Trees and lawns beat the heat
Mixed landscapes are the best way to adapt to climate change and mitigate
the heat island effect in semi-arid regions

Date:
October 13, 2020
Source:
University of Utah
Summary:
As climate change pushes many cities towards dangerous temperatures,
planners are scrambling to mitigate excessive heat. One strategy
is to replace artificial surfaces with vegetation cover. In
water-limited regions, municipalities have to balance the benefit
of cooler temperatures with using precious water for irrigation. A
new study will make those decisions easier for the semi-arid Salt
Lake Valley, the largest metropolitan area in Utah located in the
northern part of the state.



FULL STORY ==========================================================================
In cities, humans replace the natural ground cover with roofs, pavement
and other artificial materials that are impervious to water. These
surfaces significantly change how the land absorbs and releases energy
and cause the urban heat island effect, a phenomenon where developed
areas get hotter than nearby rural areas. As climate change pushes many
cities towards dangerous temperatures, planners are scrambling to mitigate excessive heat.


==========================================================================
One strategy is to replace artificial surfaces with vegetation cover. In
water- limited regions such as Utah, a state with one of the lowest annual rainfall rates in the United States, municipalities have to balance the
benefit of cooler temperatures with using precious water for irrigation.

A new University of Utah study will make those decisions easier for the
semi- arid Salt Lake Valley, the largest metropolitan area in Utah located
in the northern part of the state. The researchers used 60 sensors to
analyze the microclimate in five locations throughout the valley. They
found that neighborhoods dominated by impervious surfaces were warmer
and drier than the urban parks -- up to 2 degrees warmer in both the
daytime and nighttime.

"It's intuitive -- we've all stood in a parking lot on a hot summer day,
and you feel the heat from the ground. And when you're standing on a lawn,
it's cooler," said lead author Carolina Gomez-Navarro, postdoctoral
researcher at the U. "But we need to back up intuition with data to
determine the best strategy for our semi-arid cities." Gomez-Navarro and
the team measured the temperature and humidity inside five parks and in
their adjacent residential areas from June through August in 2016.

They also analyzed how the surrounding landscape impacted air temperature.

Surprisingly, they found that lawns reduced daytime and nighttime
temperatures even more than trees did. While trees provide shade, lawns
and turfgrass act like a swamp cooler -- water moves through the plant, evaporating from tiny holes in the leaves and cooling the air.

Much of the heat that builds up during the day dissipates at night. The
more open the land, the better heat can escape into the atmosphere. An
area with many trees acts like a greenhouse, trapping heat close to the
ground. The study concluded that a mixture of dispersed trees and grass
is the most effective way to cool temperatures in the Salt Lake Valley.



========================================================================== "Understanding how ground cover impacts temperature is crucial for
city planners to weigh the benefits and costs of its landscape design,"
said Gomez- Navarro. "This land used to be a valley of bushes and bare
soil. Any vegetation we add is going to need lots of irrigation and
modify the landscape in many ways." The paper was published on October
13, 2020 in the journal Agricultural and Forest Meteorology.

Is the grass always greener? Gomez-Navarro focused on five parks and
the adjacent neighborhoods throughout the Salt Lake Valley: Hunter
(northwest), Lone Peak (southeast), Midvale City (south central),
Southridge (northwest) and Sugar House (northeast). Each location had 12 sensors that measured temperature and humidity: six within the park and
six in the residential areas. Gomez-Navarro analyzed ground cover in a
10-meter diameter around each sensor using satellite images to estimate
the percentage of the roofs, pavement, trees or turfgrass. She found
that the more turfgrass in a given area, the lower the temperature.

She analyzed canopy cover by taking photos of the sky above each sensor
with a fish eye lens. She used software that calculated the area that
trees obstructed the sky. She found that the more open the landscape,
the hotter the daytime temperature. The more canopy cover, the more
shade reduced temperature.



==========================================================================
The authors expected there to be temperature and humidity differences
between the parks and neighborhoods. They were surprised, however, that turfgrass had nearly the same impact on air temperature as trees. It
seems counterintuitive because of the difference between air temperature
and perceived temperature.

Perceived temperature is how humans feel the environment. Wind, air temperature, humidity and solar radiation all factor into how comfortable
we are.

"We didn't measure human comfort in this study, but we know that the
amount of solar radiation on our skin has a big impact on the perceived temperature," said Gomez-Navarro. "Even if the air temperature is the
same, we feel much cooler under the shade of a tree because it blocks some
of the radiation." Smart city planning Next, Gomez-Navarro will study
how different landscapes directly affect how humans feel comfortable in
their environment, and how plant cover type affect soil water loss.

"It's going to keep getting hotter and parks can be a refuge from the
heat. But exactly how many degrees can they cool the air? And what
should we plant to maximize this cooling?" said co-author Diane Pataki, professor of biology at the U. "It's getting easier and cheaper to measure temperature all over parks and neighborhoods, and we're going to need
this information to make good decisions about future park designs."

========================================================================== Story Source: Materials provided by University_of_Utah. Original written
by Lisa Potter.

Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Carolina Go'mez-Navarro, Diane E. Pataki, Eric R. Pardyjak, David R.

Bowling. Effects of vegetation on the spatial and temporal
variation of microclimate in the urbanized Salt Lake
Valley. Agricultural and Forest Meteorology, 2021; 296: 108211 DOI:
10.1016/j.agrformet.2020.108211 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/10/201013175028.htm

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