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Towards a new understanding of carbon processing in freshwaters: methane emission hot spots and carbon burial

Final Report Summary - HYDROCARB (Towards a new understanding of carbon processing in freshwaters: methane emission hot spots and carbon burial)

The HYDROCARB project addressed the C footprint of tropical hydropower. Hydropower is the world’s largest source of renewable energy, and while most northern countries have already exploited the majority of their hydropower potential, many tropical rivers are still not dammed. Africa and Latin America, for example, use only 8 and 25% of their hydropower potential. In these regions, the freely flowing water constitutes a great and unused source of renewable energy for economic development.

Hydropower is generally regraded a ‘green’ energy source since it is renewable, and does not release fossil carbon to the atmosphere. However, hydropower reservoirs can emit the greenhouse gases carbon dioxide and methane to the atmosphere. Some reservoirs in the tropics have been pointed out as large methane sources, which is of particular concern since methane is a 34 times stronger greenhouse gas than carbon dioxide. This has triggered an intense debate about the carbon footprint of hydropower. However, current knowledge on reservoir emission is still limited. Many reservoirs are huge (hundreds or even thousands of km2), and the differences in water quality, organic matter cycling and greenhouse gas emission can be large in different parts of the reservoir. Current knowledge is limited by relatively few measurements in only a few systems, often with methods that do not allow to capture variability in space. Also, all reservoirs store carbon in their bottom sediments, but the significance of this C sink in comparison to greenhouse gas emission remains unknown.

The HYDROCARB project investigated the carbon and greenhouse gas balances of four very different tropical reservoirs in Brazil, for the first time systematically covering the spatial variability of both emission and carbon storage, in order to be able to balance carbon source and sink fluxes.

The emission of greenhouse gases was highly variable in both space and time. Neglecting this variability, by e.g. only taking measurements close to the dam, can lead to serious underestimation of reservoir emission. River inflow areas were often characterized by elevated emission, particularly of methane. Drawdown areas, i.e. areas that fall dry at low water level, can contribute significantly to carbon dioxide emission. In order to get robust estimates of reservoir emission, there is therefore no way around major field measurement campaigns that cover space at different hydrological seasons.

The more productive a reservoir, i.e. the more nutrient input it receives, the higher its methane emission. Laboratory experiments showed that aquatic plant material can be rapidly decomposed to methane, and that the methane production of reservoir sediment is related to its nitrogen content. Reducing the nutrient supply to reservoirs is therefore a possible way of reducing reservoir greenhouse gas emissions.

Carbon burial in the sediments also varied in space, thus spatially resolved methods are needed. The variability in burial between reservoirs was less variable than the variability in emission. In general, reservoirs constitute an environment that is prone to bury carbon at particularly high efficiency, and consequently, we found reservoirs were responsible for about 40% of the total carbon burial in inland waters, even though they only make up 10% of the inland water area. The buried carbon originates from both sources on land and within the reservoir, and thus reservoir carbon burial consists to a certain degree a new, anthropogenic carbon sink. The overall carbon footprint of the studied four reservoir is still being analyzed, but preliminary data suggest that greenhouse gas emission outweighs carbon burial in all studied reservoirs.

HYDROCARB has impact on both on this field of science and related to management and policy, by 1) quantifying the variability of reservoir carbon emission and burial, and recommending strategies for robust measurement and estimation; 2) quantifying and providing conceptual understanding of the importance of sediment carbon burial for estimating the overall carbon footprint; and 3) showing that eutrophication, i.e. excessive nutrient supply, is a main driver of reservoir methane emission, such that improved nutrient management will lead to reduced reservoir emissions.