Dew collection: wiping drinking water off a morning field

After shelter from the weather, water decides how long a person stays on their feet. Most adults can manage only a few days without it, and in hot conditions much less. When the only stream nearby looks foul and there is no filter in the pack, the grass underfoot can still supply a drink at dawn.

Dew forms when a clear, still night cools the ground and the leaves below the dew point of the air. Water vapor condenses on those cold surfaces in fine droplets. The water arrives as vapor, so it starts out clean, but it lands on whatever the leaf or stone is carrying: pollen, dust, bird droppings, pesticide residue, road grime. Dew is often described as distilled water. It is better to think of it as rinse water from the surface it sat on.

Where not to collect

Pick the surface before you pick up the cloth. Leave these alone:

• Poisonous plants or anything growing among them, such as poison ivy, poison oak or plants you cannot name
• Lawns, crops, verges and gardens that may have been sprayed with herbicide, pesticide or fertilizer
• Ground with droppings on it, or places where livestock or deer bed down
• Anywhere near a road, where exhaust and tire dust settle on every blade

How to collect dew

The method has three steps and needs nothing more than a cloth and something to hold water.

Step 1: Find heavy dew early

Go out in the first light, before the sun reaches the spot. Once direct sunshine hits the field the droplets start to evaporate, and within an hour there is little left. Open meadows and long grass on untreated ground tend to give the most. Broad leaves such as burdock or wild grape hold dew well, and so do smooth rocks and tarps left out overnight.

Step 2: Wipe the surface with an absorbent cloth

Drag an absorbent material across the wet surface until it is heavy with water. A cotton T-shirt, a thin towel, a bandanna or a clean rag all work, and so do the synthetic chamois cloths sold for drying cars. Some people tie the cloth around their ankles and walk slowly through the grass, which covers more ground and spares the back.

The same trick works after a shower of rain, though a tarp or a clean container catches rain far more efficiently than wiping.

Step 3: Wring it out

Twist the cloth over a pot, bottle or bag and wring it hard. Shake the cloth out, go back to the grass and repeat until the dew is gone or the container is full.

What yield to expect

On a good morning the water comes quickly. One published trial filled a 16-ounce glass in about three minutes of wiping, and extrapolated that to roughly a gallon in half an hour. That figure depends on a heavy dew and plenty of untouched grass. Research on dew condensers puts a strong night at somewhere between a tenth of a liter and about half a liter per square meter of surface, so a light dew or a small patch will give far less. Expect the water to look slightly cloudy with bits of plant matter in it.

Make it safe to drink

Treat collected dew the way you would treat any surface water. An animal carrying Giardia or another gut parasite can easily have crossed the same field in the night. Strain out the debris through a clean cloth or coffee filter, then boil it at a rolling boil for at least one minute (three minutes above 5,000 feet), or disinfect it with unscented household bleach at the doses given in the EPA’s emergency disinfection guidance. A filter rated to remove protozoa and bacteria is a useful first stage, but it will not remove chemicals, which is why choosing an unsprayed field matters more than anything done afterwards.