Low-lying, coastal Auckland is New Zealand’s largest city, with a population of 1.4 million and an average annual rainfall of 1210 mm (48 in) – slightly more than New York and twice what London usually receives in a time. The researchers estimated that 50% of Auckland’s surface was green or blue, even after excluding its harbors (London had the fewest of the seven cities, at 31%). In a heavy rainfall event – 50mm falling in 24 hours – they calculated that 35% of the water falling on Auckland would be absorbed into these spongy blue and green parts, leaving 65% to be dealt with by mechanical stormwater systems – or otherwise overflow and flood. “It’s a measure of how this city developed and its basic physical morphology,” says Fletcher. You may also like: Nairobi came a close second, with 34% spongy. The Kenyan capital has even more green and blue areas than Auckland – mainly parks and urban courtyards – but higher runoff potential due to its clay-dominated soils, which absorb less water than sand or gravel. In the lowest-ranked city, London, just 22% of the water in a similar rainfall would be absorbed – a risk highlighted in real life in July 2021, when 47.8mm of rain fell in one hour, causing widespread street flooding . houses and subway stations. Kathy Waghorn, an urban researcher at Auckland University of Technology’s School of Future Environments, isn’t surprised her city has been described as relatively spongy. “We have a low urban density, we still have a lot of single-family houses, we still have gardens,” he says. Auckland’s geomorphology also plays a role, he says: the narrow strip of land that wraps around two huge harbors, the dozens of small dormant volcanoes that criss-cross the city, the streams that run up their green sides and beneath them, their legacy of lava – basalt and scoria caves and sinkholes. “The volcanic field has shaped some of this open space,” says Waghorn. “Even our stone is kind of spongy.”
