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This idea was inspired by a number of recent occurrences.
1. Dry land salinity and desertification is increasing, worldwide.
2. Potable water shortages are becoming increasingly common.
3. Certain "pest" tree species have become invasive, in certain areas, especially in arid Australia, USA
& China.
4. Water table accession (rising) has increased in areas cleared of trees, increasing salinity problems.
5. Air pollution and CO2 levels are rising.
6. Power consumption, for desalination, is increasing. Desalination technology has progressed very little, if at all, in the last 30 years.
It occurred to me that a confluence of these problems may lead to a potential solution:
Plant large quantities of "pest" salt-tolerant species in hot areas with brackish water tables (These are, coincidentally, most of the areas with water availability problems). Build massive clear covers (preferably plastic) over the plantations, to allow humidity control.
The following outcomes should be realised:
Each tree with a 3in diameter trunk puts about 60-75 litres (16-20 gal.) of water back into the atmosphere through normal transpiration. Harvesting that humidity with blowers and dehumidifiers should be simple enough. Harvesting condensation from the underside of the clear covers is even easier.
Optimal stand density is around 460 trees/acre = 294400 trees per square mile = 5.3 million gallons of water per day. Assuming you only catch 1/2 of it, you still get 2.65 million gallons, each day, for every square mile you plant.
Fresh water becomes relatively easy to produce, without the limitations, and power requirements, of reverse osmosis plants.
Judicious pruning should see an increase in foliage area, with height control. Biomass resulting from this activity could be used for clean, sustainable power generation.
Each tree also produces 1.5 - 1.8 times as much O2 as it does H2O. This is a good thing. There is a corresponding consumption of CO2, for this benefit.
Water table levels drop, reducing salinisation of soils.
Other plants may then be easier to establish, in the humid greenhouse environment, allowing groundcovers and grasses to reclaim desert. There are wells and soaks in most deserts, where the water table coincides with the surface.
The Very Small Version
http://www.webzine....oducer/page2578.htm
.. linked elsewhere .. [bpilot, Oct 05 2004]
[link]
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//progressed very little, if at all, in the last 30 years.// what makes you think that? |
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If you're going to cover vast areas with plastic what's the point in planting trees, you might as well just harvest the rainwater run off (the water table will start to fall with no input through natural sinks.) Otherwise you'll have to find some way of watering the trees. |
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CO2 is a different matter, but that seems to be only a secondary goal, and the area planted would have to be massive to have an influence. |
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I'm thinking of areas with relatively little rainfall. There is still an underground water resource, often brackish, in large areas of the Middle East and central Asia. That water resource is sufficient to water the trees, as it flows in from other areas. |
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it looks like it could work [zanz]. + |
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Doesn't the pruning affect the water output? Or did you already account for that in your figures? |
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I'd imagine these plants would die off as the salinity decreased, to be replaced with more "freshwater" plants. Or are there plants that could thrive in both environments? |
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What sort of timescale are we looking at before the area becomes self-sustaining? |
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I agree with scuba. Wouldn't the humid air (and condensation) form with or without the plants under the plastic sheet? |
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[xaviergisz], yes, the humid air would form. The process of water collection is hastened by the addition of the plants, which grow better in the hothouse environment. Were you ever shown the method of collecting drinking water from plant cuttings, by putting cut vegetation into a big plastic bag for a day? |
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[egbert], the plants I envisage being used are particularly salt tolerant... Casuarina equisetifolia (Horsetail sheoak); C. glauca (Swamp sheoak) and Tamarix ramosissima (Salt Cedar). All of them thrive in high salt conditions, and do particularly well in littoral dune areas (oceanfront areas). The salinity problems decrease as the water table drops. The trees can be left there to ensure the water table remains low, helping to ensure a long term solution. |
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The pruning forces denser growth, increasing the leaf surface area per plant, thereby increasing the volume of H2O transpiration. What I have shown are baseline figures. |
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Seems like a lot of work to me.... |
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Very provocative - how would the cover
be supported (inflation?) |
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[bpilot] By the blowers, that introduce air to the system. That is inflation, I guess. |
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//Seems like a lot of work to me....//
So is getting a drink of fresh water, when there's none around. |
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//what makes you think that?//
The fact that we are still frittering around the edges of reverse osmosis desalination plants; That there is no major, viable proposal on the table, to replace that technology; That it's still too expensive to run reverse osmosis for large scale applications... |
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What I propose is to use nature's own solution. The plants (pun intended) build themselves, require little maintenance and provide a raft of other valuable benefits whilst in operation. |
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We don't want the trees to grow too big, so I guess they would also be millable timber, after a certain time. We're looking to maximise foliage area, remember? That's where our greatest cost:benefit kicks in. |
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Actually the price of water generation through desalination has come down considerably in the last few decades. Reverse osmosis is a very efficient way of desalinating water, which is why it's used. Advances have been in filtering technology (the filter used in reverse osmosis) which has steadily brought the cost of desalination down to the point where currently several major desalination plants are considered worldwide (Israel and the Thames estuary are 2 examples) and filtering technology is continuting to advance. The gains are through reducing the energy needed to push the water through the filter and life span of those filters. |
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The reason I know all this is that there was several articles in New Scientist recently about water desalination. Interestingly enough in the same magazine a couple of pages beforehand there was an article about a new filter derived from digital audio tape technology touted as a great way of clarifying beer (a noble and worthy pursuit) that to me seemed eminently suitable to reverse osmosis. |
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Do you have any idea how much it costs to build a plantation of trees in dusty, saline, dry areas? |
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You need a whole lot of fresh water to start with... |
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And do you have any idea why the people cut down the existing trees in the first place? |
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And why do you think that broad leafed plants would grow at all in these semi-arid dry lands? |
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Does your system work with cactuses? |
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Why does this lead to reduced soil salinity? I'd have thought the opposite. |
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Water is removed while the salt gets left behind. More (presumably salty) water flows in ... |
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[loris], that's not entirely true. If you plant trees in a dry area, the soil gets conserved and will retain more water. |
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But the problem is that you have to prepare the soil first, in order to be able to plant trees, and that costs a lot of money. |
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I still give the idea a (+) though, since it does offer some interesting possibilities. |
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I'll give this a +. If this were placed on a coastal bank, water could be pumped (or allowed to naturally flow) into one side of the facility, and then flow back out the other side. What flows OUT will be slightly saltier than what flows IN, but the trees will be living in a constant salinity. We are relying on the large amount of ocean water to act as a 'salt-sink' to prevent salt levels in the tree's water from getting too high. |
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The cover could be geodesic in nature, allowing it to be thin, but self supporting. Maybe you could start condensation by pumping cold water from farther out in the ocean (deeper, but not too deep) up though veins in the cover to cool it a bit, allowing the water to condense, and run down to the sides to be collected. |
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Someone mentioned rainwater. Why not collect that TOO? It would just run down the outside of the dome to be collected at the edges. |
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Trees are pretty amazing machines, why not use their natural abilities to our benefit? |
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Wouldn't the sun heating up the ground (or saltwater pool) under the tarp vaporise a lot of water? Couldn't this be a thermosolar system with no trees? |
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[bowing tho fellow Baker] excellent! Croissants for you! |
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The primary function of this system, as envisaged, is to reduce the natural level of the water table. A byproduct of this is the fresh water generated. Rectification of the salinity levels in the soil is taken care of by the trees. |
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This happens thus:
The trees actually pick up the salt and hold it within their systems, along with a substantial volume of water. Some species even expel the salt through pores in their leaves. Certain mangrove species maintain internal pressures as high as 800psi, for this purpose. Most living things, us included, are pretty good metaphors for the sea. We are reservoirs of salt water. Trees are no different. Excess salt is expelled onto the ground, where natural weather, or watering washes it into the alluvial system and into the sea, a huge "salt sink" as someone mentioned. |
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The salt water does not "flow" past under the ground, as such, unless you are at the edge of a land mass, i.e. A coastal, or littoral, system. The water does, however, remain salty underground. The last few feet of soil, above the water table, does not remain saturated with brackish water, so it gets a chance to rejuvenate through action by the vegetation. |
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Finally, yes, this works without the trees, as I said earlier. It works a lot better with the trees. |
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The whole idea, as posted, deals with a number of problems. The road to rectification of our environment is a long and winding one, unfortunately. |
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Spreading a giant transparent tarp dozens (hundreds?) of square miles in size, suspended (presumably) 100 or more feet in the air does pose some practical difficulties. Is it suspended by balloons, or giant pylons? Or does it rest directly on the tops of these trees? Do birds go under or over the tarp? What if they sit on the top of the tarp? How do you get dust and dirt off of the tarp? Will it tear if there is a high wind? |
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Without the tarp, the idea is a reasonable one, and a google did find other similar land desalination proposals. Even after reading these, it remains unclear to me where the salt goes. Some is locked into the trees, but this cannot account for much. |
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Fair enough. As it is expelled from the leaves of the trees, onto the ground, it can be swept up with a thin layer of soil which can be leached of the salt/s. They can be processed, for use by industry. |
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So a tarpless adaptation of this would be to intersperse salt-eating plants among crop plants - for example, in an irrigated orchard. The weedy salt-eating plants would be periodically harvested and carted off (burned for fuel?). Their presence would help maintain the soil for the crop trees. |
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That sort of thing would work. However, most agricultural techniques these days rely on broadacre monoculture. |
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I like this.. in essence it's a biological solar-powered desalinisation plant. + |
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