AI-assisted simulations show that roof-based rainwater cooling could reduce energy demand, lower urban temperatures and help cities adapt as they face more frequent and intense heatwaves.
Collecting rainwater from rooftops and using it to spray buildings during hot weather could help cities cut air conditioning use, lower urban temperatures and lessen the impact of heatwaves, according to new research from The University of Manchester.
Cities around the world are facing rising temperatures, putting pressure on public health, infrastructure and energy systems. As people rely more on air conditioning to stay cool, energy demand increases and waste heat released from buildings can make urban areas even hotter.
Urban watering technologies are becoming important ways for reducing extreme heat in cities, but their use is often limited by the availability of water.
How does rainwater keep cities cool?
In the study, published in Earth's Future, researchers used process-based numerical simulations and Artificial Intelligence (AI) to test a system that stores rainwater collected from rooftops and automatically sprays it onto buildings during hot weather.
Using Tokyo as their case study, they found that cooling rooftops, the system reduced the amount of energy needed for air conditioning. The cooler roofs transferred less heat into buildings, while lower air conditioning use meant less waste heat was released into the city. Together, these effects helped reduce urban temperatures, cutting the number of heatwave days overall and lessening the intensity of extreme heat events.
What do the reserchers say?
Lead author Dr Zhonghua Zheng, Co-Lead for Environmental Data Science & AI at Manchester Environmental Research Institute (MERI) and Senior Lecturer (Associate Professor) in Data Science and Environmental Analytics at The University of Manchester, said: "Cities around the world are facing growing challenges from extreme heat. Air conditioning can help keep people safe and comfortable, but it also consumes large amounts of energy and releases additional heat into the urban environment.
"Our study shows that harvesting rainwater from roofs and using it strategically for cooling could provide a practical way to reduce both energy demand and urban temperatures.
"What is particularly encouraging is that the benefits become even greater during hotter years, suggesting this approach could become increasingly important as the climate continues to warm."
How would the rainwater sprinklers work?
The study suggests that when the sprinklers switched on - for example, when the roof reached a certain temperature - had a greater impact than either the size of the tank or the amount of water applied. The researchers also found that bigger is not always better. Very large tanks delivered only modest additional reductions in energy use and extreme heat, while applying extra water did not always lead to more cooling because some of it remained on the roof instead of evaporating.
The researchers say the approach could help local authorities and urban planners evaluate how rainwater-based cooling systems might work in their own regions while balancing practical considerations such as cost, water availability and local regulations.
Can rainwater cooling help with other urban challenges?
As cities continue to grapple with rising temperatures, the team believes roof-based rainwater cooling systems could form part of a wider suite of urban climate adaptation measures designed to improve resilience and protect public health.
Junjie Yu, PhD researcher at The University of Manchester, added: "The rainwater tank also provides an additional co-benefit on reducing the extreme urban runoff. This approach exemplifies a 'natural solution to natural challenges', in which rainwater serves as a natural resource to mitigate both thermal stress and hydrological extremes."
