Irrigation drives urban cooling, study reports

A new study shows that irrigation and farmland greatly affect urban cooling.
By James Carlin | Oct 28, 2017
While climate scientists fear global warming will cause cities to become much hotter than rural areas, new research from researchers at Purdue University suggests some urban regions may experience a cooling effect instead.

In the study, researchers foundthat over 60 percent of urban areas in India experience a day-time cooling effect. While the process has been noted in past research, this is the first time scientists have been able to identify the cause: lack of moisture and vegetation in non-urban areas surrounding the city.

"When the areas around cities are running low on water and they aren't being irrigated, they turn into hot, dry, barren fields," said study co-author Matthew Huber, a professor of earth, atmospheric, and planetary sciences at Purdue University, according to Phys.org. "When that happens, there's actually more water available to evaporate in the cities than the surrounding countryside. It's like the cities are sweating."

Typically, the so-called "heat island" effect causes cities to be warmer than their rural surroundings. This occurs because cities lose vegetation as they develop. Without that shade and moisture, the regions become hotter over time.

However, the findings show that is not always the case.

In the study, the team collected temperature data from 89 cities in India and then used a climate model to determine the effects of irrigation. This showed that urban temperatures are largely driven by both agriculture and moisture availability from irrigation. Season and region play big factors in regional heating or cooling as well.

However, while most of the urban centers looked at in the study cooled during the day, almost all of them became warmer at night. The effects of night-time warming were especially intense in the semi-arid western region of India.

This is important because intense warming can be deadly. In May of 2015, a massiveheat waveled to over 2,000 deaths, and such temperature spikes are expected to become more frequent as time moves on.

The team hopes their new findings will show officials how to use land in a way that will create cooling effects. That could then lead to more effective urban planning and improve public health.

"[T]hat has implications for water use," added Huber. "Are you going to impoverish the countryside and leave those areas barren, and the cities lush? These are the kinds of questions we're asking: what are the tradeoffs?"

This research is detailed in Scientific Reports.

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