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Rubble Rock Fog Wall in a Swale  RSS feed

 
Dale Hodgins
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It is well known, that fog can be captured by a fog fence, and by rock piles. Certain parts of the world are blessed with regularly occurring banks of fog that blow across the landscape. Dew ponds are fog traps. There are whole plant communities in the Atacamma desert that rely almost totally on dew and fog for their moisture. Some desert creatures, carry a spiney fog fence on their backs.

Many otherwise dry areas of the world experience fog or heavy dew or clouds that blow along the ground seasonally. A swale that is heaped with rubble rock, will trap moisture that would otherwise blow on by. Some fog fence systems capture water which is piped to distant locations either for irrigation or for domestic use. A network of little hoses requires regular maintenance. With this system, the soil is our resevoir.

The system will gather more moisture if there is a fence with plenty of surface area to condense more droplets. Crops such as pole beans, mellons and other climbers could provide that surface area. Dead vines left on the fence over winter, will trap water which drips to the swale below. Sunlight would never touch the soil beneath the rocks. This should help retain moisture and provide a cooler root zone on sunny slopes.

That's all I've got right now. The idea is less than an hour old.
 
Walter McQuie
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I've read that the California redwoods get a third or so of their moisture from fog. I'm still betting that many dry areas of the world rarely see fog.
 
Dale Hodgins
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It's true that most arid places don't receive London fog or Atacama type fog, but many arid areas experience seasonal mist or heavy dew. Dew traps are used in the Kalahari.

This fog is in a dry area of Colorado.
A fog fence would condense some of that. In certain conditions, snow will sublimate and the moisture saturates the air in winter. As night falls and the temperature drops, the moisture is deposited as hoar frost onto cold surfaces. As deposits increase, the surface area available for further condensation increases. I have watched this occur on aluminum sheet metal and on piles of scrap wood, on clear days when there is no snow fall. When a thaw comes, large amounts of moisture melt and fall to the ground. Some dry mountainous areas experience warm days, then frost for long periods at both ends of the growing season. Dew can occur throughout the year and is dependent on humidity and cool night temperatures. Many very dry places get cold enough at night for dew to form. A fog, dew and frost trap like this would allow the grower to grab much more than their share of this transient resource.

Carson city gets 11 inches of precipitation annually. I absolutely could not and would not live in a place like that without setting up a means of gathering extra moisture on days like this.---


This sort of fence will also trap blowing snow within the rocks where it is less likely to sublimate. When fog, frost, dew and snow are allowed to settle naturally, they tend to be deposited in spots where that moisture is readily lost to evaporation. Moisture that is accumulated in one spot and trickles off the fence and rocks to the depths of the pile, will be hidden from the sun and dry winds.

Here are some commercially available fog traps. ---

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Someone embedded the videos. Thank you. This is something that I should learn how to do. Do instructions for that exist ?
 
Walter McQuie
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This thread and rock walls in Colorado really caught my interest. Following Robert's link to the previous thread on Dew Ponds led to several fruitful links. There's also a great Air Well - collecting water from the air thread on the permies homesteading forum. Air well (condenser) on Wikepedia is worth reading. There's even an International Organization For Dew Utilization; it's not clear their website has much practical (for pemies) information but it does list a lot of books and scholarly papers and some links to commercial products. I'm amazed at the long and ongoing history of human ingenuity applied to the prospect of harvesting atmospheric moisture. I'm most appreciative of this opportunity to increase my understanding of the possibilities here.

The ancient Greeks were very knowledgeable about harvesting atmospheric moisture. At least 50 towns had massive piles of stones to collect drinking water. In African deserts they dug tunnel into slopes to promote condensation by cooling the air. Here a photo of a reconstruction of huge mounds of rock--10,000 sq. ft. and 30-40 feet high--found in Ukraine and presumably used to harvest water from the air.
The engineer who built it estimated it would harvest 500 to 1000 gallons of water a day, but the results were less than a tenth of that at the time he abandoned the project. Study of similar piles used by the old Greeks for the water supply of the town of Theodosia on the Crimean Peninsula, indicate that such a pile would yield more than 500 gallons of water a day. In the early twentieth century, several me constructed masonry buildings with openings to foster natural air flow inside and then around masses of sandstone and slate in an attempt to maximize the operation of these ancient stone mounds. It doesn't appear that the effort paid off in improvement over the ancient ways.

There are indications that rocks aren't the best collectors--the best dew collectors are good insulators but poor conductors. Blades of grass collect more dew than pebbles on the ground. Also a smooth surface facilitates maximum drippage. Calice Courneya patented an air well using 3" pvc pipe buried up to 9 feet in the ground:
He used a fan to move air through the contraption and found that "at 90oF and 80% Relative Humidity (RH), the air well yields about 60 lb water daily. At 20% RH, the yield is only about 3 lb/day. The yield is even lower at lower temperatures." Elsewhere I read that above referenced stone constructions work best where air temperatures remain above 65.

There have been some large fog collection projects in South America. "The ideal location for fog collecters are arid or semi-arid coastal regions with cold offshore currents and a mountain range within 15 miles of the coast, rising 1,500 to 3,000 feet above sea level." They utilize an inexpensive plastic mesh and pvc piping: http://www.rexresearch.com/airwells/fogtofo.jpg Some have been in place for 10 years or more. They capture and average of 3 litres of water per square meter of fabric each day. The pictured project in Chile fell into disrepair due to lack of buy in from the local population who preferred a more conventional industrial pipeline that cost much more of someone else's money.

More encouraging is this from Rethinking Water: A Permaculture Tour of the Inland Northwest by Kyle Chamberlain:
Kyle noticed that in the steppes of southeast Washington about the only places that stayed green in summer and where he could find serviceberries were the north sides of the numerous piles of basalt. Botonists had already discovered this phenomenon and named such an area a “talus garland community”. He proposed this list of reasons why these piles of rocks work:

Shade from the southern sun minimizes evaporation and causes winter snow to melt later in the year
Drifting snow collects in the loose rock
Piled stones condense moisture from night air (thanks to the Designers Manual for the hint)
Stones protect soil moisture from sunlight and arid air
Stones minimize competition from grasses
Stones protect plants and debris from fire
Freshly eroded basalt provides ample mineral nutrients
Stone provides an ideal growing surface for lichens, which speed the breakdown of rock and fix nitrogen (lichens are the primary nitrogen fixers in some deserts)
Loose stone provides some protection from browsers, especially during early growth
Stone piles provide habitat for animal associates, like packrats, cottontail rabbits, marmots, chipmunks, snakes, lizards, etc. Animal associates distribute seeds, provide manure, control pests, ext. (Rabbits and marmots are very tasty themselves)

My conclusions. If I was in a dry area that has significant fog I'd look into those mesh collectors seriously. It's said 48 square feet of collection might yield 50 gallons of water a day at a cost for the mesh of $400. Elsewhere I like the potential for talus garland where you want some trees on a north slope and there are a bunch of rocks handy. Also rock mulches work.
 
Dale Hodgins
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That big Chilean project has been a technical success and a cultural failure. It was begun as a means of reforesting the hillsides. The local people wanted water to flush down toilets and most water was diverted toward the town. Domestic water consumption has increased to match additional units that were built and the reforestation project has not gone very far. When the hills were covered in trees historically, those trees were able to trap much more moisture than this, the world's largest man made system does. If the people had allowed the forest to come back, it is quite likely that they would now have spring water to flush down the toilet.
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It has been mentioned that this system would perform poorly in gathering summer humidity. I haven't suggested that the rocks would condense humidity on hot days. That requires a cold thermal mass. The rocks are a mulch and sun shade.

Fog and mist are already liquid. They will either settle on something or evaporate. I have piles of rock on my hillside. Banks of mist drift by during cold weather and lose moisture to my rocks and trees. I know this is happening because they drip when no rain is falling and water sheets down the trunks It's all about surface area. --- The idea of the fence in a swale is to mimic those plastic mesh collectors, using dead or living vines as the surface area instead of tons of plastic. The entire field becomes a collector. In some cool areas, climbing peas would provide a vine that could persist on the fence into the misty season. Woody vines could be grown, then cut off to provide a surface that will last longer. I don't expect to gather any water during the growing season unless it rains. Wind driven rain is not distributed evenly. Hedges and trees get what falls on them and they get the water that blows against them. The swale is a storage tank. Water that drops there is safely away from drying wind and sun, and is more valuable than water that becomes runoff. In my environment, there is no way for this to not work. Every surface that is exposed to wind driven rain and mist, gets considerably more water than areas on the leaward side of those things.

Rain trapping doesn't increase the total precipitation on a large chunk of land, other than at the property line where neighboring land can be in the lea of trees that trap lots of it.

Without intervention, most fog and mist will blow on by. When conditions are right, anything that poses a porous barrier to air flow, will gather some of that moisture.

Hoar frost collects on surfaces. Increase the surface area and more moisture will settle. As frost accumulates, it fills gaps and increases the available surface area.

In summary, if your place sometimes experiences fog, mist, hoar frost or driving rain, there are ways to gather it and store it in the ground. If you seldom see any of these common phenomena, your place is ill suited to devices designed to collect what isn't there.
 
Dale Hodgins
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Some places get most moisture in the winter as snow. The fog wall and fence should be oriented to trap blowing snow. Snow melt will flow beneath the rocks in the spring. If the soil is frozen solid when melting occurs, water can be lost to runoff. The swale will fill with water that runs beneath the rocks where percolation is more likely. On a sunny day in spring, a pile of rock will warm more quickly than the surrounding soil and this should help to create an ice free route to the bottom of the rock pile.

Soil contours and crop residue can both affect snow accumulation. The ends of swales could be capped with hugelkultur mounds. This thread is about a hugelkultur snow fence. --- http://www.permies.com/t/31728/desert/Hugelkultur-Snow-Fence --- Some combination of these ideas would seem in order if we are to to maximize moisture trapping and retention.
 
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