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Storing and Stretching Water

Tank sizing, greywater rules and keeping it clean

What a roof gives you

The arithmetic is simple: roof area in square metres, times annual rainfall in millimetres, times 0.8 for losses, gives litres per year. A 100 square metre roof in Lisbon, with about 700 mm a year, yields roughly 56000 litres. The same roof in Manchester, at 800 mm, yields 64000, and in Faro at 500 mm about 40000. That sounds like plenty until you look at when it falls: in Portugal, 80 percent arrives between October and April, and almost none in the four months you need it. Harvest is therefore not the constraint, storage is. Work out your annual yield first, then your summer demand, then see how big the gap between them is.

Sizing the tank

Size against the dry gap, not the roof. Take your garden area in square metres, multiply by the weekly demand from the water lesson, 25 litres in a temperate summer or 40 in a Mediterranean one, then by the number of weeks with no useful rain. A 100 square metre garden in Portugal needs 100 times 40 times 16 weeks, which is 64000 litres to get through from June to September on stored water alone. That is a large and expensive tank, so most holdings do a mix: a 5000 to 10000 litre tank covering the shoulder months and the shortfalls, plus a well, mains or borehole for peak summer, plus dry farming and mulch to cut demand. A 1000 litre IBC costs 80 to 150 euros used and is the usual starting unit.

Greywater, with the real rules

Greywater comes from showers, baths, hand basins and rinse water, never from toilets or kitchen sinks, which carry grease and food. The rules are not optional. Use it within 24 hours, since bacteria multiply fast in stored greywater and it turns septic and foul within a day or two. Never apply it to leaves, stems or any part of a crop eaten raw, so no salad, no strawberries, no herbs you pick fresh; put it on fruit trees, vines, ornamentals and cooked crops, at the root and under mulch. Never use laundry water containing bleach, borax or softener, and switch to a low-sodium detergent if you want to use laundry water at all. Rotate where it goes so salts do not build up, and never use greywater from nappies or from anyone with a gut infection.

Keeping stored water clean

Dark, sealed and screened is the whole recipe. Light causes algae, so use opaque tanks or shade them. Fit a mosquito-proof mesh, 1 mm or finer, on every inlet, outlet and overflow, because a tank can breed thousands of mosquitoes in a week. Use a first-flush diverter that dumps the first 0.5 to 1 litre per square metre of roof, which carries the dust, bird droppings and debris. Draw water from 10 to 15 cm above the base so sediment stays put, and clean the tank out every one to two years. Keep gutters clear and cut back overhanging branches. Rainwater off a roof is fine for irrigation without treatment but is not clean drinking water, whatever it looks like.

Drinking water is a separate system

Say it plainly: the water in this lesson is irrigation water. Roof runoff, greywater, pond and stream water are not drinking water, and no amount of clear appearance changes that. Keep the two systems physically separate, with different tanks, different pipes and different taps, and label the irrigation taps clearly so nobody fills a bottle by mistake. Never cross-connect an irrigation line to a mains supply without a proper backflow preventer, which is also a legal requirement in most places. How to make water safe to drink, boiling, filtration and chemical disinfection at correct doses, is covered in the survival lessons on water, and that is the only place you should be taking your drinking water method from.

Stretching what you have

Before spending on storage, cut demand. Mulch at 7 to 10 cm saves 20 to 30 percent. Windbreaks cut evapotranspiration by up to a quarter on exposed sites. Shade cloth at 30 to 50 percent over summer salads cuts their demand sharply and stops them bolting. Drip instead of a sprinkler saves 30 to 50 percent. Growing thirsty crops in the cool half of the year, which in Portugal means October to May, removes them from the summer budget entirely. Reuse pasta and vegetable cooking water once cooled and unsalted, and keep a bucket in the shower for the cold water before it runs warm, which is 5 to 10 litres a day for free. Together these measures halve a garden's water bill without buying a single tank.

Next lesson: Who Is Eating My Plants →

Related lessons

Watering the Right Way

Deep, early and less often than you think

Mulch and Simple Irrigation

Cut watering in half with a bale of straw and a hose

How Much Water a Crop Needs

Litres per square metre, and how to check instead of guess

When to Water and Where

Early morning, at the root, never at midday