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rock mounds in Colorado  RSS feed

 
Gilbert Fritz
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Would a rock mound really pull much moisture out of the air in Colorado? We don't have much moisture in the air to start with. How moist does the air have to be? What about a dew pond?
 
Dale Hodgins
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Gilbert Fritz
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I thought so.
 
Dale Hodgins
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There may be certain times of year when it would work. Fog which moves across the landscape is ideal. Whatever rain falls, you will get without the rocks. Fog can be intercepted.

This leads me to the idea of "The rubble rock fog wall in a swale", which I will present in a separate thread shortly. Give me an hour.
 
Gilbert Fritz
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Since I am in something of a river valley (the Platte) there is a mist on occasion. Nothing like where I used to live on the East Coast.

I was wondering because of the fact that I have heard some discussion of these things being built in very dry areas. (Though generally misty.) And Denver DOES have the huge drop between day and night temperatures which would be necessary. I know the Indians in the southwest used rock and gravel mulches.
 
Andrew Parker
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With snow on the ground, you can get a lot of water vapor from sublimation. That is how we get ice fog here in the Great Basin. I do not know if a rock pile would capture any of the water vapor from sublimation, but there may be methods to reduce sublimation of snow pack on your property. Also, during the hot summer months, humidity levels will often rise at night, even in arid and semi-arid climates.

I have been to areas of the Peruvian and Ecuadorian coast where fog and mist harvesting would be worthwhile. It doesn't rain, but you can get soaking wet with the heavy mist that blows in off the ocean.
 
Marc Troyka
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Rocks aren't particularly good for capturing water. Rocks are large thermal masses, and can hold quite a bit of solar heat as well as or better than they will hold nighttime cold temps, and I have yet to see any practical water condensers made from rocks. Water condensers are also only useful when the humidity is fairly high and aren't very useful in arid settings. "Dew ponds" are actually rain fed, and have no relationship to dew.

Rocks WERE used in arid areas as a mulch to reduce field evaporation, which does work but also makes it difficult to return organic matter to the soil. It seems to me that CO is one of those states that's very windy, so swales and windbreaks would be more important for improving soil moisture.
 
Walter McQuie
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Location: Northern New Mexico
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I don't feel like I fully understand how rock mounds work to harvest water from the atmosphere, but I've been intrigued by the idea whenever it has presented itself. As it would be quite beneficial if it happened regularly here I've considered the potential of harvesting moisture with rock piles along these lines: I assume the phenomenon would be a consequence of the fact that warmer air can hold more water vapor than cooler air. A readily observable demonstration of this process is condensation onto the surface of the container of a cold beverage outdoors on sufficiently (depending on the relative humidity) warm days. The air in or nearly in contact with the colder-than-air-temperature surface cools to the dew point, becoming saturated. If it then cools further, the water vapor changes from a gas to a liquid and condenses onto the surface. If someone has placed that beverage container on the ground in their garden, it would operate to the benefit of any nearby plants. If the presence of a rock pile causes a drop in local air temperature, it could trigger condensation of some atmospheric water vapor that wouldn't have otherwise occurred.

Days are sunny in the high desert and a rock will collect a bunch of solar energy during the day and release it back into the atmosphere after sunset. Air temperatures drop quickly, especially in spring and fall. Under some conditions, as a rock dumps its heat, it might cool sufficiently relative the the nearby air to begin acting somewhat like that cool beverage. That's the mechanism that I can envision. There may be others. I'd be interested in any thoughts about them. Some items to consider--mist and fog. Wikipedia says fog is condensed water vapor that is held in suspension. It can occur at temperatures up to 4 degrees above the dew point. It commonly occurs near bodies of water expanses of saturated soil (wetlands). So it would seem our cooler-than-air-temperature rock will have more moisture to work with in foggy weather--the relative humidity is near 100%--and the water vapor in the air is changing from a gas to a liquid. Mist can be fine precipitation--it is defined based on visibility distance--or it can be condensation heavy enough that gravity pulls it out of suspension. In either scenario the water vapor is not just condensing onto our preferred surface it is falling out of the atmosphere. Sublimation is fascinating--water goes from frozen to gaseous, without passing through liquid. I did some research looked at Wikipedia on dew point, fog... but will leave it at wondering for now how much of the local snow melt is lost to sublimation. Rocks do seem to quickly pop out of the snow, at least on their sunny side. As an aside, frost is the process of water passing from gaseous to solid states, skipping liquid in the opposite direction from sublimation.

I'll go ahead and share my thoughts on how often the conditions that would set up the mechanism I envision occur in the high desert here. Not often. Rare, even. I look at the NWS forecasts twice daily and have for about four years. I often click through to their hourly forecast to get not just temps, wind, and the occasional shot at precipitation, but relative humidity and dew point. (I also follow the maximum and minimum temps at two elevations on the property where i live and so the accuracy of NWS temperature predictions) Dew points at night are almost always at least several degrees above air temperature. We have regular frosts, almost exclusively on the days during and after a front bringing in some moisture. We have dew several time a year. I've only seen fog after significant hail, which regularly happens during the stronger monsoons, but rarely other years.

Rock pile water harvesting would be most welcome in June--the hottest driest month. The last three Junes have been brutally hot here--above ninety degrees sometimes--and dry. No precipitation. Windy, but not at night. Relative humidities down into the mid single digits during the day and upper teens at night. Dew points steer way clear of temps. I've placed some rocks along the north side of areas I like to see heat up earlier in spring to good effect, but I can't see getting much moisture from rock piles when it's needed most.
 
Gilbert Fritz
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Now, I don't think a dew pond would work here; but they did have something to do with dew!

They contained an insulating layer underneath a water proof layer to isolate them from the earth's mass. Radiational cooling could drop their temperatures below the dew point. If the waterproof layer was ever punctured, they would fail, because the insulating layer would become useless.

That is my understanding anyway.

Also, rocks on the INSIDE of the pile can stay cool into the day, thus potentially condensing water. I am not sure how well this works, though. How cool would a rock have to be to condense water out of very dry air? But they would not have to get rid of heat gained during the day. Rock piles for collecting moisture generally are large enough to have a big interior area.

I know both these things work other places. But I suspect they don't work here.
 
Walter McQuie
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Location: Northern New Mexico
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I don't actually know--but I accept that there are conditions where rock piles work. I believe that desert plants and animals, especially those adapted to the driest hottest deserts, have to rely on extreme means to harvest even a little water for survival. I feel that the plants and animals native to my less arid and not so hot locale don't adopt so many of those extreme strategies because there are costs/inefficiencies in doing so relative to the need for them. I'm trying to grow some food crops that do best with more water than the natives. I don't understand how rock piles can operate to get me the moisture I'd like to have. But I'm interested in understanding what's going on where they do work in order to make a better assessment of whether they are a tool I should have in my toolbox.

I find myself resistant to the idea that the strategy would work here during the day. Less so up here (because it doesn't get so hot), but down toward Albuquerque lots of people adopt a strategy of putting moisture in the way of air streamed into their home. Relative humidity is generally low enough that lots of that moisture evaporates, a process that sucks heat out of the air stream. It seems to me that the efficiency of evaporative/swamp coolers would be inversely proportional to the efficiency of rock piles. Swamp coolers need low relative humidity; it seems moisture harvesting works best when temperature and dew point converge--high humidity. I say seems because this conclusion is bumping up upon the limits of confidence I have in my understanding. If the process is not so efficient here as in the places where it is used, then I'm going to need a bigger or more optimally sized and placed pile of rocks. In other words there is more cost here for achieving the same gain as in foggier climes. Or less gain for the same effort.

The rocks inside a pile will certainly stay cooler (won't gain as much heat) compared to those exposed directly to the sun. The question is will they be cool enough to absorb enough heat out of the air to lower it to the dew point. They won't have a day's solar gain to dump, but it seems that laying around beneath rocks that do gain heat throughout the day will slow their nightly cooling. But I think the bigger issue is probably low relative humidity requiring significant cooling to reach the correspondingly low dew point.
 
Robert Ray
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Here is a link to an earlier thread
http://www.permies.com/t/3589/green-building/Dew-pond
 
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