Hydropower produced by turbines from water flowing down a large reservoir retained by a dam is one renewable energy source tapped by countries endowed with rivers traversing a terrain with large elevation changes. Often touted to be clean, non-polluting and sustainable, hydroelectric power generation does have environmental concerns, primarily in the area of impacting the ecology of the river where it was built as well as water availability (and, by extension, socioeconomic issues) in areas downstream of the dam, which, after operation of the dam and hydroelectric power plant, would receive less water and sediment. The second resource, sediment, is crucial for downstream areas engaged in agriculture as river sediment serves as a rich source of nutrients for fertilizing the land.
To add to hydropower’s environmental concerns, a recent news report and journal articles (Guardian, Link1; PLoS ONE, Link; BioScience, Link) highlighted the unaccounted for emissions of greenhouse gas, principally methane, from the water storage reservoirs of dams. Being at least 20 times more potent in greenhouse warming potential (GWP) than carbon dioxide, methane is released from reservoir water by biotic processes such as anoxygenic decomposition of living matter in river water by microorganisms.
Quantification of methane released from the water reservoir, necessary for assessing the impact of this emission source on greenhouse gas inventories, is difficult given the complexity and inaccessibility of measuring in situ methane release rates from water reservoir. But quantification provides the numbers by which policy makers can assess the trade-offs incurred in hydroelectric power generation against the benefits of a continuous source of relatively clean energy. Hence, while asking what and why are common questions in science, the question of how much typically influence whether a decision would be taken, and the method of providing for this quantification step must be rigorously thought about and executed to deliver useful and accurate data for assessing the germane policy questions.
Given the acknowledged release of greenhouse gas methane from water storage reservoirs of dams, where do the science takes us from here? Personally, I think there is very little that we can do to alter the rate and nature of methane emissions from hydropower projects since methane emissions depend on a variety of factors hard to control for; for example, amount and type of sediment in the water, water level in the reservoir, phytoplankton invasion and growth, and solar insolation.