Collecting and storing rainwater

Rain is the only resource that comes to you. You don't have to walk, or show yourself.

ACT

1. Spread out the largest surface you have, tilted towards a low point.
2. Let the first water go. The start of a downpour washes the surface and carries most of the dirt. Allow one litre discarded per square metre of collecting surface. That water is for washing, never for drinking.
3. Store it in something opaque, closed, in the shade or buried. Light makes algae grow. Open water attracts mosquitoes, which lay their eggs in any accessible standing water within a few days.
4. Split it between several containers. A single stock is a single point of failure: one contamination, one break or one discovery costs you everything.
5. Treat it before drinking. Rainwater isn't safe to drink as it is: it goes through Making water safe to drink like any other water.

ADAPT

You're in a city. Gutter downpipes deliver a considerable volume and only need a container under the outlet. But the roof is unknown: that's water for washing and for treating, not raw drinking water.

You're on the move. A tarp does double duty: Building an emergency shelter at night, a collector during the downpour. A poncho stretched between four points fills a water bottle in a few minutes of steady rain.

It isn't raining. On a clear night, a clean cloth spread over low vegetation soaks up dew and can be wrung out in the morning. The yield is small and it only costs you sleep. Condensation bags over branches produce real but modest amounts, and tie up the plants you use. These are extras, never a foundation.

You're staying a long time. Burying a container fifty centimetres (about 20 inches) down stabilises its temperature better than any surface insulation, and keeps it out of sight.

UNDERSTAND

One millimetre of rain on one square metre gives one litre, minus losses. Count on 0.8 litres in practice: some of it wets the surface and evaporates.

A three-by-four-metre tarp under a ten-millimetre downpour produces around a hundred litres. Several days of water, gathered in an hour without moving. The collecting surface matters more than how hard it rains: doubling the surface doubles the yield, while waiting for rain twice as heavy is down to luck.

The surface contaminates the water, never the other way round. A clean food-grade tarp is the best support: neutral, washable, foldable. An unknown roof is a gamble — lead from old roofing, bitumen compounds from shingles, treatment residues on farm roofs. And on any exposed horizontal surface, bird droppings are the main source of germs.

Rainwater itself carries very little to begin with. That's what makes it valuable: it needs less treatment, and so less fuel, than any surface water. All the care goes into what it touches between the sky and your throat.