Well, it's been a few days since closure of the forests has NOT been extended, so I am planning on hitting the trail about mid' next week. It will be a new course for me, so I looked at the route very closely (as I always do).
Much of it appears to be an arid landscape, however, I will be zigzaging my way northwest (the major fire area near here is northeast) with the help of several canyon systems, two major ones appear to be wet and shaded (a good thing for the current heat!), but those trails are "UNMAINTAINED", and there is ALWAYS tree fall! So, this could take a few more days than planned (one of the differences between distance hiking and WILDERNESS hiking!!)
I will be joining the PCT at North Fork Saddle, then on to Vazquez Rocks, a favorite location for television shows (like the original Star Trek and West World), and movies. But that whole area is arid and likely dry. So, I'm thinking of hedging my odds with a large water bladder (to take advantage of 'last chance water') and a large plastic trash bag to be used as a transpiration bag.
"Transpiration" is the process by which moisture is carried through plants, from the roots to small pores on the underside of the leaves called stomata (and lenticles in the stem) where it changes to vapor and is released into the atmosphere (this is NOT "photosynthesis"!!!).
By capturing the water from the leaves, you CAN collect enough water to sustain life... but ONLY if you do it RIGHT!!
TRANSPORTATION ON T.V.
This is not a new method, in fact, you’d probably be able to find this method illustrated in many books. Several years ago, Les Stroud demonstrated it on his television show, "Survivorman" in the Kalahari Desert. By tying a plastic grocery bag over a branch, the leaves transpirated water, which was collected by the bag, resulting in maybe enough water for a single swallow. It worked, but was it ENOUGH? As Les demonstrated it, NO!
The same goes for methods involving digging a shallow pit in the sand/dirt, tossing in some green leaves/branches (or peeing), then covering the pit with a sheet of plastic and letting the condensation drip into a cup. Same results.
GETTING IT RIGHT
To have ANY chance at making water appear out of thin air, start with standard water foraging observation, look for the greenest foliage, typically in/along terrain depressions. Then, you need the RIGHT kind of plastic bags! The type Les used was a typical semi-opaque, white, grocery bag. What you need are plastic bags that are completely CLEAR and fairly tough.
Second, they need to be BIG! Think of your average garbage can, a bag that could be that can’s liner is big enough (the bigger the better!).
Third, you need at LEAST two of these bags PER PERSON to have a good chance at results that may make a difference. And of course, you need cordage.
Next, you need to pick the side of the bushes/trees that face the sun all day, THAT is where you set up. Then, you need to gather as many branches with as many leaves as will fit into the bag as possible, and either tie them down so you can do the rest of the steps without fuss or have someone hold the branches down for you.
With the leaves in the bag, gather the bag at the opening around the branches, and trim any tiny branches/leaves at the opening before tying it closed to make it as air tight as possible. Now pull/bend the branches downward with one corner of the bag lower than the rest of the bag and anchor it that way.
That lowest corner will be your water collection point, you can narrow the corner, as pictured, to make a reservoir, but I don’t find it essential (though it does keep the leaves out of the water).
Check for any holes that have poked through and patch with a square of duct tape. Repeat on as MANY trees as you have bags!! A variation of a transpiration bag on a tree involves a bush.
USING A BUSH
Again, you NEED a LARGE, clear, durable plastic bag! Envelope the leaves of the bush in the bag, then gather and tie at its base.
Next, if the bush is on a slope, your collection point will be on the lowest side of the bush, AND that collection point MUST be lower than the base of the bush where you gathered and tied the bag. If the bush is on flat ground, then you'll have to dig a hold right next to the bush for a collection point.
In either case, placing a rock inside the bag will help weigh-down your collection point. If possible, it's also advisable to place something against the bush so it leans over the collection point.
Whether you use a bush or a tree, vegetation growing at lower points will likely be closer to the water table. Ideally, you want to set-up early in the morning. Then, just stay out of the sun and don't move around too much!
By evening, using this method under typical arid/desert conditions, this may produce at least a half-liter of water per setup, under ideal conditions, maybe up to about a liter, although in one experiment, a single set-up produced a GALLON of water a day for THREE STRAIT DAYS with no damage to the tree! Certainly enough to make a difference and probably life-sustaining! (which is the CORRECT standard you're looking for!!).
Just carefully untie the bag, keeping the trapped water low, DON'T SPILL IT!, carefully remove the bag and carefully pour the water into a container, and replace the bag. Of course, this means you have to PREPARE your gear accordingly with the correct plastic bags and enough cordage!!
NOTE: Some trees give-off chemicals called alkaloids (plant metabolites that have a nitrogen-containing chemical ring structure, alkali-like chemical reactivity, and pharmacologic activity) that may make the water toxic (or even "mind-altering" in some instances), eucalypts don’t, so always do your research.
Other waste products including tannins, mucilage, gum, crystals and anthocyanin are converted into insoluble compounds and deposited in dead flora tissues like the bark of trees, leaves and petals, which are shed periodically.
Always a fun experiment, tho I hope I'm never in a position to rely on it for survival!
Note: as @SurvivalGal points out, go for nice green leaves. To flesh this out: First, healthy leaves are going to be doing more work, and producing more water. But, also, plants transpire at different rates - large soft leaves are likely to be grown by plants that throw their water around and are less efficient in retaining it, and thus provide you with more. Waxy, tough, arid adapted leaves are probably going to be releasing less water, and conifers are much less water-giving than many other trees.
Individual plants can also vary: some plants have both "shade" and "sun" leaves. Typically sun leaves will be at the top/outside of the plant, smaller and tougher, and will transpire less, giving you less water. Shade leaves, because they are sheltered, are used by the plant to do the grunt work and will be giving up more water under the same heat/humidity conditions (but are overall more efficient for the plant). Also as mentioned above, being exposed to sun is important as plants use transpiration to cool their leaves and will up the rate if they are hot, so it can be extra helpful to make sure that you clear any obstructions from the leaves you bag, including exposing shade leaves if you have them. The rate at which leaves let off water can be measured and can drop to almost nothing even when the sun goes behind a cloud, so don't underestimate exposure to sunlight in increasing your yield!