Researchers from the University of Manchester have developed a computer model showing how rooftop rainwater harvesting tanks combined with smart sprinkler systems could help reduce extreme heat in cities. Using Tokyo as a case study, the study found that the system could lower rooftop and outdoor air temperatures while also reducing heat exposure inside buildings during heat waves.
The proposed system collects rainwater in rooftop tanks and automatically sprays it over roofs when outdoor temperatures reach a specified threshold. As the water evaporates, it cools the roof surface and can reduce heat entering buildings. Researchers say the approach could provide an additional way for cities to manage rising temperatures without putting extra pressure on municipal water supplies.
The model showed that larger rainwater tanks generally provided greater cooling benefits because they stored more water for use during hot periods. The temperature at which sprinklers were activated was also important, while increasing sprinkler intensity beyond a certain level produced smaller additional benefits. The researchers also found reductions in surface runoff, suggesting that rainwater storage could provide benefits during periods of heavy rainfall.
Urban heat islands are a growing concern because built-up areas can become significantly hotter than surrounding rural areas. Concrete, asphalt and buildings absorb and retain heat, while reduced vegetation and evaporation limit natural cooling. Human activities such as transportation, industry and air-conditioning can add further heat.
The researchers said the system could be particularly relevant for people who do not have access to air-conditioning, including some older adults and low-income urban residents. Reducing indoor and outdoor temperatures could help lower heat exposure, while reduced demand for air-conditioning could potentially decrease electricity consumption and associated emissions.
The proposed system could also help address another climate-related urban problem: heavy rainfall and flooding. By capturing rainwater before it reaches drainage systems, rooftop tanks could reduce surface runoff during intense storms. The researchers said this potential benefit requires further investigation.
However, the system has not yet been tested extensively in real-world conditions. The study was based on computer modelling, and the researchers acknowledge that its effectiveness will vary according to local climate. In tropical regions, for example, high humidity could reduce the cooling effect of evaporation. Questions also remain about installation costs and whether systems installed on individual buildings would provide the same benefits as widespread, community-level deployment.
Researchers say the model can be adapted for cities in different climate zones to assess where rooftop rainwater sprinklers could be most effective. Field testing on actual buildings is expected to be an important next step before wider implementation.
The study, published in Earth’s Future, suggests that combining rainwater harvesting with rooftop cooling could provide cities with another tool to manage extreme heat while potentially delivering additional benefits for stormwater management.