A lot of rice production involves standing water (and the burning of rice straw between seasons), and so rice farming is a major contributor to GHG emissions:
It also happens to be a staple for over 60% of the world's population. And unlike transportation and energy, obvious substitutes are much harder to identify.
The more I read about novel sources of GHG the more intractable this problem appears absent geoengineering.
>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.
> 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.
>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).
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?
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."
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.
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.
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.
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.
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.
According to the report, they supposedly underestimated methane production by 25%.
What surprised me more though was that even going be the previous value (which I'll simply assume to be 1.5%/1.25=1.2%), that sounds like a _lot_ of greenhouse gas contribution for a resource which in 2014 "only" contributed 3.8% to global energy consumption, according to [1].
but energy isn't their main benefit. flood control and proving water to cities and even farms is the main benefit. throw in the recreational opportunities, increased tax base because all that lake front is worth something, and it seems they are more than worth it.
to be honest I walked into that article expecting another dam killer tirade with an all new reason to take down more and was pleasantly surprised they defend the use of them in the end
Methane isn't the most potent greenhouse gas as stated in the article. Water vapor is both the most potent and the most abundant. It also has a positive feedback loop so the hotter the Earth gets, the more water vapor goes into the atmosphere!
A reason to not include water vapor in a list of "greenhouse gases" is its short lifetime in the atmosphere. If you inject extra water vapor there, it rains down and exits the atmosphere in a matter of days, whereas methane etc stay for years --- humans emitting water vapor vs. CO2/etc to the atmosphere has very different relevance for the physics. Of course, the vapor does have effect on the radiation physics, and the fact that the equilibrium concentration depends on temperature leads to the positive feedbacks.
People leave out water vapor because, as you note, its concentration changes depending on temperature. It's an effect, not a cause. Emit lots of steam and you don't increase the temperature, you add to the rain.
Water vapor does amplify the climate effect of other greenhouse gases.
Methane is about 25-100 times more potent than CO2. Nitrous oxide about 300 times than CO2.
Although, the biggest worry is the stored methane. [1]
The amounts stored are equivalent (in CO2) to about 20-35 years of human activity (2014 CO2 output). If that gets quickly released, who knows what will happen.
I would add that presumably water vapour, methane and carbon dioxide have been in the atmosphere influencing temperatures since before anthropogenic climate change. I imagine rising concentrations of atmospheric water vapour are partly a consequence of emissions of carbon dioxide and methane emissions. (I believe that methane breaks down into chemicals including water.)
I imagine that humans are more responsible for levels of atmospheric water vapour indirectly (through carbon dioxide and methane emissions) than directly.
I imagine temperature and concentrations of atmospheric water vapour are in equilibrium in the absence of changes in the concentrations of other greenhouse gases.
Maybe I should have written my comment as questions. I would have phrased it differently if I had sources.
My main reason for answering the parent comment was to suggest that most of any increase in water in the atmosphere might largely result from warming due to other greenhouse gases. It might be the case that global warming is almost solely a function of greenhouse gases other than water vapour, even if it largely had its effect via water vapour.
The most potent greenhouse gas is actually SF6 (Sulfur hexafluoride), a chemical used in some industry processes. Water vapor is however the most common one.
SF6 is used as an insulator in high voltage breakers and switchgear. It is heavier than air so it sinks. There is a YouTube video where they float a paper boat on a basin of sf6 gas and also breathe it and it makes your voice low, opposite of helium
I hotly disagree. Reservoir who empty at drought season typically have immeasurable methane output during good season. -- Lake bed dries up and bakes in the sun. For that, we can rule out Lake Folsom, California.
This can be seen as good news, I suspect. Now we have identified a substantial source of the gasses it gives us an opportunity to look at how we can fix it. Oygenation? Stirring?
I think ruminants including cows do significantly contribute to methane emissions by burping.
"Cows don't emit 400 quarts of daily flatulence, as the term is usually understood. According to Professor Johnson, they emit 400 quarts' worth of burps,known in polite circles as eructation."
http://www.straightdope.com/columns/read/832/do-cow-and-term...
And it is fairly self-evident when you think of it: cows are ruminants, and the process of rumination means fermentation before digestion. Methane is released in this fermentation, so it naturally comes as burping, not flatulence.
http://www.southasia.ox.ac.uk/sites/sias/files/documents/GHG...
It also happens to be a staple for over 60% of the world's population. And unlike transportation and energy, obvious substitutes are much harder to identify.
The more I read about novel sources of GHG the more intractable this problem appears absent geoengineering.