How Do You Store a Year of Rain? A Tank Under the House

Groundwater requires permeable rock holding water at a reachable depth, and a great many places have neither. Limestone islands drain too fast to hold anything. Volcanic islands frequently have groundwater that is brackish. Arid regions may have water far too deep to reach with pre-industrial methods. Where those conditions apply, the only fresh water available is whatever falls from the sky, which arrives unpredictably and must therefore be caught and stored against the period when none falls. A roof is already a collecting surface, so the arrangement requires only guttering and a tank, and the whole problem becomes one of sizing that tank correctly.

The calculation is straightforward and the answers are frequently uncomfortable:

• •Roof area, which sets how much is collected per millimetre of rain
• •Annual rainfall, which sets the total available
• •The length of the longest reliable dry period, which sets the storage needed
• •Daily consumption, which multiplies through that period
• •Losses to evaporation, overflow and the first flush diverted away
• •A safety margin for years drier than average

Several societies built at scales that dwarf domestic arrangements. Roman cities across the drier parts of the empire depended on enormous covered reservoirs, and the one beneath Constantinople, built in the sixth century with hundreds of columns supporting its roof, held tens of millions of litres and remains standing. Bermuda requires by law that every house have a white-painted limestone roof and a tank beneath it, since the island has no fresh groundwater at all, and the arrangement has supplied the population for centuries. Indian stepwells combined storage with access by descending staircases and are architecturally extraordinary. Nabataean settlements in the desert channelled runoff from bare rock into carved underground reservoirs.

Collecting rain is being adopted in places with perfectly good mains water, for reasons that have nothing to do with necessity. Urban drainage is overwhelmed when heavy rain runs off roofs and paving into sewers all at once, and holding that water in tanks and releasing it slowly reduces flooding downstream, which is why several countries now require attenuation storage on new buildings. Using stored rain for flushing, washing and irrigation reduces demand on treated supply, which matters where supply is stressed. Charges based on the area draining to a sewer give a direct financial incentive. The water is soft, which suits plants and appliances. And the installations are frequently mandated by building regulations rather than chosen.

Stored water spoils unless the system is designed against it, and the measures are consistent. The first portion of runoff carries dust, droppings and debris from the roof and is diverted away rather than collected, which removes most of the contamination. Screens exclude leaves and insects, and a sealed lid excludes light, which prevents algae, and mosquitoes, which otherwise breed in any standing water. The tank is kept dark and cool underground where possible. Sediment settles and is periodically removed, and the outlet draws from above the bottom. Roofing material matters, since some surfaces shed chemicals. And treatment before drinking is standard practice wherever the water is used for that.

Limestone islands, volcanic islands and arid regions frequently have no usable groundwater, so rain caught from a roof is the only supply, and the problem reduces to sizing a tank against the longest dry period. Roman reservoirs, Bermudian law requiring a tank per house and Indian stepwells are the historical scale. Diverting the first flush, excluding light and insects and drawing from above the sediment keep the water usable.