Among other things, dams serve as reservoirs for drinking water, agricultural irrigation or the operation of hydroelectric power stations. Until now, it has been assumed that dams act as net carbon stores. Researchers from the Helmholtz Center for Environmental Research (UFZ) as well as Spanish scientists from the Catalan Institute for Water Research (ICRA) in Girona and the University of Barcelona have shown that dams release twice as much carbon that they store. The study was published in Geosciences of nature.
Whether they are leaves, branches or algae, streams carry large amounts of carbon-containing material. If the water is dammed, the material gradually settles and accumulates at the bottom of the body of water. “Due to the lack of oxygen, the degradation processes are much slower there. Therefore, less carbon dioxide is released. The carbon contained is stored in the sediment of the dam for a longer time,” explains Dr Matthias Koschorreck, a biologist in the lake research department at UFZ. “It was assumed that the dams store roughly the same amount of carbon as they release as greenhouse gases.”
However, for the carbon footprint of water bodies, not only areas covered by water – but also those that temporarily dry up due to a drop in water level – play a role. Koschorreck’s task force demonstrated this in previous studies. If the carbonaceous material previously covered with water comes into contact with atmospheric oxygen, degradation processes and therefore the formation of carbon dioxide are strongly involved. “Areas of water that dry up therefore release considerably more carbon than areas covered by water,” explains Philipp Keller, a former doctoral student at the UFZ lake research department. “If large amounts of water are released from a dam, large areas are suddenly exposed. But these areas were not taken into account when calculating the carbon footprint. It is the knowledge gap that we are filling with. our work.”
For their investigations, the researchers used a database based on satellite imagery. It contains monthly data on the size of the water surface of approximately 6,800 dams worldwide between 1985 and 2015. During these 30 years, scientists were able to determine exactly when, where and for how long the dams did not. were not completely filled and how big the dry areas were. On average, 15% of the total surface of the reservoir was not covered with water. The scientists used this figure to further calculate the carbon releases from these areas. “Our calculations show that the carbon emissions from the dams have been grossly underestimated. On a global average, they release twice as much carbon as they store, ”Koschorreck explains. “Their image as a net carbon stock in the global carbon cycle needs to be reconsidered.”
The data also shows that the magnitude of fluctuations in the water level of dams depends on both their use and their geographic location. “The fluctuations were more pronounced in dams used for irrigation than in those used for hydroelectric generation,” Keller said. “And in places where the annual precipitation regime is more uniform – such as near the poles and around the equator – there were less large fluctuations in water levels than in intermediate latitudes, where larger areas of the dams were often dry for much longer periods. “
Using the example of dams, the research team demonstrates the influence of drying areas on the global carbon footprint of water bodies. “We hope our study raises awareness that areas that dry up should also be taken into account when balancing the carbon flows of natural inland waters,” says Koschorreck. The new findings could also be incorporated into more climate-friendly management of dams. If, for example, water needs to be drained for maintenance, it makes sense to consider the best time for carbon release. If the work is done in the cold season instead of the summer, the degradation processes of exposed carbon-containing materials are much slower and carbon emissions are much lower.
In order to better understand the carbon footprint of dams, Koschorreck’s research team plans to take a closer look at carbon dioxide and methane releases as well as the role of vegetation on the carbon cycle of areas that have become dry.
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