24-06-2025
Rising evaporative demand spotlights India's data and research gap
M. S. Kukal, M. Hobbins,'Thirstwaves: prolonged periods of agricultural exposure to extreme atmospheric evaporative demand for water', Earth's Future, March 20, 2025.
The air itself has become more thirsty due to global warming. Quenching this thirst has meant more water is coming off the land, including from plants and trees, leaving them drier.
Evaporative demand is a measure of how thirsty the atmosphere is. Meetpal Kukal of the University of Idaho and Mike Hobbins of the University of Colorado and the U.S. National Oceanic and Atmospheric Administration coined the term 'thirstwave' to denote three or more contiguous days of intense evaporative demand — which they recently found to be increasing over the U.S.
Their research was published in Earth's Future, where they wrote: 'Not only have thirstwaves increased in severity, but the likelihood of no thirstwaves occurring during the growing season has significantly decreased.'
More water leaving
While heat waves are caused by particular temperature and wind patterns, a thirstwave is the product of temperature, humidity, solar radiation, and wind speed. When temperatures rise, the consequences include more heat as well as the mechanics of water exchange between land and atmosphere, which in turn alters humidity, wind, and solar radiation.
'If you are a farmer growing rice or wheat, and your crop is irrigated sufficiently, its water use on any given day will be dictated by what the atmospheric evaporative demand is on that day,' said Mr. Kukal, assistant professor of hydrologic science and water management at the University of Idaho. Evaporative demand determines the near-maximum of how much water will evaporate from a given piece of land if sufficient water is available.
In a warming world, the researchers found that thirstwaves have grown more intense, are more frequent, and are lasting longer, especially in seasons when crops are grown. While previous studies examined the mean or total evaporative demand, the new one focused on extremes.
A simplified measure
Mr. Kukal said evaporative demand is measured using standardised short-crop evapotranspiration — defined as the amount of water a grass surface 12 cm high and which has continuous access to sufficient water and is free of any stress will use (evapotranspiration itself refers to the two processes by which water moves from land to the atmosphere: evaporation from surfaces and transpiration from plant leaves.)
Mr. Kukal called standardised short-crop evapotranspiration 'a core concept that is recommended to be used in deciding how much and when to irrigate a crop', adding that it is 'a simplification of [an] otherwise very complicated process, where we are assuming the vegetation properties to be constant, so water use is only a function of weather.'
An increasing standardised short-crop evapotranspiration means the ambient temperature is increasing, the humidity dropping, wind speeds picking up, and the amount of solar radiation picking up as well.
Effect of humidity
In a paper published in Agricultural and Forest Meteorology in 1997, Nabansu Chattopadhyay and M. Hulme had suggested that both evaporation and potential evapotranspiration — the maximum amount of water that can be evaporated from any surface — decreased in India during the 30 years before the publication of their paper. However, they added, future warming was likely to lead to more potential evapotranspiration over the country, with regional and seasonal disparities.
Chattopadhyay, who worked in the agricultural meteorology division of the India Meteorological Department (IMD), Pune, before his retirement, said that he and his co0author had analysed 30 years' worth of data from the IMD's network of evaporation stations and estimated potential evapotranspiration. But while warming over India should have increased evaporation, he added, their analysis found the opposite.
When they rechecked the data, he said humidity had nullified the effect of rise in temperature. Using global circulation models, they also found that future temperature increases would supersede the effect of humidity and increasing evaporative demand.
'Great direction'
In 2022, researchers from IIT-Roorkee, the National Institute of Hydrology (Roorkee) and institutes in France and The Netherlands, reported recent changes in evaporative demand across 100 river sub-basins in India. Their paper, published in the Journal of Cleaner Production, stated that 'the largest increase in actual evapotranspiration is found in Northern India, Western Himalayas, and several areas in Eastern Himalayas, which could be a sign of either increased vegetation or agricultural expansion.'
This said, according to experts, there is essentially no data about extreme thirstwaves over India.
'The sensitivity of different crops, ecosystems, and regions to evaporative demand will likely be different, but this has not been investigated much yet. This is a great direction for future research,' Mr. Kukal said.
A familiar pattern
While Mr. Kukal's and Mr. Hobbins's study was the first to characterise thirstwaves in the U.S., Mr. Kukal said there is considerable scope to investigate this phenomenon in the Global South, where societies are generally more vulnerable to the consequences of climate change.
As a step in this direction, Mr. Kukal is currently hosting Shailza Sharma, a PhD scholar from the National Institute of Technology, Jalandhar, to investigate thirstwave behaviour over South Asia with financial help from the Water Advanced Research and Innovation Program. They hope to publish their findings about thirstwaves in climate-vulnerable countries — with important implications for global food and water security — soon.
One particularly intriguing aspect of their research is that they found that 'the worst thirstwaves happened in places that do not experience the highest [evaporative] demand.' This means there may be a need to reevaluate how governments prioritise different regions of their countries for climate change preparedness and climate mitigation, using the lens of thirstwaves.
As the world warms further, tracking, measuring and reporting and sensitising farmers and water managers is of paramount importance, experts added.
G.B.S.N.P. Varma is a freelance science journalist.
