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> 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.

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.

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)

>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

http://permaculturenews.org/category/earthworks-earth-resour...

Small hydroelectric is extremely efficient.

You can get efficiencies of above 80%. In larger installations 90% or more. It's expensive but beats the shite out of solar and wind over time. You can also, if you have an application for physical energy, roughly double the output.

In addition almost all energy storage capacity for the grid is pumped storage from hydroelectric stations. I think it is massively overlooked merely because most people are interesting in scaling and these people overlook the fact that A: most things do not scale and B: long term maintenance matters. You can run a hydro plant with next to no maintenance for 100 years. I can't think of anything close to it and they can easily be fixed or repaired.

It annoys me that micro and small hydro aren't talked about more.

You can get efficiencies of above 80%. In larger installations 90% or more.

You can also, if you have an application for physical energy, roughly double the output.

Could you explain the apparent contradiction? How are you calculating a 90% efficiency such that it's possible to get another 90% of "physical" energy?

Sure.

With a hydroelectric turbine you can produce electrical power (by black magic as far as I'm concerned) or mechanical power.

There is a terrific documentary called Ben's Mill on such a mechanical application here:

https://www.youtube.com/watch?v=l2KJbRHO76s

That will explain visually how mechanical power can be coached into performing useful work. It's also worth watching just to see the cows reaction to their new water vessel at the end, they are like animals out of Disneyland.

Mechanical power when converted to electrical power undergoes a drop in performance (similar to other power conversions, something is lost).

There is a fascinating article here on the subject, I quote:

"The hydro power installations in use today are actually less efficient than those of earlier centuries"

"Direct hydropower is at least twice as economically viable as a hydroelectric operation with the same rate of energy production."

http://www.lowtechmagazine.com/2013/08/direct-hydropower.htm...

Also take a look at this:

http://www.lowtechmagazine.com/2013/09/power-from-the-tap-wa...

That is hacking if I ever saw it! I intend to construct a micro hydroelectric plant myself, so ask away if you've any questions. I love the idea of marrying up old school technology with modern technology. I also have a terrific idea for lighting using heliostats, which combines tech literally thousands of years old with the latest in machine learning.

That's not even close to an explanation.

If 1kg of water descends 1m the most power you can hope to extract from that is U = m x g x h or 1 * 1 * 9.8 ~ 10 J (https://en.wikipedia.org/wiki/Potential_energy)

If you can extract 90% of that, it's 9J. If you can double that efficiency, it's 18J. Which would make the "over unity" generator folks on YouTube pretty happy, but the universe generally says you can't really do.

So which is it? Is hydro 90% efficient, or can you get double the power output if you want mechanical energy? You can't have both.

Hmmm. I'm not claiming to defy physics although I see how it came across that way. That was not my intention.

If you read the literature you can see why I was phrasing it like that so perhaps I too was misled.

http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/eng...

The potential energy harvest from a hydro setup must be 90% of available energy.

In practice half is lost due to the reasons outlined in the low tech magazine article I linked to. So the electricity production is lower than if we had the turbine hooked up to a motor. The other possibility is that small scale hydro is of a lower efficiency and that this explains the paradox. That is: the literature is talking of a large scale operation that can achieve 90% efficiency but smaller schemes incur losses that explain why converting a small hydro setup back to providing mechanical energy can double the output.

Assembling and paraphrasing your information:

Turning the shaft horsepower from a hydro turbine into electricity, transmitting it through wires, transformers, etc, and turning it back into shaft horsepower through motors, etc is inefficient due to conversion losses. Just using the shaft horsepower directly incurs none of those conversion losses. They can be substantial enough in some situations to approach 50%. This doesn't mean you can harvest more energy than the water can provide, it means that by eliminating losses you can USE more of the energy that the water provides, instead of losing it to heat or noise or whatever other conversion losses.

That's an explanation. What you have linked to and hinted at is only the start of one.

Hydropower is great.

Pumping water back "up the hill" for another pass through a circuitous swale system (ie a space filling curve) is also a good way to recycle nutrients. When compared to the embodied energy of fertilizer I think you'll come out ahead.