The Indo-Gangetic Plain stretches across the north of the Indian subcontinent. Oppressive heat and humidity build up during the early summer months across the plain, stressing the roughly 400 million people who live there and the crops they grow. Because agriculture is practiced throughout the region, some scientists thought irrigation was responsible for a combination of cooler temperatures and higher humidity in the premonsoon season, but a new study published in Nature Communications has said otherwise.
Previous research has suggested that large-scale irrigation in the region causes a cooling effect of 3°C–4°C in premonsoon summer months and increases humidity that results in moist heat stress. Moist heat stress limits humans’ ability to cool off through sweating.
However, the irrigation patterns simulated in previous models do not reflect agricultural reality in the Indo-Gangetic Plain, according to the new study. Throughout the plain, irrigation is common from June to September and November to February, when most crops are grown. But irrigation is rare in April through early June—a period that also experiences intense heat and humidity. By ignoring seasonal agricultural practices, previous research overestimated the role irrigation plays in premonsoon moist heat stress, according to the new study.
“We set out to understand the degree to which irrigation contributes to heat stress in the summer season when major agriculture activities do not take place,” said Arpita Mondal, a hydrology and climate scientist at the Indian Institute of Technology (IIT) Bombay and one of the authors of the study.
Using governmental irrigation estimates and satellite data of temperature, evapotranspiration, and vegetation, the authors found that models that do not incorporate real irrigation patterns overestimate the land surface cooling effects of irrigation during premonsoon months by about 1.7°C, nearly a factor of 5. In addition, humidity increased by only 2.5% in the data-based models, rather than by the 10% found by more general models, suggesting irrigation plays an inconsequential role in moist heat stress during this time.
To explore what factors aside from irrigation could cause the premonsoon heat stress, the study used satellite measurements of aerosols to examine the impacts of aerosols on cooling. During premonsoon months in the Indo-Gangetic Plain, biomass burning results in high atmospheric concentrations of aerosols such as black carbon. These aerosols can change the region’s radiative balance.
Since the 1980s, aerosol concentrations have been increasing in the premonsoon season, whereas irrigation increased only in the monsoon season, highlighting the importance of aerosols as a control for land surface temperature during the premonsoon months. The study found a strong relationship between summer heat and aerosols—not irrigation—over the Indo-Gangetic Plain.
To determine the drivers of premonsoon humidity, the researchers tested the role of moisture coming from distant sources. They found that moisture from the ocean and the northern Himalayas, rather than irrigation, could be responsible for the increase in moist heat stress in the plain.
Aditi Mukherji, a climate scientist at the International Water Management Institute who was not involved with the study, said the paper is an “important one and is quite consistent” with irrigation data in the summary for policymakers in Working Group II’s (WGII) contribution to the Intergovernmental Panel on Climate Change’s Sixth Assessment Report (AR6). Mukherji is one of the authors of WGII’s contribution to the AR6.
Policymaking for Climate Adaptation
Overall, the new study improves scientists’ understanding of how irrigation and atmospheric factors contribute to heat stress in the region. But there’s a bigger issue to consider.
“Most climate models and their various components are tailored for the Western world,” said Subimal Ghosh, a climate scientist at IIT Bombay and a coauthor of the paper. For example, many irrigation models assume farmers will test soil moisture and decide how much water to use in irrigation. But in India and across South Asia, practices like flood irrigation for paddy farming are widely used, and that assumption would not apply.
“We need models suited to regional conditions.”
“For the longest time, paddy [farming] was not even represented in global models because of the dominance of crops like corn,” which use methods like drip or pivot irrigation, Ghosh explained. Such exclusions and “one-model-fits-all” approaches have led to inaccuracies in climate models, whose results could then negatively affect policies and farmers in regions with unique seasonal practices.
“Imagine a farmer engaging in some small premonsoon cropping activities. Now, if research shows heat waves are linked to irrigation in the Indo-Gangetic basin, he might be forced to stop irrigation. But blaming irrigation was incorrect [in the first place],” Ghosh said.
“For atmospheric and ocean sciences and for hydrology, where human components play a big role, we need models suited to regional conditions,” Ghosh said. One way to deal with this, he added, is scientific peer review by regional experts to ensure that on-the-ground realities are accurately reflected in models.
—Rishika Pardikar (@rishpardikar), Science Writer
15 April 2024: This article has been updated to correct the amount of overestimation in irrigation-related moist heat stress.