Research Unearths Water-Saving Nature of Cool Roofs

The advantages of cool roofs in terms of saving energy and climate change have already been confirmed: light-colored roofs reflect the sunlight instead of absorbing, thereby keeping buildings, cities, and ultimately the earth much cooler.

At present, innovative research performed by the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) has discovered that cool roofs also have the ability to save water since they decrease the amount of water required for irrigation in cities.

Researchers Pouya Vahmani and Andrew Jones from Berkeley Lab used regional climate simulations from 18 counties in California and discovered that extensive use of cool roofs can decrease the consumption of outdoor water by nearly 9%. If reflective roofs were installed on all buildings in Los Angeles County, the combined water savings could be almost 83 million gallons in a single day. The study titled “Water conservation benefits of urban heat mitigation,” has been published in the Nature Communications journal.

“This is the first study to look at the link between water and heat mitigation strategies in urban areas,”

Pouya Vashami, Researcher, Department of Energy, Berkley Lab

“You might not do cool roofs just to save water, but it’s another previously unrecognized benefit of having cool roofs. And from a water management standpoint, it’s an entirely different way of thinking—to manipulate the local climate in order to manipulate water demand.”

The intial aim of the research was to analyze how a warmer climate in the future will impact water requirements due to increasing number of cities exploring various climate mitigation and adaptation approaches. “While urban heat mitigation strategies have been shown to have beneficial effects on health, energy consumption, and greenhouse gas emissions, their implications for water conservation have not been widely examined,” Vahmani and Jones write in their study.

As cool roofs reduce the ambient air temperature—which in this research was found to be nearly 1 °C-1.5 °C—they decrease the water demand, thereby reducing the amount of water required for lawns and other landscaping. Both the researchers, working in the Lab’s Climate and Ecosystem Sciences Division, agree that changes in human behavior are inevitable to achieve the water-savings advantages.

“There is a key assumption in here that people would adjust their irrigation behavior in response,” stated Jones. “In order to reap the benefits, we would need people to be aware of the appropriate amount of water, or else use sensors or smart irrigation systems, which are a good idea anyway.”

In addition, they also discovered that the water-savings advantage was much higher on hotter days. “So that’s an indication that in a future climate, where hot days are occurring much more frequently, the added benefit of doing cool roofs might be even more dramatic,” Jones said. “But that has yet to be investigated.”

Both the researchers adopted a high-resolution regional climate model for their investigation; Then, Vahmani modified the model by adding a component to calculate the amount of irrigation water. “It basically adjusts soil moisture to mimic irrigation events,” stated Vahmani. “We also used remote sensing data to improve the representation of physical characteristics of the land surface, which resulted in improved model performance.”

The data for irrigation-only users from Northern California’s Contra Costa Water District was used to validate the model. “The irrigation water demands simulated by the model were matched quite well by the customer data, given the complex nature of urban irrigation,” stated Vahmani.

For nearly one and a half decades, model simulations were carried out in 18 counties in Northern and Southern California, presuming a control scenario that indicates the prevalent status of the urban areas, as well as a cool roof scenario where commercially obtainable cool roofs were installed in all buildings.

The irrigation water savings for individual counties were seen to be 4%-9%, where per capita water savings were highest in the case of medium-density environments (i.e. environments including a mix of landscaping and buildings). “It’s in the suburban areas where you see the most water savings,” stated Jones.

The research also validated an unfolding discovery that water conservation approaches, such as drought-tolerant landscaping (which directly decrease irrigation), can lead to the unfavorable outcome of rising temperatures in urban regions. Both researchers carried out a simulation of the most extreme case (i.e. a complete discontinuance of irrigation) and deduced a mean daytime warming of 1 °C equated over the San Francisco Bay Area.

“These results show that the warming signal from strategies that focus only on outdoor water-use reductions can meaningfully offset the cooling effects of a major heat mitigation strategy, such as citywide cool roof deployment,”

Pouya Vashami and Andrew Jones, Researcher, Department of Energy, Berkley Lab

Modeling microclimates

As the impacts of climate change have increased in the urban areas, Berkeley Lab researchers are hoping to use their models in urban areas that have higher population levels and concentrated infrastructure. “This study is part of a larger effort to improve our ability to model microclimates in urban areas and other climate phenomena at decision-relevant scales,” stated Jones. “For example, we’re also interested in using this to look at the role of fog in the microclimates of the Bay Area.”

These attempts also form part of Berkeley Lab’s Water Resilience Initiative where one of its goals is to develop strategies to estimate hydroclimate at scales that can be applied to direct water-energy approaches.

Vahmani and Jones are intending to continue this research by applying it to agriculture and also analyzing approaches for alleviating increasingly hot climate and ever increasing water requirements. “First we want to see how much climate change will increase water demand. Next will be to come up with strategies to counter that,” stated Vahmani. “In urban areas, we’ll look at how cool roofs can ameliorate both extreme heat demand and irrigation demands associated with future warming. Whereas in agricultural areas, the strategies will have to do with irrigation technology and what kind of crops you’re growing.”

Berkeley Lab’s Laboratory Directed Research and Development program, drafted to initiate novel science and innovative research directions, funded this research.