WEST LAFAYETTE, Ind. — Agricultural irrigation is an important practice for sustaining food production in many regions of the world. Besides improving agricultural yields, irrigation also affects climate conditions, typically lowering land surface temperatures in a field and the surrounding areas. But a new study shows that in some parts of the world, irrigation increases moist heat stress, putting millions of people living in one of the most densely populated regions in the world at risk of adverse health impacts.
A new study published today in the journal Nature Geoscience indicates that decades of intensive irrigation in India’s Indo-Gangetic Plain have reduced average daily temperatures and lowered the probability of extreme heat conditions in the region, effectively dampening the impact of the global warming that has been observed in non-irrigated parts of the country.
Vimal Mishra, professor of civil engineering and earth sciences at the Indian Institute of Technology and lead author of the study, said “Irrigation has tremendously helped India’s efforts to achieve food security, but it has also led to rapid groundwater depletion and other negative environmental consequences. Furthermore, our analysis shows that while intensive irrigation does lower regional temperatures, it has significantly increased heat stress, affecting millions of people living in one of the most densely populated regions in the world.”
In addition to a cooling effect, the researchers find that irrigation increases near-surface humidity, which results in a dramatic increase in moist heat stress. This weather mix, the combination of high heat and high humidity, has a strong effect on human health. High humidity makes it more difficult for the human body to shed heat through the evaporation of sweat, making moist heat more dangerous than dry heat.
According to the study, heat stress projections in India and other regions dominated by semiarid and monsoon climates that do not include the role of irrigation could be underestimating important health risks. Although this study focused on agricultural irrigation, the findings have direct implications for the reported benefits of green infrastructure used to mitigate heat and urban heat island effects, especially as global temperatures continue to rise.
“As we all have experienced, when it comes to feeling hot, it’s not just the heat, it’s the humidity. In this case we found that under some situations in the future in India, the world’s most populous democracy, irrigation and the associated humidification of the air make conditions feel worse,” said Matthew Huber, co-author of the paper and Purdue professor of earth, atmospheric and planetary sciences. “Our results show that this simple fact must be given serious consideration when adding water to the air via evaporation in a hot region whether through irrigation or even ‘green roofs’ — will this reduce or increase heat stress?”
Moist heat stress extremes in India enhanced by irrigation
Vimal Mishra, Anukesh Krishnankutty Ambika, Akarsh Asoka, Saran Aadhar, Jonathan Buzan, Rohini Kumar, and Matthew Huber
Intensive irrigation in India has been demonstrated to decrease surface temperature, but the influence of irrigation on humidity and extreme moist heat stress is not well understood. Here we analysed a combination of in situ and satellite-based datasets and conducted meteorological model simulations to show that irrigation modulates extreme moist heat. We found that intensive irrigation in the region cools the land surface by 1 °C and the air by 0.5 °C. However, the decreased sensible heat flux due to irrigation reduces the planetary boundary layer height, which increases low-level moist enthalpy. Thus, irrigation increases the specific and relative humidity, which raises the moist heat stress metrics. Intense irrigation over the region results in increased moist heat stress in India, Pakistan, and parts of Afghanistan—affecting about 37–46 million people in South Asia—despite a cooler land surface. We suggest that heat stress projections in India and other regions dominated by semi-arid and monsoon climates that do not include the role of irrigation overestimate the benefits of irrigation on dry heat stress and underestimate the risks.