Swiss cities store rainwater to tackle drought and heat

Swiss cities are increasingly treating rainwater as a resource, storing it to help manage drought, heat and heavy rainfall, SRF reported.

At the Wolkenwerk development in Zurich Oerlikon, water still trickles from roof gutters into a pond after three days without rain. The project uses the “sponge city” principle: instead of quickly sending rainwater into drains, it holds water back, lets it seep into the ground or evaporate through plants. This can ease pressure on drainage systems during heavy rain, water green spaces and cool the surroundings on hot days.

Experts at a media conference in Zurich on Monday said the summer of 2026, with long dry spells and high temperatures, had put Switzerland’s water supply under pressure. Heavy rainfall is also becoming more frequent. “We still have the same amount of rain, but at different times and with greater intensity,” said Anja Herlyn, co-president of the Association of Swiss Wastewater and Water Protection Experts. “If we want our landscape to function as before, we have to hold water back.”

At Wolkenwerk, rain flows from roofs on buildings up to 70 metres tall across planted terraces and onto the roofs of lower buildings before reaching the park. Along the way, it waters plants. Roof gardens have a layer of growing medium up to 70 centimetres deep to absorb and store water; excess water flows onward when capacity is reached. During extreme rainfall, the development can hold back about 280 cubic metres of water, roughly the volume of six tanker trucks.

Experts say individual showcase projects are not enough and that rainwater needs to be considered systematically in planning. Green roofs can retain about 70 per cent of annual rainfall. A large-crowned tree can cool its immediate surroundings by four to seven degrees Celsius on a summer day.

But sponge cities also have limits during extreme heat and drought. Wolkenwerk project manager Hendrik Stump said that eventually “even a sponge dries out”. Without watering, plants can suffer and eventually dry out despite the water-storage measures.