ASU, Phoenix work together to lower temperatures, conserve water by 'greening' urban spaces

It’s summer. You’re walking down a familiar street in Phoenix and notice that a dirt lot that had previously been empty is now filled with trees with huge canopies, sprawling grass, and dense brush. Not only is the green vegetation pleasing to the eye, it’s doing two other key things: cooling the city and reducing air pollution. 

New joint research between Arizona State University and the city of Phoenix is taking an in-depth look at the best ways to incorporate more trees, shrubs and grass around the city to curb urban heat and pollution — all while conserving water.   

The study is the most comprehensive to date in Phoenix that is aimed at quantifying vegetation growth, its water use, and the impact on land surface temperature. 

Led by Soe Myint, professor in the School of Geographical Sciences and Urban Planning, the study is designed to provide Phoenix decision-makers with the critical information necessary to forge new public policy and implement tree beautification and cooling programs around the city where they are needed most. 

“This research will allow Phoenix’s city managers to identify areas of concern and make informed decisions for effective water conservation practices to lower heat stress and improve the health and wellness of the Phoenix community,” said Myint, an expert on urban climate in desert cities and principal investigator of the study. “This is real-world research conducted in close partnership with key stakeholders that can impact our local community.” 

From research to reality 

It’s been long understood that plants and urban vegetation contribute to lowering land surface temperatures, however, increasing green spaces in a desert city can lead to higher outdoor water use, which is not ideally practical for urban sustainability. 

The absence of localized data quantifying this phenomenon has limited our understanding of exactly how much water is necessary to cool Phoenix and whether vegetation cooling strategies are realistically and efficiently achievable. 

Enabling this new joint research is newly available NASA ECOSTRESS data, the most detailed temperature images of Earth’s surface ever acquired from space that measure land surface temperature, evapotranspiration (water use), vegetated stress, and growth conditions of vegetative features. 

Using the data and advanced mapping analysis, Myint and his team are able to tackle key research questions that have previously remained unanswered.

Among a number of things, the study seeks to identify with specificity the number of degrees Celsius various types of vegetation — trees, grass and shrubs — lower the land surface temperature at different points in the day, in the summer, and in the winter. The team will also determine how much water each plant will use based on its land-use category; and how much it would cost the city of Phoenix to maintain. 

“If we convert open land and open soil in the city of Phoenix to 10% of grass features, 10% of trees or 10% of shrubs, how many degrees Celsius can that lower within the city of Phoenix?” Myint said. “This research will identify all opportunity areas of hot spots, cold spots, stressed areas, and how much water is required to grow vegetation in certain land-use areas.” 

“Decision-makers will know that if they increase this much vegetation, it will need this much water and they will need to spend this much to maintain it annually.” 

ASU and the city of Phoenix together will do in-depth scenario planning. 

“There are a lot of open spaces in the city that are just left open; what if we can use that open land and convert it into trees it can help our city fare better than other cities,” Myint said. 

The benefits of more urban vegetation extend beyond reducing heat stress: Plants mitigate air pollution that is linked to respiratory and cardiovascular morbidity and mortality rates within the city.

“We are in one of the most (air) polluted cities in the United States, like LA, but in addition, we live in one of the warmest areas,” Myint said. “We need a lot more incentives to self-increase vegetated features, not only to lower surface temperatures but to absorb the carbon dioxide and release oxygen to help us not only cool the environment but help the health conditions of all people within the city.” 

Impacting meaningful change  

Myint is the recipient of the highly-competitive, NASA ECOSTRESS program grant. Of more than 120 submissions from universities across the country, only 15 projects were funded and the ASU-led project is the sole research working on city heat islands and water use. 

“It was perfect timing that NASA produced brand new satellite data, it has water use, it has land surface temperature, it has evaporative stress and it has water use efficiency,” Myint said. “I knew we could use all these available data and make something really meaningful for evidence-based decision making and policy formulation.” 

Myint, who has studied urban heat island warming and cooling in desert cities around the globe for almost two decades, says that being able to conduct real-world research in close partnership with key stakeholders to benefit the people of Phoenix is rewarding. 

“It’s an honor for me to work with the city of Phoenix, to collaborate together to inform actual decision-making based on the results,” Myint said. “With this knowledge, Phoenix city planners can make decisions properly for urban greening development of a sustainable future.”

 “The city of Phoenix can be a model city, among others.”