Solar still and transpiration bag

Both of these appear in every survival book. Both are real. Both are routinely oversold, and one of them is close to useless in the British climate.

This page gives the actual yields first, because deciding whether to build one at all matters more than the technique.

The honest numbers

Read that table honestly. A ground still in average British conditions may return less water than you lost sweating to dig it. Several experienced survival instructors regard it as not worth building at all, and in a cool cloudy climate with no strong sun and no hot dry soil, they have a point.

The transpiration bag is different. It costs almost nothing to set up, it works from living plant tissue rather than solar heating of damp soil, and Britain has an enormous amount of leafy vegetation. That is the one to know.

Neither is a water supply. Both are a supplement, or a last resort where nothing else exists.

The transpiration bag, which is the one worth building

A plant pulls water up from the ground and releases it through its leaves. Bag the leaves and you catch it as it condenses.

What you need: a clear plastic bag, as large as you have, and something to tie it with. That is all.

1. Choose a healthy, leafy, non-poisonous branch in full sun. Green and vigorous, plenty of leaves, ideally a broadleaf tree or shrub. Hazel, willow, birch, sycamore and most fruit trees are all fine.
2. Slip the bag over the leafy end and enclose as many leaves as you can.
3. Seal it at the branch with cord, tape or a twist tie. It does not have to be airtight, just close.
4. Weight the lowest corner so water collects in one place. A small stone inside the bag, in the corner, is enough. Arrange the branch so that corner hangs lowest.
5. Leave it in the sun all day. Collect in the evening, or cut a small corner, drain, and retie.

Practical notes that make the difference:

• Clear bags beat black. You want light in to drive photosynthesis and transpiration.
• Bigger bags and more leaves beat clever technique. Yield scales with enclosed leaf area more than anything else.
• Rotate branches. A bagged branch stops transpiring effectively after a day or two and the leaves begin to cook. Move to a fresh branch daily and the first will recover.
• Several small bags beat one big one, because you can site them all in sun and spread the load across the plant.

The water tastes faintly of the plant. That is normal and harmless from a non-toxic species.

The ground still

Build it if you have surplus time and energy, water-bearing material to put in it, and no better option. Not otherwise.

1. Dig a pit roughly a metre across and half a metre deep, in the sunniest spot with the dampest soil you can find.
2. Put a container in the centre, at the bottom.
3. Load the pit with green vegetation around the container. In most British conditions this matters more than the soil moisture: cut green leaves, grass, or non-toxic foliage packed around the sides. You can also add urine or contaminated water to the pit soil, which is one of the method's genuine strengths.
4. Cover with a plastic sheet, weighted around the rim with soil so it seals.
5. Place one small stone in the centre of the sheet, directly above the container, so the sheet forms a cone dipping toward it.
6. Wait. Condensation forms on the underside, runs down to the low point and drips into the container.

Add a length of tubing from the container out under the sheet edge and you can drink without breaking the seal, which matters because every opening costs you an hour of rebuilt humidity.

Why it disappoints in Britain: it needs strong sun to drive a temperature difference between the sheet and the soil. Under British cloud, in cool air, the difference is small and so is the yield. It performs far better in a desert, which is where the technique was developed and where most of the writing about it comes from.

What both of them do remove

This is the genuinely valuable property, and the reason to know these methods at all.

Both are distillation. Water evaporates and recondenses, and everything that does not evaporate with it gets left behind. That means both remove salt, most chemicals, heavy metals and every organism.

So they are the correct answer to the two problems nothing else on this site solves:

• Salt water, including road-salted snow and coastal water.
• Chemically contaminated water, where boiling, filtering and chlorinating all fail.

If the problem is dirty fresh water, use a biosand filter and boil it: vastly more output for vastly less work. If the problem is salt or chemicals, these methods are among the very few that help, and their poor yield stops being the point.

What distillation does not remove

The section above needs one important qualification, because "removes chemicals" is not quite true and the exception is the dangerous one.

Where distillation genuinely is the answer: salt water, road-salted snow and meltwater, brackish water, hard mineral water, and water contaminated with metals or with organisms alone.

How many you would actually need

The yields in the table above are per bag, and the requirement is per person, so the arithmetic is worth doing before you rely on this.

And the bags must be rotated, because a bagged branch stops transpiring usefully after a day or two and the leaves cook. So the working figure is more like eight to twelve branches per adult in rotation, in sun, on plants you have identified.

That is a lot of bags for one person's drinking water, and stating it plainly is the point of this page. It is why both methods are described here as a supplement or a last resort rather than a supply.

What it is genuinely good for:

• Topping up a supply that is nearly adequate.
• The specific salt or chemical problem where nothing else works, accepting that you will be short.
• Somewhere hot and sunny, where both methods perform far better than they do here.
• Keeping somebody alive while you walk to a real source, which is the situation the survival books were describing.

Stock the plastic bags. They weigh nothing, cost almost nothing, store indefinitely, and there is no way to improvise a clear waterproof sheet. A roll of clear bags in the kit is the whole equipment list for the method that actually works.