Older property in the eastern Algarve often comes with a cistern, a brick or concrete tank sunk beside the house that once held the household’s water through the dry months. Most are no longer the main supply, but plenty still work, and the idea behind them is worth borrowing. Rain is free, and a cistern is really just a very large water butt.
What an old cistern does
Rain runs off the roof into a gutter or channel and then into a covered tank, which is often partly underground so that the water stays cool and dark. Cool and dark keeps algae down. An overflow leads any excess away from the house. In a region where rain falls in a few months and the summers are long and dry, a tank like that bridged the gap between one wet season and the next. Many gardens also had a well beside the cistern, used to irrigate a small vegetable plot.
The arithmetic of a roof
One millimetre of rain falling on one square metre of roof gives one litre of water. A roof with a footprint of 50 square metres therefore collects 50 litres from a single millimetre, and a 15 millimetre downpour delivers around 750 litres before losses. In practice you lose some to evaporation and to the first rinse of the roof, so allow for roughly four-fifths of the theoretical figure.
The same sum works for a garage or shed. A garage roof of 18 square metres sends 18 litres down the pipe for every millimetre of rain, which is enough to fill a 200-litre butt in about eleven millimetres. That is one decent shower.
Water butts at home
Standard butts hold somewhere between 100 and 250 litres. A diverter kit fitted into the downpipe fills the butt and then sends the overflow back down the drain, which stops the butt flooding the path. Raise it on a sturdy stand so that a watering can fits under the tap, and link two or three together if you have the space. Keep the lid on. It stops mosquitoes breeding and keeps the light out.
Keeping the water clean
Fit a mesh filter at the gutter outlet and clear leaves from the gutters a couple of times a year. Some diverters let the first rain after a dry spell run to waste, which carries off the dust and bird droppings from the roof before the tank starts to fill. Rainwater is fine for the garden, for washing the car and for topping up a pond. It is not drinking water unless it has been properly treated, and it is best used within a few weeks in summer, before it warms up and turns stale.
Why plants prefer it
Rainwater is soft and free of the lime found in hard tap water, which matters if you grow camellias, blueberries or other acid-loving plants. It is also free of chlorine. Watering with it costs nothing, and during a hosepipe ban a full butt is the difference between a green garden and a brown one.
What to check in an old house
If a property has a cistern, ask whether it is still connected, whether it leaks and when it was last cleaned. A cracked lining can lose water, and a tank with a dead animal in it is an unpleasant discovery. A sound cistern is an asset, because it lowers the water bill and gives you a reserve when restrictions bite.
