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Why harvest and use pure rainwater?

Harvesting rainwater has long been the norm in rural areas that reticulated water doesn’t reach, yet it’s still not common in New Zealand’s urban areas. We lag far behind Australia, where drought and pressure on water supply are motivating changes in household water use and government subsidies for rainwater collection. More than half of Adelaide households have rainwater tanks for example.

There’s a lot to consider about how you harvest rainwater. But for urban households already connected to reticulated water, first you need to consider why you might do that.

Water security

Your family needs water to survive an emergency. That’s the single most compelling reason to have some means to harvest rainwater. Urban water researcher Martin Payne lives in Wellington city, where water comes from Kaitoke in the Hutt Valley. “The pipeline is about 45 km long and it crosses a major fault line three times. In an earthquake, it is very likely the network will be significantly affected. It may be years before some houses can be connected to the water network again.”

How much water do you need in an emergency? Some guidelines say three litres per person per day, but that’s the absolute bare minimum for short-term survival, says Martin. WHO guidelines call for 20–40 litres.

A four-person household may need at least 80 litres of water per day. “That’s 80 kilos you have to transport, every day, in whatever containers you have available,” warns Martin. “In my opinion, that’s what will drive half the population out of the city following an emergency.”

Water conservation

Harvesting rainwater from your own roof is a way to get around water restrictions applied by local bodies in many parts of the country. Many local bodies have introduced water meters, so reducing your use of town supply will save money on water charges.

However, there are bigger issues that go beyond a simple dollar calculation. “City surfaces are increasingly impermeable, so rainwater becomes stormwater and it’s shed quickly,” says Martin. High volumes are released quickly into urban streams, disturbing the ecosystem and reducing water quality.

There’s a lot of infrastructure and cost involved in treating reticulated water to drinking water quality – but very little of that water is used for drinking. Much of it is used to flush toilets.

Martin’s modelling suggests that a Wellington household with a modest 1000–3000 litre rainwater tank (plumbed to the laundry, toilet and outside taps), combined with reasonably efficient water use, could still halve its use of potable water.

Water quality

Rainwater may be preferable to town supply in places where reticulated water is poor quality. And some organic gardeners prefer to use rainwater rather than chlorinated town water on their gardens.