In simple terms, a transpiration bag is simply a kind of solar still that is useful for collecting water in the wilderness. It is effective and takes very little effort on your part. All you need is some large clear trash bags and some cordage.
You are going to need to do a little research as to what trees and bushes in your area may be poisonous so that they can be avoided. Good trees for this technique are oak, maple and birch. The larger the plant the larger the root system is going to be which is where the water will be coming from so try to pick the largest tree or bush that you can. The transpiration bags should also be setup where they will get the most sun throughout the day.
I also mentioned that it is clear plastic bags that you need. It is important that they are clear rather than black or colored bags as clear will allow the sun to shine through the bag properly. I have found the following to be very useful:
How To Use Transpiration Bags To Gather Water
I have seen a countless number of videos that will show the guy just grab a leafy branch and tie a bag around the end. Although in theory, this works, for best results you want to pull a group of these leafy branches together to get as many branches and leaves in your bag as possible. 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.
We mentioned before about needing a small rock to place in the corner of your bag. This rock will pull the bag down and give a collection area for the water.
As the sun heats the bag up, water will be drawn from the leaves. It will first evaporate and then it will condensate on the inside walls of the bag. This condensation will start to travel down the inside wall of the bag and accumulate in the lowered corner.
That collected water will be pure and can be drank as it is but we do still recommend that you have a means of purification as the leaves themselves may have pollen and insects on it which will end up in the water. We recommend the Sawyer Mini Filter for the job.
When it comes down to how much water you can get using this technique, the average is about a cup of water a day but the more bags that you use, the more water that you will collect with hardly any effort on your part at all.
Check out the following video for a more detailaed look at the transpiration bag.