Harvested rainwater could help cities beat the heat
A Tokyo modeling study suggests spraying stored rainwater on rooftops could reduce extreme heat and air-conditioning demand.
When temperatures climb into the 90s and higher, roads expand, and the expanding concrete can have nowhere to go. So instead of spreading out, it pushes up.
A new study suggests that spraying harvested rainwater onto rooftops could help cities reduce the impact of heat waves.
Urban watering techniques help reduce extreme heat, but their use is often limited by water availability. The research proposes a system that collects rainwater and uses it to cool roofs, potentially reducing air conditioning energy demand, high city temperatures, extreme urban surface runoff and the number of heat wave days.
Published in Earth’s Future, the study modeled a system in a Tokyo, Japan, neighborhood. The model captured runoff in rooftop tanks and sprayed the stored rainwater onto roofs when their surface temperature exceeded a set threshold. The sprinkled water increased evaporation, cooling the roof and reducing the amount of heat that entered the building.
Light rain and clouds are seen over downtown on Sept. 8, 2026, in Los Angeles, California. (Photo by Apu Gomes/Getty Images)
"As the roof surface becomes increasingly wet, the excess water may remain on the roof without being effectively used for evaporation,” study authors explained. “Further, under limited water availability, a longer sprinkling duration with an appropriate intensity may sustainably wet the roofs and reduce total AC energy consumption more than short, high-intensity sprinkling.”
Results also indicated that the temperature threshold used to activate the sprinklers was more important than either sprinkling intensity or water tank size. When the threshold was set lower, sprinkling intensity had a greater influence on energy saving.
The researchers explained they chose Tokyo as the research area due to its hot climate and Japan’s success in promoting rainwater tanks. The sprinkling system was only activated in the city’s hottest months, from June to September.
“The climate in Tokyo is warm and temperate, which is a subtropical monsoon climate with distinct seasonal variations,” the study authors wrote. “Tokyo experiences hot and humid summers and cold winters, and receives abundant rainfall throughout the year.”
This picture taken from Azabudai Hills on August 28, 2026, shows the Tokyo Tower and the city skyline in Tokyo. (Photo by Philip FONG / AFP via Getty Images)
Sprinkling usually began at around 9 a.m. and ended at 7 p.m., with the strategy particularly useful around noon, when extreme temperatures occurred most often. The model also produced greater air-conditioning savings during warmer years, suggesting it may be effective as temperatures rise. Rainfall levels, however, did not show a strong correlation with the reduction in air-conditioning use.
Over the 10-year simulation period, the hours of extreme heat on roof surfaces were cut by about 592.5 to 1,221 hours. Exposure to urban canyon air temperatures of at least 35 degrees Celsius (95 F) also fell by 86 to 297.5 hours. In the model, urban canyon air refers to the air within the spaces between buildings, rather than the atmosphere over a city.
While larger rainwater tanks generally produced greater reductions in air-conditioning use and heat wave days, the additional benefits diminished as tank size increased. Increasing tank size also did not continually produce greater reductions in heat wave intensity.
The authors noted that the experiment was a process-based computer modeling study, and they lacked observational data to directly validate the proposed system. The feasibility of the system also depends on rainfall, climate, costs and rainwater-harvesting regulations.
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