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After years of uncertainty, the science around hydropower’s carbon footprint is now clearer than ever, write Alain Kilajian, Deputy Executive Director of the Â鶹ÊÓƵ Sustainability Alliance and Sara Mercier-Blais, Research Associate for the UNESCO Chair in Global Environmental Change at the University of Quebec, Montreal (UQAM).  

Alain and Sara are part of the G-res Tool team, a multi-stakeholder research project looking at the carbon emissions of hydropower reservoirs around the world. In this blog, they look at the most up-to-date science to address frequently asked questions and common myths around emissions from hydropower, in particular reservoirs created by dams.

We need hydropower to address climate change and reduce global carbon emissions. True or false?

True. Global action against climate change is centred around a need to reduce carbon emissions. For the energy sector, this means a rapid switch to, and increase of, renewable and low carbon sources of electricity such as solar, wind and hydropower.  

By replacing coal and natural gas thermal plants, these low carbon alternatives provide the ability to offset a huge amount of emissions destined for the atmosphere. Today, hydropower’s flexibility and storage capacity are integral to tackle climate change, as they can help stabilise energy production when coupled to variable renewables, such as when solar and wind power are unavailable due to a lack of sunshine or wind.

Â鶹ÊÓƵ median emissions intensity is comparable to other renewable energies such as wind and solar. True or false?

True. The emissions intensity of any energy source is the amount of GHG emitted per unit of energy produced (mostly expressed in gCO2-eq/kWh). A study of nearly 500 global hydropower reservoirs using the G-res Tool published in Water Security and Climate Change: Â鶹ÊÓƵ Reservoir Greenhouse Gas Emissions (2021) found the median value for hydropower to be 23 gCO2-eq/kWh, which aligns with the Intergovernmental Panel on Climate Change (IPCC) estimate of .

When we compare this value with other energy sources, only nuclear and wind power have a lower average lifecycle GHG emission intensities than hydropower, both about 12 gCO2-eq/kWh. For solar energy, the value is 48 gCO2-eq/kWh. For gas and coal, the values are respectively.

Of course, this is only a median value. In a few rare and extreme cases, hydropower reservoirs have been documented to produce significantly higher emissions, while others have close to zero emissions or can act as carbon sinks.

Â鶹ÊÓƵ reservoirs do not release greenhouse gas (GHG) emissions. True or false?

False. It was long believed that inland waters (in other words, rivers, lakes and artificial reservoirs) did not release GHG emissions but instead simply acted like a pipe transporting carbon between the land and the ocean.

We now know that inland waters are actually . Lakes and rivers collectively emit as much CO2 than oceans take up. By square metre, lakes are 80 times more active than oceans and 30 times more active than terrestrial landscapes. As reservoirs are human-made lakes, they are also a source for GHG transformation.  

More up-to-date knowledge showed that in reality, despite common belief, hydropower reservoirs do emit GHG. The emissions primarily come from microbial processes that decompose organic matter into GHG. It should also be noted that the carbon processing in reservoirs can work two ways, both emitting and absorbing emissions. This explains why, in a limited number of cases, reservoirs can act as .

Â鶹ÊÓƵ storage projects tend to release more emissions than run-of-river plants. True or false?

True. The main difference between storage and run-of-river projects is the creation of a reservoir. A storage project requires a reservoir to store water to use when electricity is needed while a run-of-river plant, which generates energy using the natural flow of a river, has no or minimal water accumulation.

When a reservoir is created, a terrestrial environment is flooded and transformed into an aquatic environment. This flooding has multiple effects on the GHG emissions of the system.

First, when new land is flooded, more carbon, mostly from the flooded soil, is available to be transformed into GHG emissions. Second, as the water slows down and begins to accumulate in the reservoir, aquatic bacteria have more time to transform the available carbon into GHG emissions. More carbon and more time equals more emissions.

Finally, the longer the water stays in the reservoir (i.e. its residence time), the more it tends to warm up at the surface but stay cold near the bottom. This temperature gradient can create something we call a thermocline that acts as a physical barrier for small molecules like CO2 and CH4. Above the thermocline, you will have well oxygenated water where carbon can mix with oxygen from the atmosphere to create CO2. Below the thermocline, you can have an anoxic environment (no oxygen) where carbon is transformed into CH4 and because of it acts as a barrier, the CH4 produced in the deeper parts of the reservoir stays there. So, if a project’s water intake is located under the thermocline, CH4 will be released downstream through the turbine – a process we call 'degassing'.  

Degassing is one of four emissions pathways associated to reservoirs. The other three include CH4 bubbling, CO2 diffusion and CH4 diffusion. While run-of-river plants may still have varying emissions associated to the different pathways, these will tend to be lower than storage projects due to the lower water residence time of run-of-river plants and the small amount of impounded land.

All tropical reservoirs have high emissions. True or false?

False. It is well known and that GHG emissions from reservoirs tend to be higher in the tropics due to higher annual average temperature. This created the myth that all tropical reservoirs have high emissions.  

Temperature is only one of the many elements influencing the carbon footprint of reservoirs. The time water spends in the reservoir, the amount of carbon contained in the flooded soil and the amount of shallow area in the reservoir all contribute to the GHG emissions profile of hydropower reservoirs.  

For example, a large but very deep reservoir in the tropics producing a large amount of energy is most likely to have a low carbon footprint per unit of energy generated.  

Clearing vegetation in the impoundment area significantly reduces reservoir emissions. True or false?

False. While most people think that flooded trees and vegetation are the main source of carbon in the impoundment area, this is in fact not the case. Most of the carbon actually comes from the soil and more specifically the top 10cm of soil.  

The main reason is that the carbon from above ground biomass is much harder for bacteria to breakdown than the carbon available in the soil. A clear example of this is the Petit Saut reservoir in French Guiana where flooded trees are still being harvested 30 years after impoundment. Sometimes the flooded forest can even provide important habitat areas for the fish species in the reservoir.

Clearing vegetation has been documented to improve water quality in the reservoir. But for the clearing to be effective, all the material would need to be disposed. This brings its own challenges as burning, the option most often considered, has very significant impacts and exporting the biomass is not practically feasible, especially for large reservoirs.  

Field sampling is the only way to estimate emissions from hydropower. True or False?

False. With advances in and data availability related to GHG emissions from reservoirs, we now have the knowledge to predict emissions without going directly on site.  

One example is the GHG Reservoir (G-res) Tool. The uses a conceptual framework created by experts in the field that integrates up-to-date science in an online interface to estimate the GHG emissions from reservoirs. Such tools help hydropower companies and researchers estimate and report the net GHG emissions of a reservoir without the need to conduct expensive field sampling campaigns. They are especially valuable in the prefeasibility stage as a screening tool to avoid high-emitting projects.  

It is impossible to reduce reservoir emissions after a hydropower project is constructed. True or false?

False. Although it’s easier to implement carbon reduction measures in the design phase of a project, there are a few innovative ways hydropower owners can reduce reservoir emissions even after their projects are constructed.  

Below are a few examples:

• Changing operating level. The amount of shallow littoral area is one of the multiple factors that influence the GHG emissions of reservoirs. More littoral equals more CH4 production which equals more GHG emissions. In some cases, changing the operating level can reduce the amount of shallow littoral area which in turn will reduce the amount of GHG emissions from the reservoir.

• Installing aerating devices. Aerating devices can be installed to increase the dissolved oxygen in the water and reduce the amount of CH4 being released downstream.

• Adding a secondary intake above thermocline. Â鶹ÊÓƵ operators will often need to refurbish their asset over time. Any major refurbishment project provides an opportunity to add a secondary intake above the thermocline (or a multi-level water intake) to circulate oxygenated water through the turbines and reduce the amount of degassing.

• Converting methane emissions into energy. A has explored the possibility to recover biogenic methane release from reservoirs and transforming it into a potential source of energy. They suggest that the recovered methane could be pumped directly to large consuming centres, stored locally and burned by gas turbines for electricity generation or purified for use in transport. The approach both reduces emissions and provides a additional source of energy generation.  

There is no defined acceptable level of GHG emissions from reservoirs. True or False?

False. The Â鶹ÊÓƵ Sustainability Standard, as well as the Guidelines on Good International Industry Practice, state that a project with low emissions should have an emissions intensity less than 100 gCO2e/kWh. This emissions level can and should guide future new hydropower development.  

Similarly, the states that hydropower facilities in operation before 2020 are eligible for financing if their emission intensity is less than 100g CO2e/kWh. The corresponding criteria for facilities in operation after 2020 is 50g CO2e/kWh. These can be estimated using either the G-res Tool or a site-specific assessment.

Further reading:

• Climate change
• Factsheet about hydropower’s carbon footprint