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Do hydro-electic dam reservoirs really fart?


Do hydro-electic dam reservoirs really fart?08-10-2015 00:20
trafnProfile picture★★★☆☆
(779)
I read an interesting article online last night about how the reservoirs behind hydro-electric dams are emitting huge amounts of methane from all the accumulated organic matter that is decomposing in them. This organic matter comes from dead plant material that washes into, and then sinks to the bottom of these reservoirs.

My first thought was that even though there was a lot of methane coming from these reservoirs, the same thing would have occurred if that organic matter hadn't washed into them. It would still have decomposed, except on dry land, and probably have exuded roughly the same amount of methane.

Then I read about how there tends to be agriculture near most of these reservoirs (I guess it makes sense to grow things near water) and that fertilizer runoff into the reservoirs was speeding up the decomposition process by causing more microorganisms that feed on the decomposing plant matter to grow in the water.

In the end, I found the whole thing intriguing, because the implication was that hydro-electric power was "dirty energy."

Farting reservoirs. Who knew?
08-10-2015 00:33
drm
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(67)
It applies to reservoirs in places with intense amounts of foliage, i.e. the tropics. Most larger dams in the US are in the desert so there wasn't much foliage to decompose.

And if there was no dam, the foliage would decompose, but then regrowth would pull that greenhouse gas back in again. Technically, it is the lack of regrowth and not the decay that is the problem.
Edited on 08-10-2015 00:34
08-10-2015 00:56
trafnProfile picture★★★☆☆
(779)
I guess you're probably right about some of the decomposing organic matter releasing some of its carbon back to the soil, but wouldn't some it also be released as methane into the air as it was being decomposed by microorganisms on dry land?

Also, just like some of the land-based decomposing carbon would be re-trapped in the soil, I'd imagine that some of the carbon from the organic matter at the bottom of the reservoirs would be trapped underneath overlying sedimentary layers.

I'm not trying in any way to diminish their findings, but I'm just wondering what the results would be of a comparative study of land-based to reservoir-based decomposition methane.

You'd probably also have to take into account all of the methane that was not being produced because of the submerged land that was no longer supporting vegetation and therefore no longer adding to the decomposition methane total.

Tricky questions, but interesting.
08-10-2015 01:10
drm
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(67)
I'm not saying that decomposing gas gets held in the soil. I'm saying that without a dam, a new plant regrows and absorbs greenhouse gases similar to what was emitted before.
Edited on 08-10-2015 01:11
08-10-2015 01:23
trafnProfile picture★★★☆☆
(779)
Oh, I see now, but this is about methane, and plants absorb CO2, not methane. But, yes, that CO2 absorption could help counterbalance the methane factor.

Either way, I think we both agree that there are probably a lot of mitigating and aggravating factors to this story which we're not aware of, yet. It's great, though, that there are people willing to think outside the box to find important climate change factors which others might not have even thought of.
08-10-2015 04:10
trafnProfile picture★★★☆☆
(779)
Hey drm,

I was really intrigued about the interaction of both methane and CO2 in the reservoir question, so I created the following figures to summarize it. This is, of course a gross oversimplification of what's actually happening, but I think it's an effective way of isolating the CO2 and methane GHG components.



In the top figure there's the valley between two hills before the reservoir is built, with the following variables:

A = Methane released from vegetation decay on hill A.
B = CO2 absorbed from plant respiration on hill A.
C = Methane released from vegetation decay in the valley.
D = CO2 absorbed from plant respiration in the valley.
E = Methane released from vegetation decay on hill B.
F = CO2 absorbed from plant respiration on hill B.

The overall atmospheric balance of the 2 GHG's for this figure would be:

A - B + C - D + E - F = net gain or loss of GHG's




In the bottom figure there's the valley between two hills after the reservoir is built, with the following variables:

A = Methane released from vegetation decay on hill A.
B = CO2 absorbed from plant respiration on hill A.
G = Methane released from vegetation decay in the reservoir.
H = CO2 absorbed by water in the reservoir.
E = Methane released from vegetation decay on hill B.
F = CO2 absorbed from plant respiration on hill B.

The overall atmospheric balance of the 2 GHG's for this figure would be:

A - B + G - H + E - F = net gain or loss of GHG's




Overall, if we assume that the effects from the two hills would be the same in both figures, then variables A, B, E and F would be the same for both figures. Thus, it would be the difference between "C-D" and "G-H" which would determine the overall net gain or loss between the two figures.

On the methane side, C would probably be greater than G, since C would be derived from both dead vegetation from the valley as well as dead vegetation which had traveled into the valley through gravitational pull and runoff. It's possible that G might be somewhat bolstered by decaying water born vegetation, but probably not enough to compensate for the loss of the natural valley floor covering.

On the CO2 side, we might be able to presume that D and H are fairly close, since oceans are known to be powerful CO2 absorbers. Thus, the CO2 absorption factor lost from the plants being absent in the reservoir filled valley might be counterbalanced by the CO2 absorbing capacity of the reservoir water itself.

If the above assumptions are correct with D equaling H, and A, B, E and F being the same in both figures, then this means (drum roll please for a surprise ending) that the reservoir free valley would actually be emitting more methane (C) than would be the reservoir filled valley (G).

Now that's not what we expected, is it? Comments and corrections are most welcome.
Edited on 08-10-2015 04:11
08-10-2015 18:34
trafnProfile picture★★★☆☆
(779)
(AUTHOR'S NOTE: I apologize for this showing up as a separate post as I was merely trying to edit the one above (I must have hit the wrong button), but this updates the above original post by expanding the explanation about the G variable in the paragraph which starts "On the methane side...")

Hey drm,

I was really intrigued about the interaction of both methane and CO2 in the reservoir question, so I created the following figures to summarize it. This is, of course a gross oversimplification of what's actually happening, but I think it's an effective way of isolating the CO2 and methane GHG components.



In the top figure there's the valley between two hills before the reservoir is built, with the following variables:

A = Methane released from vegetation decay on hill A.
B = CO2 absorbed from plant respiration on hill A.
C = Methane released from vegetation decay in the valley.
D = CO2 absorbed from plant respiration in the valley.
E = Methane released from vegetation decay on hill B.
F = CO2 absorbed from plant respiration on hill B.

The overall atmospheric balance of the 2 GHG's for this figure would be:

A - B + C - D + E - F = net gain or loss of GHG's




In the bottom figure there's the valley between two hills after the reservoir is built, with the following variables:

A = Methane released from vegetation decay on hill A.
B = CO2 absorbed from plant respiration on hill A.
G = Methane released from vegetation decay in the reservoir.
H = CO2 absorbed by water in the reservoir.
E = Methane released from vegetation decay on hill B.
F = CO2 absorbed from plant respiration on hill B.

The overall atmospheric balance of the 2 GHG's for this figure would be:

A - B + G - H + E - F = net gain or loss of GHG's




Overall, if we assume that the effects from the two hills would be the same in both figures, then variables A, B, E and F would be the same for both figures. Thus, it would be the difference between "C-D" and "G-H" which would determine the overall net gain or loss between the two figures.

On the methane side, C would probably be greater than G, since C would be continually derived from both dead vegetation from the valley as well as dead vegetation which had traveled into the valley through gravitational pull and runoff. As for G, after the reservoir had filled and the original valley plant life had drowned and decayed, it would only receive ongoing organic matter input from dead vegetation which had traveled into the valley through gravitational pull and runoff. It's possible that G might be somewhat bolstered by decaying water born vegetation, but probably not enough to compensate for the loss of the naturally recurring valley floor covering.

On the CO2 side, we might be able to presume that D and H are fairly close, since oceans are known to be powerful CO2 absorbers. Thus, the CO2 absorption factor lost from the plants being absent in the reservoir filled valley might be counterbalanced by the CO2 absorbing capacity of the reservoir water itself.

If the above assumptions are correct with D equaling H, and A, B, E and F being the same in both figures, then this means (drum roll please for a surprise ending) that the reservoir free valley would actually be emitting more methane (C) than would be the reservoir filled valley (G).

Now that's not what we expected, is it? Comments and corrections are most welcome.[/quote]




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