Vesistöpäästöt

Vesistöpäästöt, which translates from Finnish as “water reservoir emissions”, refers to the greenhouse gas emissions that originate from artificial lakes and reservoirs created for hydroelectric power, irrigation, and water supply. These emissions primarily consist of carbon dioxide and methane released by the decomposition of submerged organic material and aquatic vegetation when a reservoir is first filled or when water levels fluctuate significantly. The process has been compared to peatland and wetland oxidation, where oxygen depletion in stagnant water encourages anaerobic microbial activity that generates methane.

The magnitude of vesistöpäästöt depends on several factors: the amount and type of organic material present before inundation, temperature, watershed characteristics, and the size of the reservoir. Tropical reservoirs tend to produce higher emissions than temperate ones, largely due to warmer temperatures and richer vegetation. In some large hydroelectric projects in the Amazon basin and Southeast Asia, research has shown annual emissions on the order of several hundred to a few thousand metric tons of CO₂-equivalent per megawatt of installed capacity.

Many environmental assessments now include vesistöpäästöt in life‑cycle analyses of hydroelectric potential. Mitigation strategies are being explored, such as pre‑drainage of vegetation, construction of atmospheric return pipes, and selective harvesting of detritus before inundation. While hydroelectric power remains a low‑carbon source in terms of grid electricity, accurately accounting for reservoir emissions is essential to ensure that the overall climate impact of hydropower projects is assessed fairly.