Most people who write about solar stills can be divided into two camps. Proponents and opponents. The first group usually are those that talk about the in ground solar still, borrow diagrams from old military manuals, and tout their effectiveness. Many of these people have never built one. The second group, usually have built one, and not only have determined they don’t work very well, but you lose more water from your body through sweating, than you gain from the still.
However, the Transpiration Bag IS a solar still and IS an effective way to collect water, with very little effort on your part! All you have to do is carry some large, clear plastic bags and a small amount of cordage in your kit or pack.
The first thing you have to do is find some deciduous trees or bushes (be sure you do not use a poisonous plant – this is when being able to identify specific trees and bushes comes in handy). The larger the better, as the larger the plant the larger the root system. It is the roots that gathers the water. Clear bags work better than colored bags as the sun can shine through the bag. This allows for photosynthesis, which provides for the transpiration.
I find that the clear trash bags, measuring 30″ x 33″, work well and are inexpensive to purchase and you can carry several in a small space. It should be noted that the transpiration bags should be setup where they will get the most sun throughout the day. At our school, we have a large field with a woods line on the north side. As the sun rises in the east, and sets in the west, it allows the transpiration bags to get continuous sun for most of the day.
You want to pull a bunch of branches together so you can fit as many branches, and leaves, into the size bag you have (the larger the bag the better). Pull the bag up over the branches and tie the bag off, around the limb, using a small amount of cordage. Make sure the seal is tight as you do not want any air leaks.
You want one corner of the bag pulled down to have a place for the water to collect. You can use a small stone to hold this corner down, or you can tie the corner of the bag down to a stake in the ground.
The sun causes the clear bag to heat up which will draw water from the leaves. This water will evaporate and then condense on the inside walls of the bag. With the corner of the bag held down lower than the rest of the bag, with the small rock or cordage, the drops of water condensing on the inside walls, will start to travel down the inside wall of the bag and accumulate in the lowered corner. The transpired water is now purified and ready to drink.
If you take the bag off in order to retrieve the water, you must start all over again. So, one way to get the water out of the corner is to use a tube. You place this in the corner of the bag when you first set it up, and have it exit where you tie the bag to the limb. However, you must make sure that you seal this tube with a small plug, such as whittled from a piece of wood.
I don’t prefer this technique as the water often has debris, bugs, etc. in the water that accumulates in the corner. Therefore, we use another technique, whereby you just snip off the very corner of the bag (the corner that is lower for accumulating the water), and fill your container with the water. You then tie a tight knot in the corner and let the process continue. You can then filter the water through a bandana, coffee filter, etc., to eliminate debris.
John also demonstrated the transpiration bag for a class when teaching in the Sierra Nevada mountains in California. This was a desert environment and the water collection was about a quart in one day.
So how much water do you get? Well, we average between a cup and a quart a day, depending on the sun and cloud cover. That’s quite a bit of purified water for little effort. The tree should continue to provide water for 2-3 days, at which point you should move the bags. This is a very effective, yet effortless, way to collect water in a survival situation. Give it a try, and be prepared to survive!