According to the study, the “oasis effect” in urban parks could contribute to greenhouse gas emissions

Unsurprisingly, those living in Phoenix, Arizona, that 2020 was a record year for high temperatures. According to the National Weather Service, the Phoenix area outperformed all previous years in 2020 for high average temperatures and excessive heat warnings.

With the oppressive heat combined and COVID-19 restrictions on travel and indoor activities like movies and restaurants, many people have turned to urban parks for outdoor recreation.

And while Phoenix area parks can serve as oasis for residents, the irrigation needed to keep parks thriving and fresh comes at a high cost of water consumption.

To determine this cost, a team of scientists led by hydrologist Enrique R. Vivoni of Arizona State University’s School of Earth and Space Exploration and School of Sustainable Engineering and the Built Environment (ASU) spent a year collecting data on conditions. weather, evapotranspiration and soil water at the Encanto golf course in Phoenix. The results of their study were recently published in Geophysical Research Letters.

Studying the Oasis effect

Vivoni and her team identified that the park exhibited what meteorologists call the “oasis effect,” which refers to creating a cooler microclimate of a surrounding dry area due to evaporation from a water source.

“The word ‘oasis’ can evoke the image of a pool of water surrounded by palm trees in a sandy desert,” says Vivoni. “But this same effect occurs in urban parks when heat from the surrounding neighborhood is carried by winds into the park, increasing evaporation rates.”

To measure the oasis effect at the Encanto golf course, Vivoni and her team used special sensors at a weather station located inside the park. These sensors measured the flows of water and energy along with the carbon dioxide exchanges that represent photosynthesis and respiration of plants.

Vivoni and graduate students Mercedes Kindler, Zhaocheng Wang and Eli Pérez-Ruiz worked together on sensor distribution, data analysis and the use of satellite products to monitor park conditions.

“We settled at Encanto Park in February 2019 and all of our tools were up and running within the next month,” says Kindler. “For a year, we conducted weekly or biweekly visits to the sites for data collection and station maintenance. It was a great experience working on this research project, as it allowed us to learn more about water and energy flows at an urban golf course. “

Their remarkable results were twofold. First, the oasis effect led to unexpectedly high evaporative losses overnight. Second, the oasis effect was related to the evaporation of soil water and irrigation water, and not to the activity of plants and herbs in the park.

“Because of the oasis effect, when we irrigate our urban parks at night, we lose large amounts of water and see an increase in carbon dioxide emissions, which could lead to greater global warming potential,” says Vivoni. “This has important implications for water conservation and management of greenhouse gas emissions in desert cities like Phoenix.”

Although more studies are needed to determine when during the day it would be preferable to irrigate, making this management change would reduce evaporative loss and carbon dioxide emissions (which contribute to global warming) during hot, dry and windy days.

It also remains to be determined how widespread this effect is within Phoenix. Given the large number of parks and golf courses, however, the results of this study are expected to have important regional consequences that will need to be considered by state, provincial and city agencies.