It’s Not the Heat—It’s the Irrigation

Where there’s heat, there’s often humidity. When high values of these factors combine, it leads to heat stress—aptly named as this experience makes it difficult for the human body to cool down. 

A new study led by Pacific Northwest National Laboratory (PNNL) researcher TC Chakraborty provides some answers about how irrigated agricultural land near cities affects urban heat stress. 

Irrigation’s cooling—and complicating—influence

Irrigation typically decreases air temperature and increases humidity. But researchers have debated what effect irrigation has on heat stress for some time. This is largely due to the difficulty of resolving urban areas, where most of us now live, because common methods to evaluate heat stress typically use coarse scale data.

Chakraborty and his colleagues tackled this challenge using high-resolution urban-resolving and convection-permitting model simulations to isolate the contributions of different factors that affect heat stress, including temperature, humidity, and other variables.

They found that irrigation has a substantial effect on moist heat stress, primarily by reducing daytime temperatures and slightly increasing relative humidity. This makes predicting future population-scale human heat risks difficult and is an open area of research.

Not all heat stress metrics tell the same story

An important contribution of this study lies in its assessment of how heat stress is quantified. Different scientific fields often rely on different indices, and those choices matter.

“We found that the wet-bulb temperature, which is a commonly used proxy for moist heat stress in Earth system studies, is likely an unreliable proxy for real-world human health impacts,” said Chakraborty. “This index is also rarely used in epidemiological research, which means there’s a need for greater convergence across disciplines to improve how we understand and predict heat stress impacts on people across the globe.”

Beyond irrigation: urban vegetation matters

In addition to irrigation, vegetation loss in cities—or "urban browning"—can also affect heat stress. Another of Chakraborty’s recent endeavors was a study led by colleagues from Nanjing University that examined this phenomenon globally. Their results showed that exacerbated heat stress was particularly prevalent in the Global South—which encompasses countries such as Nigeria, Botswana, Saudi Arabia, and Vietnam, as defined by the United Nations—where rates of urban browning were highest.

In a related study, Chakraborty and fellow PNNL researcher, Yun Qian, were part of another team also looking at urban vegetation differences between the Global North and Global South. They found that when cities emphasize urban greening, they can partially offset the negative impacts of initial physical vegetation loss due to urbanization. This effect was particularly clear in the Global North and underscores the difficulty of capturing these nuanced, localized dynamics in global Earth system models, which rarely represent urban vegetation.

Small areas, big modeling challenges

Chakraborty’s research interests go beyond heat stress. His Department of Energy Early Career Project focuses on improving the representation of urban areas in Earth system models, with a goal of better capturing the microscale dynamics relevant to city-scale processes and their interactions with energy systems. For instance, temperature and humidity changes due to urbanization controls building energy demand with impacts on grid resilience and indoor thermal comfort.

In another recent work, Chakraborty tackled the very basis of how global estimates of urban land are made. “By analyzing a variety of global urban land estimates made using different approaches, my coauthors and I found substantial discrepancies in the available estimates across scales,” he said. “This clearly highlighted the importance of intentionally choosing the most appropriate urban land estimate depending on consistency of urban definition with the goals of the study.” 

Chakraborty will continue his efforts on this topic as the co-investigator of a recently funded NASA Land-Cover and Land-Use Change project. For this project, he and his colleagues will look to better understand the impact of using different inputs of urban land estimates on urban-scale processes simulated by Earth system models.

Together, these select works from Chakraborty and his collaborators show the importance of and challenges with incorporating urban processes into large-scale Earth system models. Improving city representation in models is essential—both for predicting how heat stress and other environmental stressors will affect growing urban populations and for anticipating demands on the critical energy infrastructure which urban life depends.