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I found this article confusing: >There are three main differences between man-made reservoirs and lakes: >1. Man-made reservoirs require flooding terrestri

I found this article confusing: >There are three main differences between man-made reservoirs and lakes: >1. Man-made reservoirs require flooding terrestrial land, supplying a large pulse of dead organic matter from trees and grasslands. The timescale of how this happens to natural lakes is much longer. >2. Man-made reservoirs experience greater fluctuations than natural lakes. As reservoirs reduce in volume, the weight of the water over the sediments drop freeing even more methane molecules from the confines of their origin. >3. Man-made reservoirs are often closer to human activities, such as agricultural run-off containing fertilizers that can promote the growth of organic matter in the water. Here one-off effects are mixed with ongoing effects which makes limited sense. Also agricultural run-off is mixed in. But if that is not producing methane in the reservoir then in will be producing methane elsewhere - cause and effect is mixed up. Then later on the alternative: > Groundwater recharge basins are regularly maintained to have less organic matter at the bottom, since it can clog the percolation of water into groundwater aquifers beneath. In addition, recharge basins are shallower than man-made reservoirs and periodically become empty during dry periods when less water is available to recharge groundwater. I'm confused about a shallower basin being less problematic as the methane is produced by the wet sediment. A shallower basin for the same amount of hydro power would be much larger. What is miss is putting the generated methane in relationship with the generated energy.

ckastner 10y ago
> But if that is not producing methane in the reservoir then in will be producing methane elsewhere Not necessarily. The article explains methane production as so: [...] bacteria eating this dead organic matter to depend on other molecules containing oxygen. These include nitrate, phosphate, sulfate, and carbon dioxide. The least appealing molecule to breathe for these microbes is carbon dioxide because it requires the greatest energy to break the molecule apart to access the oxygen. When these microbes have no choice than to breathe carbon dioxide, a process called “methanogenesis” occurs.
Brakenshire 10y ago
Methane is produced by anaerobic digestion, which I would think is much more likely to happen when a whole area is flooded at once. The same biomass bring broken down in atmospheric oxygen would have a significantly lower effect. Agree the mix of ongoing and one-off effects is confusing.
schiffern 10y ago
To borrow a phrase from Sheldon Brown, "As always, Bill Mollison has the answer." :) (for those unfamiliar with what I affectionately call the "Perma-cult": https://en.wikipedia.org/wiki/Bill_Mollison https://en.wikipedia.org/wiki/Bill_Mollison) >So, it seems as though hydropower is not as clean as we once thought. But that’s isn’t a good reason to be ripping out dams. Hydropower generation often replaces much dirtier sources of energy, such as coal and even natural gas. In addition, dams provide us a wide range of public benefits such as flood control, recreation, and water management. It sounds like the Permaculture strategy of disfavoring singular large dams (huge concrete construction projects) and favoring smaller "farm dams" (cheap earthen construction) is the way to go. The smaller scale means more surface area:volume, increasing the equilibrium oxygen level in the water and reducing methanogenesis. Also, the typical Permaculture dam construction practice is to scrape off and conserve all the topsoil from the inside of the dam. This should further reduce methane production (which also represents squandered biomass, ie stored carbon). This technique is practical on the scale of a farm dam, but totally impractical for a huge reservoir. Farm dams have cost advantages in flood control and water management over monolithic dam projects, since they're near to the big consumers of water (farms). So the largest diversion canals and tunnels never need to be built. Simple gravity-fed water run through plastic pipes serves for irrigation purposes, so it improves on-farm water security. >groundwater storage may be a ‘cleaner’ alternative. Groundwater recharge basins are regularly maintained to have less organic matter at the bottom, since it can clog the percolation of water into groundwater aquifers beneath. In addition, recharge basins are shallower than man-made reservoirs and periodically become empty during dry periods when less water is available to recharge groundwater. This periodic drying out helps keep soils aerated, whereas man-made reservoirs can stay inundated with water for longer periods of time. I wonder how "groundwater recharge basins" differ from swales (long on-contour ditches)? They also intercept large quantities of runoff and divert it to recharging groundwater. They also require minimal earth movement, since it's a small surface feature compared to farm dams. Swales can also feed into dams, dramatically increasing their total catchment and infiltration capacity (since before the dam overflows, it floods and soaks the length of the swale). https://www.youtube.com/watch?v=E79C9tdcgEA https://www.youtube.com/watch?v=E79C9tdcgEA http://permaculturenews.org/category/earthworks-earth-resources/dams/ http://permaculturenews.org/category/earthworks-earth-resour...

ckastner 10y ago

> But if that is not producing methane in the reservoir then in will be producing methane elsewhere Not necessarily. The article explains methane production as so: [...] bacteria eating this dead organic matter to depend on other molecules containing oxygen. These include nitrate, phosphate, sulfate, and carbon dioxide. The least appealing molecule to breathe for these microbes is carbon dioxide because it requires the greatest energy to break the molecule apart to access the oxygen. When these microbes have no choice than to breathe carbon dioxide, a process called “methanogenesis” occurs.

Brakenshire 10y ago

Methane is produced by anaerobic digestion, which I would think is much more likely to happen when a whole area is flooded at once. The same biomass bring broken down in atmospheric oxygen would have a significantly lower effect. Agree the mix of ongoing and one-off effects is confusing.

schiffern 10y ago

To borrow a phrase from Sheldon Brown, "As always, Bill Mollison has the answer." :) (for those unfamiliar with what I affectionately call the "Perma-cult": https://en.wikipedia.org/wiki/Bill_Mollison https://en.wikipedia.org/wiki/Bill_Mollison) >So, it seems as though hydropower is not as clean as we once thought. But that’s isn’t a good reason to be ripping out dams. Hydropower generation often replaces much dirtier sources of energy, such as coal and even natural gas. In addition, dams provide us a wide range of public benefits such as flood control, recreation, and water management. It sounds like the Permaculture strategy of disfavoring singular large dams (huge concrete construction projects) and favoring smaller "farm dams" (cheap earthen construction) is the way to go. The smaller scale means more surface area:volume, increasing the equilibrium oxygen level in the water and reducing methanogenesis. Also, the typical Permaculture dam construction practice is to scrape off and conserve all the topsoil from the inside of the dam. This should further reduce methane production (which also represents squandered biomass, ie stored carbon). This technique is practical on the scale of a farm dam, but totally impractical for a huge reservoir. Farm dams have cost advantages in flood control and water management over monolithic dam projects, since they're near to the big consumers of water (farms). So the largest diversion canals and tunnels never need to be built. Simple gravity-fed water run through plastic pipes serves for irrigation purposes, so it improves on-farm water security. >groundwater storage may be a ‘cleaner’ alternative. Groundwater recharge basins are regularly maintained to have less organic matter at the bottom, since it can clog the percolation of water into groundwater aquifers beneath. In addition, recharge basins are shallower than man-made reservoirs and periodically become empty during dry periods when less water is available to recharge groundwater. This periodic drying out helps keep soils aerated, whereas man-made reservoirs can stay inundated with water for longer periods of time. I wonder how "groundwater recharge basins" differ from swales (long on-contour ditches)? They also intercept large quantities of runoff and divert it to recharging groundwater. They also require minimal earth movement, since it's a small surface feature compared to farm dams. Swales can also feed into dams, dramatically increasing their total catchment and infiltration capacity (since before the dam overflows, it floods and soaks the length of the swale). https://www.youtube.com/watch?v=E79C9tdcgEA https://www.youtube.com/watch?v=E79C9tdcgEA http://permaculturenews.org/category/earthworks-earth-resources/dams/ http://permaculturenews.org/category/earthworks-earth-resour...