Enormous amounts of fresh water is lost from open air reservoirs by evaporation every year. Could this be reduced by covering the reservoirs with a clear plastic sheet?
I don’t propose just laying a big poly tarp on top of the water. But something like a large reinforced plastic sheet that is suspended on floating pylons in the water. The sheet would attach to the pylons to form peaks with air vents, and in beween the plastic would dip down to the water surface and have drain holes there for rain water and condensation to pass through. If the plastic passed a reasonable portion of the light spectrum, or maybe even filtering to something beneficial to the aquatic environment, and enough air transfer were maintained could this significantly reduce evaporation in reservoirs? Also, if anyone knows, could this be practical?
The fishing would suffer. And migrating birds would be upset.
You would still lose water since you are allowing air transfer, but you would probably reduce it a fair amount. But the initial cost and ongoing maintenance costs would be more than the value of the water.
I would guess that increasing the depth of the reservoir (so that the ratio of total volume to surface area is increased) might be a better course of action.
This would never happen in the US, as most dams (that created those reservoirs) were only approaching cost-effective if you factored in “recreational benefits.” In other words, people supposedly happily frolicking on the lake thus formed. Remove those benefits with Super Saran Wrap and the reservoir/lake becomes, retroactively, a bad idea–and possibly a candidate for dismantling.
Whether it would work is an engineering problem, but I would guess a formidable one even for a relatively small reservoir. The initial cost+maintenance could be prohibitive. I think it would be much easier to regulate downstream usage more closely with an eye to conservation; many farmers downstream from Bureau of Reclamation dams, for example, irrigate with the equivalent of 120+ inches of rainfall per year. There are rice paddies all over the Sacramento and San Joaquin valleys.
Dag Otto, fishing would take a hit. I think the fish themselves would be a bigger concern. Some birds would have trouble, others could adapt. But birds might take the biggest hit. I guess those reservoirs which have a sufficient source could be deepened. Why isn’t this being done more?
Not all reservoirs are supported by recreation, or even allow it. The NYC reservoirs only allow fishing as part of the deal where the land was donated to them. No other recreation is allowed. Many others allow no recreation at all. After all, they are drinking water sources. And it is the ones supporting metropolitan areas I was thinking of. There aren’t any downstream agricultural uses. But certainly more reservoirs could be made, even if they were just giant underground storage tanks, And as suggested above, the reservoirs could be deepened.
I do understand the annual water crises in many areas are highly political. Perhaps there are plenty of existing means of conserving water and annual summertime water crises are just advantageous for politicans. Maybe there isn’t even a problem to be solved.
You’d have problems keeping the water oxygenated. Without allowing contact between the atmosphere and the water surface, there’s no way for additional oxygen to get into the system, and the plastic sheet is likely blocking enough light to reduce primary productivity (photosynthesis). You’d end up with an anoxic water column that resulted in fish kills and other problems (particularly odors) pretty quickly. It would be a tough sell both to folks downstream and to the communities who pull their water supply from the reservoir.
Evaporation is a surface phenomenon, and it’ll happen at roughly a uniform rate over the entire reservoir. to reduce the losses from evaporation, you’d have to reduce the surface area. Dredging to increase depth doesn’t get you much benefit in this respect, and it’s awfully expensive. Based on the numbers here, for example, the cost to increase the average depth of Lake Mead by six inches would be in the neighborhood of a billion dollars plus whatever it costs to dispose of 100M cubic yards of muck. For that much money, you’d get little change in the water loss.
Evaporation is something that water managers in the west have struggled with for decades, but it’s just an unavoidable fact of life for open water surfaces. If you want to reduce evaporation, you’re better off looking at irrigation practices - it’s certainly feasible to cover diversion ditches (see this project in India) and you can irrigation systems can be designed to reduce evaporative losses. Unfortunately, you’ll run into political concerns here, as well - water laws were written to control quantity and ensure that the available water was used, but there’s little incentive in the law or administration of it for conservation practices. It’s catching up, but it’s slow going.
We stayed at a motel in Boise that, at least in January, had the outdoor surface covered with peanuts. It was not really to reduce evaporation, per se, but to reduce heating costs (though evaporation does lead to heat loss). Probably not an ideal choice for an impounded reservoir, but who knows. Maybe just use a bunch of tiny Japanese-style glass fishing floats.
The plastic sheet/tarp idea in the OP is impractical due to wind. It would just flap around in the wind and tear apart. Unless it was really strong material with a lot of anchors. In which case it would cost too much to be effective in most situations.
So people look at large number of floating things (small plastic balls/beads and such) or stable “coatings” of an oil/plastic nature. Lots of problems there (including wind, of course) and widescale use is decades away. Pools are one thing, reservoirs hundreds of acres in surface area are another.
In desert areas where evaporation is most of a concern, the reservoir is probably completely unnatural to begin with and the wildlife in and around it wasn’t there to begin with so if it goes away after a coating, that might not be a problem in some cases. Some reservoirs are seasonal anyway, designed to just hold water for the growing season and refill the next winter/spring. Fish don’t survive in those anyway.
It’s not just the top, but the bottom that’s a big problem. Lot of water is lost into the ground in the reservoir or in canals.
I recall a case where a reservoir was covered with plastic balls for similar purposes…
Ah, here it is. This reservoir in LA was covered with floating plastic balls, though the express purpose is to block sunlight that is reacting with chemicals in the water and producing a potential carcinogen. Still, the balls might be preventing evaporation as well. Though white or silver balls would better reflect light and keep temperatures down, and do a better job and reducing evaporation.
I’m guessing that this sort of solution is only cost effective in comparison to purification measures that would remove the chemicals in question. But where water is much more scarce, such measures could conceivably be used as a stopgap measure to help protect freshwater supplies. For example, I bet it’s cheaper than a desalination plant that can replace a lot of evaporated water.
I think such a scheme would have limited applications due to concerns around wildlife and aquatic vegetation.
Also, in my city we have a few covered reservoirs. They create a cement cavern (massive cistern, I guess) and cover it. They then put athletic fields on top of it. I imagine they do so to prevent tampering with drinking water more than to discourage evaporation.
Also, keep in mind that evaporation is at least potentially mitigated by increased rainfall.
Regarding the message from tripolar on 08-15-2012 10:50 PM. NYCDEP allows fishing and boating (no motors) on all NYC resrvoirs upstate as part of the watershed and land use agreements made with the communities. This includes sailboating and kayaking as well as fishing. It has nothing to do with land donation. See link for more: http://www.nyc.gov/html/dep/html/recreation/index.shtml
Yes, boating and fishing are allowed, but boats must be kept at the reservoirs and can’t be used other places to prevent transfer of undesired species. Do you have any info on the origin of the regulations?
Without knowing for sure, I would guess that they were a response to the introduction of zebra mussels and quagga mussels to US waters in the late 1980s. These species colonize everything, but they’re particularly hard on infrastructure and they seem to prefer structures like intakes that have a constant supply of flowing water. They’re also suspected of being a disease vector for birds, and they’re certainly present in some of the upstate reservoirs for NYC.