using floating chambers for gas collection placed on the water
surface. that collect the gases. These Gases are measured
in situ or in the laboratory by infrared chromatography. Of
course, these methods require a large number of measure-
ments in both space and time in order to obtain be repre-
sentative.
Table 5. Hydromorphological and physico-chemical features of the Susqueda Reservoir during the 309 days of the study.
Tabla 5. xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Hydromorphological features
Physico-chemical features
April 2002 March 2003
Reservoir water volume at maximum water level (hm3)
Reservoir water volume (hm3)
Maximum operative water level elevation (m a.s.l.)
Incoming water in hm3 during the study
Reservoir water level (m a.s.l.)
Maximum depth (m)
233
86,48
351
376,46
Outgoing water in hm3 during the study 305,73
Thermal characteristics Summer stratification. Permanent deep cold water mass
Electric conductivity (mS/cm) 373-756
Alkalinity (meq/L) 1,76-2,93
Water transparency in m (Secchi disc) 0,6-6,47
pH (und) 7-9
Dissolved oxygen Permanent hypoxia or anoxia in deep water
316,64
56
348,20
>95
215,12
In the project carried out by ENDESA, it was decided to use a “black box”
model, measuring or estimating carbon fluxes at the inflows and out-
flows of the reservoir. This model is based on the assumption that the
incoming amount is equal to the outgoing amount, which includes the
amount of carbon stored in the sediment plus the amount stored in the
body of water due to the increase in volume during the study period.
The inputs are:
• TCin = total carbon in the water flowing into the reservoir from the
Sau Reservoir (measured).
• TCdi = total carbon in the diffuse inflow to the reservoir from the
main basin (estimated).
The outputs are:
• TCout = total carbon of the outgoing water from the reservoir into
the Ter River (measured).
• TCsed = carbon accumulated in the sediment of the reservoir (estimated
from the measurements obtained in the surface layer of sediment).
The air-water exchange of CO2 is :
• CO2 = net exchange of CO2 between the water and the air (calcu-
lated from the difference in concentration of CO2 concentration in
between the water and in the atmosphere).
The amount stored is:
• TCsto = total carbon stored in the water of the reservoir (calculated from
the difference between the initial and final volume of the reservoir in the
study period, multiplied by the carbon concentration of the outflow).
The balance can be expressed as:
TCin+TCdi = TCout
+ TCsed + DTCsto
+ DCO2
A CASE STUDY: THE SUSQUEDA RESERVOIR (TER RIVER, GIRONA)34
The measured or estimated forms of carbon that were measured or
estimated were:
• In the water flowing into and out of the reservoir:
– Dissolved inorganic carbon (DIC) based on alkalinity and pH.
– Organic carbon: dissolved (DOC) and particulate (POC).
• In the sediment traps: Particulate organic carbon (POC) and par-
ticulate inorganic carbon (PIC).
• In the air-water interface: exchange of CO2.
• In the sediment: Particulate organic carbon and particulate inor-
ganic carbon (POC and PIC).
In this study the dissolved inorganic carbon (DIC) is equivalent to total
inorganic carbon, because the alkalinity was measured by acidimetric
titration of unfiltered samples. It is thus assumed that the resulting
alkalinity corresponds to the sum of DIC and PIC (carbonates).The
diagram in Figure 15 shows the black box model used to determine
the carbon balance in the Susqueda Reservoir. The initial assump-
tions were as follows:
(1) (TCin + TCdi) > (TCout + TCsto) and TCsed ↑ ==> heterotrophy with
incorporation of carbon.
(2) (TCen + TCdi) > (TCout + TCsto) and TCsed ↓ ==> heterotropy and
emission of CO2.
(3) (TCin + TCdi) ≤ (TCout + TCsto) and TCsed ↑ ==> autotrophy with fixa-
tion of CO2.
(4) (TCin + TCdi) ≤ (TCout + TCsto) y TCsed ↓ ==> autotrophy and exporta-
tion of carbon, or oligotrophy.
(5) (TCin + TCdi) = (TCout + TCsto) and no change in TCsed ==> Inert sys-
tem or autotrophy with fixation of CO2 adjusted to the oxidation of
CTin + CTdi; always without net emission of CO2.
3. FIELD METHODS
Campaigns and sampling stationsSix limnological sampling campaigns were performed during an annual
cycle: April, July, September and November 2002, and January and
March 2003. Four sampling points were established (Figure 16): the in-
flow to the reservoir from the Sau Dam, the outflow from the reservoir to
El Pasteral, the fluvial zone of the reservoir (tail) and the lake area of the
reservoir (dam).
Carbon was not analyzed in the water entering the reservoir through
the tributaries because reliable data on input were already available.
These tributaries represent a lower inflow than that of the Ter River
through the Sau Reservoir. The total area of the Susqueda Basin is
POCsed
PICsed
Figure 15. Scheme of the “black-box” model used to determine the carbon balance in the Susqueda Reservoir (TC: Total Carbon; OC: Organic carbon; IC: Inorganic carbon; POC: Particulate organic carbon; DOC: dissolved organic carbon; PIC: Particulate inorganic carbon).
1,850 km2, whereas the sub-basin has only 286 km2, which repre-
sents 18% of the whole basin. The Susqueda sub-basin is character-
ized by dense forests with low anthropic pressure. According to the
definition of the Catalan Water Agency (2002), the streams that flow
into the left bank belong to the “Calcareous Mediterranean Moun-
tain” ecotype (with conductivities of about 500 mS/cm), whereas
those that flow into the right bank belong to the “Siliceous Mediter-
ranean Mountain” ecotype (with conductivities of about 100 mS/cm).
Both ecotypes have a low annual water inflow (<40 hm3).
Measurements and observations • In the reservoir (tail and dam areas):
— Vertical profiles (measured each metre) of temperature, electri-
cal conductivity, pH, redox potential, dissolved oxygen and turbidity.
Figure 17. Field work carried out in the Susqueda Reservoir.
A CASE STUDY: THE SUSQUEDA RESERVOIR (TER RIVER, GIRONA)35
— Measurements of the Secchi disc depth.
— Water sampling for analysisto determine of the alkalinity at dif-
ferent water levels: surface, Secchi Disc depth (2.7 SD), thermo-
cline, hypolimnion and bed.
— Water sampling of the sedimentation traps for analysis of the
matter sedimented (particulate organic and inorganic carbon
and undissolved solids) during the time between two consecu-
tive campaigns.
— Extraction of sediment samples at the dam station through a
Phleger Corer sampling unit with a tube 3 cm in diameter and
60 cm long for subsequent dating and determination of the or-
ganic and inorganic carbon content. In an initial campaign, sedi-
ment samples were taken at the tail and dam with an Ekman-type
dredge for a preliminary characterization of sediment (color,
smell, texture, presence of gases, etc.).
• At the inflows and outflows:
— In situ measurements of temperature, conductivity, pH, redox
potential, turbidity and dissolved oxygen in water.
— Water sampling for analysis of alkalinity and organic carbon.
Installation of the sedimentation traps
Two sedimentation traps were placed at each sampling point of the
reservoir (dam and tail): a fixed one to calculate the sedimented mat-
ter throughout the entire study cycle and anone other containing the
matter sedimented between two consecutive campaigns (every two
months). The traps consisted of an opaque PVC tube 1 m long and
9 cm in diameter, closed at the lower end. They were, anchored about
5 m from the bottom, and kept vertical by submerged floats located
about 15 m above the trap and tied to a surface buoy by a line whose
length allowed for the fluctuation in the water level of the reservoir.
Figure 16. Approximate location of sampling points.
4. ANALYTICAL METHODS
Table 6 shows the test methods used on the samples. In the core
collected at the deepest part of the reservoir, the sedimentation
processes were dated using absolute radiometric techniques
(210Pb, 226Ra and 137Cs) and the amount of organic and inorganic
carbon accumulated throughout the history of the reservoir was
measured.
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A CASE STUDY: THE SUSQUEDA RESERVOIR (TER RIVER, GIRONA)37
Table 6. Assay methods used to analyze the samples taken in the Susqueda reservoir.
Samples
Water
Alkalinity Acidimetry. NAQUADAT 10101 method.
Total organic carbon Catalytic oxidation. Detection with IR analyzer. EPA 9069 method.
Dissolved organic carbonCatalytic oxidation. Detection with IR analyze after filtering through glass fibre filters. EPA 9060 method.
Sedimented matter
Sedimented matter
Total Carbon
Particulate organic carbon
Total inorganic carbon
Carbono inorgánico total
Filtration with a GF/C filter. Dry residue is oxidized with oxygen and cobalt oxide as catalyzer. CO2 measured with IR analyzer.
Filtration with a GF/C filter. The dry residue is previously treated with HCl to remove organic carbon from it. DHROMANN 190 analyzer equipped with a module for solid matter (MOD. 183).
Oxidation.
Acid attack.
Parameter Method
Table 7. Dissolved inorganic (DIC), organic (DOC) and particulate organic (POC) carbon in the incoming and outcoming water of the Susqueda Reservoir.
Carbon
Incoming
DIC 34,94 32,90 33,41 36,20 36,13 37,14 35,08
DOC 3,59 3,67 3,40 2,93 3,42 4,28 3,48
POC 0,85 0,30 0,16 0,39 0,73 0,66 0,47
DIC 34,68 31,45 30,89 29,95 31,61 33,74 31,90
DOC 3,18 4,07 4,90 3,44 3,33 3,56 3,83
POC 0,72 1,06 0,43 0,18 0,77 0,70 0,64
Outgoing
Apr 29 Jul 22 Sep 10 Nov 14 Jan 14 Mar 4 Annual weighted mean
5. RESULTS
Carbon in the inflowing water from Sau (CTin) and the outflow from the reservoir (CTout)All forms of carbon measured at the inflow and outflow of the
reservoir showed little variation during the study period. The tem-
poral pattern of POC is very similar to that described for the sedi-
mentation traps (Table 7). The sum of the annual weighted averages
(for the time between campaigns) of all forms of carbon in the in-
flowing and outflowing water reveals that during the course of study
the average carbon concentration in water from Sau (inflowing wa-
ter: 39.04 mg/L) was greater than that in the water flowing out to
El Pasteral (outflow water: 36.36 mg/L).
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Carbon from the sub-basin of the reservoir (TCdi)
During the study period there was an increase of 128.67 hm3 in the
volume of water stored, 70.73 coming from Sau and 57.94 from the
Susqueda sub-basin. The carbon load of the sub-basin was estimated
from existing data for the Riera Major creek (Butturini, 1997; Martí
i Sabaté, 1996), whose water inflow is approximately 50% of the an-
nual total, and assuming that similar carbon concentrations in all the
tributaries. According to the studies cited, the average concentra-
tion in this creek is 18.1 mgC/L, so the total load is estimated to be
1048.7 t .
Carbon incorporated in the water (TCsto)
The carbon incorporated in the water of the reservoir was calculated
as the difference between the final and initial volume (128.67 hm3)
multiplied by the average outflow concentration (36.36 mg/L). We
estimated 4678 t of carbon by this procedure.
Carbon accumulated in the sedimentation traps
The annual sedimentation of organic carbon (POC) and inorganic
carbon (PIC) was calculated from the sediment that accumulated
in the traps during the 309 days of the study (Figure 18). The
rate of sedimentation was 4 times greater at the tail of the res-
Figure 18. Particulate organic (POC) and inorganic (PIC) carbon sediment-ed in the traps during the study.
Figure 19. Evolution of the daily sedimentation rate of organic (POC) and inorganic (PIC) carbon in the sedimentation traps installed at the tail and the dam of the Susqueda Reservoir. The content of the traps was removed every two months.
ervoir than in the dam area; furthermore, in both zones the rate
of sedimentation of POC was greater than that of PIC. From the
data of the traps, whose content was removed every two months,
we determined the evolution of the carbon sedimentation rate,
which followed the pattern of water inflow from Sau in the tail
area and the pattern of thermal stratification of the reservoir in
the dam area.
Table 9. Organic and inorganic carbon sedimented during one year on the dam area of the Susqueda Reservoir.
Sedimented carbon on the dam area
Carbon sedimentation rate (bi-monthly traps) (g C cm2 year-1) 0,0211
0,0151
0,0138
0,0874Carbon sedimentation rate (monthly trap) (g C cm2 year-1)
Carbon
organic inorganic
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A CASE STUDY: THE SUSQUEDA RESERVOIR (TER RIVER, GIRONA)39
thermal stability (vertical mixing of the water column) but higher
hydrodynamic stability (lower water flow through the reservoir). In
the tail area, under far more complex hydrodynamic conditions due
to the influence of incoming water, the sedimentation rate should
be even more correlated to the incoming flow pulses. The larger in-
coming flows in January increased the sediment loads in the traps
placed in the tail area.
As the Susqueda reservoir has a narrow section and the water flow-
ing into it from the bottom of the Sau Reservoir is normally cold, tur-
bidity currents are quite often formed. Indeed, in February a mass
of turbid water (15 NTU) circulating at depth and generated by an
increase in the turbine discharge from the Sau Reservoir was de-
tected in the tail area.
The total carbon settled in one year (Table 9), calculated from the
data of traps placed in the dam area, will be compared later with the
data of carbon accumulation in the sediment at the bottom of the
reservoir, collected by core sampler.
Figure 19 and Table 8 show the low sedimentation levels during
the months of thermal stratification. When the mixing of the res-
ervoir starts, the sedimentation rate increases in the tail area,
which loses its vertical stability first, and then rises sharply in the
dam area. In February the rate of sedimentation in the tail area
increased significantly again. This evolution seems contradictory
because the highest rates of sedimentation are associated with
the times of vertical mixing of the water column. This shows the im-
portance of the hydrodynamic behaviour of the water mass stored
at each moment. In the dam area, the highest rate of sedimenta-
tion occurred in December, with low incoming and outgoing flows
both before and during the sampling (Table 8); when there is lower
Carbon (g C m-2 day-1)
DamPOC 0,38 0,21 0,15 2,14 –
PIC 0,26 0,09 0,07 1,45 –
POC – 0,29 0,81 0,34 1,79
PIC – 0,17 0,46 0,21 1,24Tail
inicial date
Final date
Nov 14 Jan 14
Mar 4Jan 14
Sep 10
Nov 14
Jul 22
Sep 10
Apr 29
Jul 22
Incoming flor Vol. (m3/s) 20,16 12,15 11,89 6,73 17,73
Incoming Vol. (hm3) 148,09 52,50 66,81 35,52 73,55
Outgoing Vol. (hm3) 67,55 49,36 74,10 47,41 67,31
Table 8. Daily rate of organic (POC) and inorganic carbon deposition in the sedimentation traps installed in the tail and the dam of the Susqueda Reservoir.
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A CASE STUDY: THE SUSQUEDA RESERVOIR (TER RIVER, GIRONA)39
Table 10. Sediment age at different depths in the Susqueda Reservoir, determined by absolute radiometric dating, together with sedimentation rate, carbon content and carbon accumulation rate in the sediments of each time interval.
Depth (cm) Year
Sedimentation rate
(cm year-1) (g C cm-2 year-1)
Carbon content
organic
(g C cm-2 year-1)(% C dw)
inorganic
Carbon accumulation rate
organic inorganic
1,3
3,5
5,5
7,5
9,5
11,5
13,5
2002
2000
1998
1996
1994
1991
1987
1,17
1,06
1,00
1,00
0,80
0,57
0,50
0,29
0,27
0,28
0,26
0,19
0,23
0,19
1,85
1,61
1,55
1,80
1,43
1,55
1,38
0,0209
0,0186
0,0190
0,0182
0,0131
0,0152
0,0120
0,0054
0,0042
0,0043
0,0047
0,0027
0,0036
0,0026
7,20
6,90
6,80
7,00
6,91
6,60
6,30
Carbon incorporated in the sediment (Ctsed) The content of the core sample obtained in the dam area was taken
as an estimation of the carbon incorporated in the sediment during
an interval in the life of the Susqueda Reservoir. Table 10 shows the
results of the core dating, and the carbon content of the different
sections of the sediment column. The table also indicates the cor-
respondence between the depth of the sediment and the year of
deposition, and the sedimentation rate calculated for each section.
Table 11. Particulate organic (POC), inorganic (PIC) and total carbon accumulation rate in the Susqueda Reservoir.
Inorganic carbon
Total carbon (t C year-1)
POC sedimentation rate
Total carbon (tC in 309 days)
Dam (core)
0,0209
149,2
0,0054
126,3
Tail (estimated)
0,0847
606,4 755,6
0,0220
513,4 639,6
Reservoir
The average sedimentation rate in the Susqueda Reservoir amounted
towas 0.84 cm/year, with a maximum of 1.17 cm/year in the most
recent layer (surface) and 0.50 cm/yr in the oldest layer (deep). In the
Sau Reservoir, using a 46.5 cm core, Armengol et al. (1984) estimated
the sedimentation rate by , correlating the apatite phosphorus concen-
tration with the monthly inflow of water into the reservoir. The resulting
sedimentation rates were 1.5 cm/yr in dry periods and 3-4.5 cm/year
in rainy periods.
Reservoirs act as decanters of sediment, so the rate of sedimenta-
tion was expected to be lower in the Susqueda Reservoir than in Sau,
which is located upstream. Since the most recent sediment sections
have higher organic and inorganic carbon content, we considered
the carbon accumulation rate of the uppermost layer of sediment,
which includes the last two years. Furthermore, data from the an-
nual traps were used to correct the extrapolation of the core result
to the whole reservoir, considering that, according to the results,
the sedimentation rate is four times higher in the tail area than in
the dam area.
The total volume of the sediment layer was determined as the prod-
uct of the sediment rate by the area of the reservoir bed potentially
occupied by sediment, i.e. that with a slope of less than 14% (Hakan-
son, 1981). By analyzing detailed maps of the dam area (1:10,000)
and the tail area (1:50,000) before the flooding of the reservoir, we
obtained a sediment area of 113.66 ha. The accumulation rates at
the tail (estimated) and dam (the core sample) were allocated to 50%
Net CO2 exchange (t C in 309 days)
Tail
11,0
Dam
74,9
Reservoir
85,9 (emission)
Table 13. CO2 exchange during different time intervals in the Susqueda Reservoir.
CO2 exchange Apr 29 Nov 14 Jul 22 Jan 14 Sep 10 Mar 4
Tail(mg C m-2 day -1)
(mmol C m-2 day -1)
-80,9
-6.742
59,8
4.983
-78,3
-6,527
106,7
8.890
-21,2
-1.766
173,5
14.461
Dam
Surface
(mg C m-2 day -1)
(mmol C m-2 day -1)
(ha)
-104,5
-8,711
317
81,2
6.768
434
8,5
711
432
512,5
42.708
430
-26,5
-2.211
438
222,0
18.503
451
of the total sediment area in accordance with the apparent hydro-
dynamic organization of the reservoir (Table 11). The accumulation
rates obtained by the core are slightly lower for organic carbon and
higher for inorganic carbon than those obtained from sediment traps
in the dam area (Table 12).
The data obtained from the core seem the best suited to perform
the final carbon balance, as they are an estimate of the carbon that
has actually accumulated in the sediment. On the other hand, the
data from the sediment traps are an estimate of the carbon that
could potentially end up in the sediment but may still be subject to
a variety of biological and physico-chemical processes. The estimate
could also be affected by situations of resuspension or accumulation
of carbonated matter through physical processes, according to the
hydrodynamics of the water body at any given time.
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A CASE STUDY: THE SUSQUEDA RESERVOIR (TER RIVER, GIRONA)41
Carbon incorporation to sediment in the dam
Accumulation rate in superficial sediment (core)
Carbon sedimentation rate (yearly trap)
Carbon
organic
0,0209
0,0151
inorganic
0,0054
0,0874
Table 12. Comparison between the rates of carbon incorporation to the sediments estimated by sedimentation traps and cores.
Exchange of CO2 between water and airThe diffusive flux of CO2 from the water into the atmosphere, or vice
versa, was calculated according to the following expression:
Flow = DG x (C - CO2 )a - (C - CO2)W
where:
• DG is the molecular diffusivity of CO2 in water, corrected for water
temperature (Broecker & Peng, 1982).
• [C-CO2]a is the carbon concentration (weight/volume) of CO2 in the
air, calculated as a function of atmospheric pressure at the altitude
of the reservoir, the air temperature and the amount of CO2 in the air;
• [C-CO2]w is the carbon concentration (weight/volume) of CO2 in the
layer corresponding to the epilimnion in summer (between 1.5 and
5 m from the surface) or the uppermost layer in winter (top 5-6 m).
It was calculated from the dissolved inorganic carbon data obtained
through the data on alkalinity, water temperature and pH (Stumm
& Morgan, 1996).
• Z is the boundary water layer, which was estimated based on the
average wind speed during sampling, from the calculation expres-
sionsformulae of Broecker and Peng (1982), applying a polynomial
relationship between the two variables.
The meteorological data (air temperature, atmospheric pressure
and wind speed) for calculating the exchange of CO2 between wa-
ter and air was taken at the meteorological station of the University
of Barcelona located at the Sau Yacht Club (coordinates: 41º 58’
32.15’’ N; 2º 23’ 54.44’’ E).
The evolution of the exchange of CO2 between water and air shows
that in the most productive months there is greater incorporation of
Z
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A CASE STUDY: THE SUSQUEDA RESERVOIR (TER RIVER, GIRONA)41
PERIOD
Inorganic carbon
Inorganic carbon
SUSQUEDA RESERVOIR
Studied period
Elapsed days
WATER OF TER RIVER
Volume (hm3)
Difference between input and output (hm3)
WATER OF TRIBUTARIES
Direct input from the reservoir basin (hm3)
CARBONO EN EL AGUA DEL TER
DIC load in the water (t)
DOC load in the water (t)
POC load in the water (t)
Total C load in the water (t)
Total C concentration in the water (mg/L)
CARBON IN THE WATER OF TRIBUTARIES
Total C load in the water (L)
STORED CARBON
Total C incorporated in the reservoir water (t)
AIR-TO-WATER CO2 EXCHANGE
CO2 emission (t C)
CARBON IN SEDIMENT
POC incorporated in the sediment (t)
PIC incorporated in the sediment (t)
C incorporated in the sediment (t)
Initial
Inorganic carbon
309
Initial
86,45
128,67
Input
376,46
70,3
Input
57,49
Input
13.068,0
1.329,6
185,8
14.583,4
39,04
Input
1.048,7
Final
Carbono ino
Final
215,12
Output
305,73
86,45
Output
9.684,9
1.167,5
190,9
11.043,2
36,36
Output
4.678,0
Output
85,9
Output
507,9
131,7
639,6
Table 14. Data for carbon balance (t) calculations in the Susqueda Reservoir for the 309 period.
CO2 into the water (negative values from April to September), where-
as in the least productive months (positive values from November to
January) CO2 is emitted into the atmosphere (Table 13). The month
of highest incorporation of CO2 in the study period was April, with a
value of -104.5 mg C m-2 day-1 in the dam area. The month of highest
emission of CO2 into the atmosphere was January, with a value of
512.5 mg C m-2 day-1.
The flows of exchange of CO2 were extrapolated to the total area of
the reservoir in each sampling campaign. The values obtained in the
tail and dam area were each assigned to 50% of the total surface of
the water layer; the sum of the two over the study period gave the net
exchange of CO2 (Table 13).
6. FINAL BALANCE
The results of the calculations for the carbon balance in the Susque-
da Reservoir during the study period are presented in Table 14.
With the performed approximations (Table 15) the inputs (15,632.1 t)
and outputs (16,444.7 t) in the final carbon balance are not entirely
equivalent. First, the errors associated with the local measurements
and partial estimations can be increased in the final extrapolations,
e.g. when transforming daily rates at a specific point into annual
rates for the whole reservoir. Second, the incorporation of carbon in
the sediment is low (639.6 t), but the emission into the atmosphere
is even lower (85.9 t) compared with the carbon inputs and outputs
from the flowing water. The emission of CO2 may be underestimated.
The air-water exchange is calculated from in situ measurements
during daylight hours, so the processes of respiration during the
hours when there is insufficient light for photosynthesis are under-
estimated.
7. DISCUSSION OF RESULTS
Carbon balanceDuring the study period, the amount of carbon that the Susqueda
Reservoir receives from the Sau Reservoir (14,583 t) plus the input
from the main basin of the reservoir (1,049 t) add up to 15,632 t,
whereas the amount of carbon stored in the water (4,679 t) plus the
amount exported to El Pasteral (11,043 t) add up to 15,722 t.
The operating model of the Susqueda Reservoir is as follows:
(CTin + CTdi= 15.632 t) ≤ (CTout + CTsto = 15.722 t) and CTsed ^=
640 t ==> autotrophic reservoir with CO2 fixation.
Most of the carbon entering the reservoir from the basin is exported
downstream. The sediment accumulates 640 tonnes. The orga-
nic fraction (508 t) is significantly higher than the inorganic fraction
(132 t). There is a net emission of about 86 t of carbon into the at-
mosphere as CO2. This low emission implies a net retention of carbon
in the reservoir. However, the carbon accumulated in the sediment
and the emitted into the atmosphere are much less than the carbon
entering the reservoir by the inflow (15,632 t).
DIC is the form of carbon that most contributes to the inflow
(13,068 t) and outflow (9,685 t) load in the Ter River. POC has
a lower contribution, with 186 t in the inflow water and 191 t
in the outflow water. Notably, dissolved carbon (DIC and DOC)
is greater and particulate carbon (POC) is slightly lower in the
inflow than in the outflow. This would indicate that the system
is exporting particulate organic matter, which could come from
the main basin of the reservoir or even might be matter fixed by
the reservoir.
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A CASE STUDY: THE SUSQUEDA RESERVOIR (TER RIVER, GIRONA)43
Table 15. Carbon balance in the Susqueda Reservoir during the study (309 days).
Total 15.632,1 16.444,7
TCen
TCed
TCsal
TCsed
C from CO2 atm
DCTwater
Input from Sau Reservoir
Input from tributaries
Output into El Pasteral
Incorporation to sediment
Emission into the atmosphere
Stored in the reservoir
Inputs(t)
14.583,4
1.048,7
Outputs(t)
11.043,2
85,9
Accumulated(t)
639,6
4.679,0
Figure 20. Carbon balance in the Susqueda Reservoir during the study period of 309 days (TC: Total carbon; OC: Organic Carbon; IC: Inorganic carbon; POC: Particulate carbon; DOC: Dissolved organic carbon; PIC: Particulate inorganic carbon).
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A CASE STUDY: THE SUSQUEDA RESERVOIR (TER RIVER, GIRONA)43
Emission of CO2 into the atmosphereThe rate of carbon emitted into the atmosphere as CO2 (86 t in the
study period) for the Susqueda Reservoir, expressed as a rate, gives
the figuresis shown in Table 16.
ever, the Susqueda Reservoir retains carbon in the sediment, and
its amount (665 g/m2.year) is far higher than that of the replaced
Mediterranean woodlands and shrublands. (between 140 and 80
g/m2.year; Table 4). Therefore, the dam has significantly increased
the carbon sink capacity of the occupied land.
Overall, the contribution of the Susqueda Reservoir to the green-
house effect is as follows (values rounded to unity):
• Net emission (NE) of the water body = +102 t/year
• Difference between the maximum fixation capacity in the reservoir
and in the replaced terrestrial ecosystem (EcF) = - 597 t / year
• Carbon retained in the sediment coming from the basin
(CRs) = -756 t/year.
It can be deduced from the above data that the Susqueda Reservoir:
a) Emits some carbon as CO2 into the atmosphere.
b) Retains in the sediment some of the carbon incoming from its
own basin and from the Sau Reservoir (the fixed amount is about
7.5 times greater than the amount emitted into the atmosphere).
c) Fixes more carbon (almost 5 times) than could be fixed by the origi-
nal Mediterranean terrestrial ecosystem before flooding.
With the data obtained, we conclude that the Susqueda Reservoir
acts as a net carbon sink in absolute terms in comparison with the
terrestrial system that it replaced.
Mean CO2 emission rate
Susqueda dam
Tail area in the Susqueda Reservoir
mg CO2 m-2 day-1
398
48
g C m-2 year-1
39,6
4,9
Table16. Rate of CO2 emission into the atmosphere during the study.
Fixation of carbon in the sedimentsThe total carbon retained in sediment is about 640 t in the
309 studied days; i.e. 756 t per year. This accounts for 665
g/m2.year in the 113.66 ha of the reservoir bed..The calculated
fixation rate of dry organic matter in the seston (about 1663
g/m2.year), it is well above the normal range of sedimentation in
lakes (60 to 200 g/m2.year).
Contribution of the reservoir to the greenhouse effectAAt the gas exchange level, the contribution of CO2 emission from
the Susqueda Reservoir to the greenhouse effect is modest. Moreo-
ver, we must keep in mind that this emission forms part of the cur-
rent carbon cycle, i.e. of the ecosystem respiration in the basin. How-
IMPORTANT FINAL QUESTIONS45
Between 1993 and 1999, comparative studies of emissions of
greenhouse gases in boreal reservoirs (Finland, Canada) and humid
tropics (French Guiana, Brazil) were carried out. Based on the results
of these studies, and also on those obtained in the Susqueda Reser-
voir, we will try to answer some frequently debated questions about
the role of reservoirs in climate change:
Is it true that reservoirs emit more greenhouse gases than the
natural ecosystems they replace?
In boreal climates forests play a limited role as carbon sinks. Carbon
is emitted during forest fires and after carbon leaching from soils and
forest cover to the watercourses and water bodies, where carbon
compounds are broken down and emitted into the atmosphere. Fur-
thermore, boreal soils fix little carbon.
IIn humid tropical climates, forests may be even more limited sinks
than in boreal climates, since the carbon is rapidly recycled and re-
used by the dense biomass. In these environments, seasonal rains
can flood large areas of forest for long periods (up to 6 months) to a
great depth (2-18 m), so these areas act as tropical reservoirs with
regard to CH4 emission.
The creation of a dam does not reduce the carbon stock in a basin,
but stores it in sediments, where is in a more sealed and stable form
than in forest soils. A biased way of explaining the balance of green-
house gases from reservoirs is to ignore the emissions of ecosys-
tems before the flooding of the valleys. Unfortunately, this type of ap-
proach is still used widely, but it seems clear the inaccuracy of carbon
balance analyses that only consider gross emissions from reservoirs
and not those from the replaced terrestrial ecosystems.
Is it true that reservoirs produce more emissions than natural
lakes and rivers?
In boreal climates emissions of CO2 and CH4 from natural lakes are
similar to those of reservoirs after the initial maturation stage as-
sociated with the flooding of the valleys, which usually takes 6-10
years. In this initial phase, the emissions from reservoirs may be
higher due to the decomposition of the more degradable organic
matter (dead leaves, humus, etc.) covered by the waters. Hence
the importance of taking into account the maturation state of the
reservoirs in greenhouse gas emission studies. In humid tropical
climates methane emissions from rivers and floodplains are similar
to those of reservoirs.
With our current knowledge, what can be said about the net emis-
sions from reservoirs?
There is considerable variability in results depending on several factors.
(trophic characteristics of the reservoir, persistence of thermal strati-
fication, water residence time, surface water layer, basin morphology,
degree of anoxia, etc.). However, in general it can be concluded that
for medium-sized boreal hydroelectric reservoirs (63 km2/TWh) the
emission factor of greenhouse gases per unit of energy produced is
about 11 kt CO2 equivalent/TWh (Gagnon, 2002). This accounts for
1% of the equivalent emissions of a coal power plant and 3% of a
natural gas power plant. The Susqueda Reservoir, with 24 km2/Twh,
only has an emission factor of 2 kt CO2 equivalent/Twh.
In the case of tropical reservoirs, for a comparable surface water
layer the net emission can be up to 10% of that of a coal power plant.
In extreme cases of large, permanently anoxic reservoirs with meth-
ane production, the net emission per unit of energy produced may
be similar to the emission of a coal power plant. Finally, it should be
recalled that all the carbon processed by reservoirs belongs to cur-
rent biogeochemical cycles, in which reservoirs are merely compart-
ments. The role of this carbon emission in the atmospheric balance
related to climate change is not comparable to that resulting from
the use of fossil fuels.
BIBLIOGRAPHY47
BIBLIOGRAPHY
• Agència Catalana de l’Aigua (2002). Regionalització del sistema
fluvial a les Conques Internes de Catalunya. Departament de Medi
Ambient, Generalitat de Catalunya. 92 pp.
• Armengol, J., Crespo, M. & Morguí, J.P. (1984). Phosphorus com-
pounds in the sediment of the Sau reservoir (Barcelona, NE Spain)
throughout its twenty-year existence. Verh. Internat. Verein. Limnol.,
22: 1536-1540.
• Broecker, W.S. & Peng, T.H. (1982). Tracers in the sea. Lamont-
Doherty Geological Observatory. New York. 690 pp.
• Butturini, A. (1997). Contribution of the boundary zones on nutrient
dynamics in a stream with Mediterranean regime. Tesis doctoral.
Departament d’Ecologia, Universitat de Barcelona.
• Duchemin, E., Lucotte, M. & Canuel, R. (1995). Production of the
greenhouse gases CH4 and CO2 by hydroelectric reservoirs of the
Boreal region. Global Biogeochemical Cycles, 9(4): 529-540.
• Duchemin, E., Canuel, R., Ferland, P. & Lucotte, M. (1999). Étude sur
la production et l’émission de gaz à effet de serre par les réservoirs
hydroélectriques de l’entreprise et de lacs naturels (Volet 2), Univer-
sité du Québec a Montreal. 47 pp.
• Gagnon, L. (2002). The International Rivers Network Statement on
GHG emissions from reservoirs, a use of misleading science. Inter-
national Hydropower Association. Sutton (UK). 9 pp.
• Hakanson, L. (1981). A manual of lake morphometry. Springer-
Verlag. Berlin. 78 pp.
• Lean, J. (2004). Solar Irradiance Reconstruction. IGBP PAGES/ World
Data Center for paleoclimatology. Data Contributión Series 2004-35.
NOAA/NGDC Paleoclimatology Program, Boulder (CO). USA.
• Leman, E. & Leman, D. (1981). Nitrous oxide concentrations in fresh
waters of the Great Lakes basin. Limnol. Oceanogr., 26: 867-879.
• Martí, E. & Sabater, F. (1996). High variability in temporal and
spatial nutrient retention in Mediterranean streams. Ecology, 77:
854-869.
• Mengis, M., Gachter, R. & Wehrli, B. (1997). Sources and Sinks of
nitrous oxide (N2O) in deep lokes. Bigeochemistry, 38: 281-301.
• Rosa, L.P., Sikar, B.M., Sikar, E.M., & Santos, M.A. (1997). A model
for CH4 and CO
2 emission mean life in reservoir based on data from
an Amazonian hydroplan. In: Hydropower plants and greenhouse
gas emissions, Rosa, L.P. & Santos, M.A. (eds.). COPPE.
• Rosa, L.P., Matvienko, B., Santos, M.A. & Sikar, E. (1999). Relatório
Eletrobrás/Fundaçao Coppetec. Inventário das emissoes de gases de
effeito estufa derivadas de hidrelétricas. COPPE report to Eletrobras.
• Stumm, W. & Morgan, J.J. (1996). Aquatic Chemistry. Chemical equi-
libria in natural waters. 3rd Edition. John Willey & Sons Inc, N.Y.
• Therrien, J., Tremblay, A. & Jacques, R. (2005). Emissions from
Semiarid Reservoirs and Natural Aquatic Ecosystems. In: Tremblay,
A., Varfalvy, L., Roehm, C. & Garneau, M. (2005). Greenhouse Gas
Emissions: Fluxes and Processes, Hydroelectric Reservoirs and
natural Environments. Environmental Science Series. Springer. New
York. 233-250 pp.
• Tremblay, A. Therrien, J. Hamlin, B. Wichmann, E. & LeDrew, L.
(2005). GHG Emissions from Boreal Reservoirs and Natural Aquat-
ic Ecosystems. In: Tremblay, A., Varfalvy, L., Roehm, C. & Garneau, M.
(2005). Greenhouse Gas Emissions: Fluxes and Processes, Hydroe-
lectric Reservoirs and natural Environments. Environmental Science
Series. Springer. New York. 209-231 pp.
• Tremblay, A., Varfalvy, L., Roehm, C. & Garneau, M. (2005). Green-
house Gas Emissions: Fluxes and Processes, Hydroelectric Res-
ervoirs and natural Environ ments. Environmental Science Series.
Springer. New York. 732 pp.
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