Irrigation linked to reduced rainfall in northern India

India remains one of the most highly irrigated regions in the world. Although the rise in irrigation over the decades has boosted crop productivity, it has also caused the depletion of groundwater. At the same time, irrigation affects regional climate by changing the water and energy balance between the land surface and the atmosphere.

This results in a drop in temperatures over heavily irrigated parts of India such as in the Indo-Gangetic Plains. The cooling, in turn, reduces the overall summer monsoonal rainfall over the region, finds a new model-based study by researchers from the Indian Institute of Technology (IIT) Delhi. 

“Although the larger amount of irrigation is in the western part of the Indo-Gangetic Plain, much of the rainfall decrease is over the eastern parts of the Indo-Gangetic plains and Central India, said Professor Krishna AchutaRao, senior author of the study. “So there are implications for rainfed agriculture as well as water security over large parts of India.”

Anomalously, temperatures (especially the extreme-high temperatures) over the northern plains of India have not been increasing as fast as over the rest of the world, observed AchutaRao. In this region, he said there could be two possible reasons that could mask the effects of global warming as suggested by previous studies: irrigation or aerosols (from air pollution). 

The fraction of agriculture under irrigation has grown substantially from a mere 20 percent in the 1960s when the Green Revolution began to 45 percent now, according to the India Economy Survey 2017-18. While many studies have explored the impact of irrigation on regional and global climate, few have investigated the impact of varying levels of irrigation. Understanding the impact is important because 60 percent of the sown area in India is rain-fed. 

Groundwater has played a key role in revolutionising irrigation in India: a staggering 89 percent of the groundwater that is extracted in India is used for irrigation as per the Asian Water Development Outlook of 2016. But under the backdrop of global warming, more water will be required for irrigation in the future, and this has raised concerns among scientists about the sustainability of using groundwater for irrigation. “If the rainfall is reduced in some parts of the country, it will impact groundwater recharge,” noted AchutaRao. 

Cooler and drier Indo-Gangetic Plains 

So how do varying levels of irrigation in India affect temperatures and rainfall? 

To find out, AchutaoRao and his Ph.D. student Roshini Mathur used a global atmospheric circulation model coupled with a land surface model and a slab ocean model to simulate the impact of a range of irrigation possibilities from the bare minimum to avoid water stress in crops to saturated soil. These simulations were carried out while holding all other conditions constant at year 2000 levels. They then compared the results from different levels of irrigation to a control simulation without irrigation. 

The model simulations showed lower maximum and minimum temperatures at all levels of irrigation and over heavily irrigated parts of India, temperatures drop by about 3 and 4 degrees Celsius respectively when compared with no irrigation. This cooling, in turn, reduces the overall summer monsoonal rainfall over India by 1.46–4.17 percent. Rainfall over the eastern part of the Indo-Gangetic Plains is lower while it is higher over the Punjab region in Pakistan. 

How does irrigation mediate cooling? When the Earth’s surface is wet, some of the sun’s energy reaching the Earth is used up in evaporating the water in the soil, which is known as the latent heat of vaporisation, explained AchutaRao. “This is like on a hot summer day you feel cool when you wet your skin. If the surface is dry, there is not enough energy diverted as latent heat for evaporation and all of the heat is felt as sensible heat.”

Cooling affects monsoonal circulation and moisture transport, which lowers the amount of monsoonal rainfall. AchutaRao explains: “During the months leading up to the monsoon, the land surface of India heats up leading to a temperature gradient between the cooler oceans and the warm land. This temperature gradient translates into a pressure gradient drawing in winds from the ocean onto land. When land is irrigated, it does not heat up as much, resulting in a reduced temperature gradient and consequently, weaker winds transporting moisture from the oceans. This reduced moisture transport results in reduction in monsoon rainfall.”

Sonali McDermid, assistant professor at New York University, who was not connected to this study, said that “the monsoon is a highly complex system and there are more interactions to consider here with respect to rainfall.” Apart from the transport of moisture, we need more studies to “understand the influence of irrigation on water and energy distributions high into the atmosphere; the presence and evolution of low-pressure centers; land-atmosphere coupling; and the overall interaction between widespread irrigation and the movement of the tropical convergence zone (among other things),” she elaborated. 

Other model-based studies have also reported a dip in temperatures due to irrigation but estimates of the cooling magnitude vary widely. The 3 and 4 degrees Celsius cooling for minimum and maximum temperatures respectively is on the stronger side, “but it is likely still within the uncertainty estimates across models,” said McDermid. 

Simulating rainfall impacts from irrigation is tricky for a number of reasons, states McDermid, noting that it depends on the area of consideration. “Most of the studies I’ve seen show declines in monsoonal rainfall as a result of irrigation across the entire South Asian domain. But this doesn’t mean you can’t have an increase in rainfall locally (particularly above heavily irrigated areas) or at other times of the year.” 

Irrigation uncertainties 

“This new study is highly useful and important in that it both confirms that irrigation may be an important regional climate influence in South Asia,” said McDermid, adding that “there is still considerable uncertainty in understanding the timing, seasonal effects, how strong this forcing is compared to others (air pollution/aerosols; greenhouse gas concentrations), and our ability to control these irrigation impacts by moderating our water use.” 

“What is pretty consistent across the many studies on this topic, inclusive of this current study, is that irrigation can not only change the rainfall amounts over South Asia but also the spatial distribution of rainfall,” said McDermid, stressing how important this can be. “If irrigation in one area does indeed impact rainfall over another, then we need to think about who this impacts and what the tradeoffs are in water use (and, ultimately, livelihood via production).” 

AchutaRao also pointed out that this study showed gaps in our understanding of the seasonality of irrigation—we don’t know how much water is actually added over the seasons. As a result, it is difficult to precisely model the magnitudes of rainfall and temperature changes, he cautioned. These concerns are echoed by McDermid, who highlighted the need for better observations and data to understand how much irrigation water is actually being applied and how it varies across space and time. 

In future, satellite remote sensing, AchutaRao suggested, could enable scientists to better estimate irrigation water use.

Read more: Is Indian monsoon turning into a string of extreme weather events?

CITATION:

Mathur, R., & AchutaRao, K. (2019). A modelling exploration of the sensitivity of India’s climate to irrigation. Climate Dynamics, 1-22.

Banner Image: Majority of India’s groundwater is used for irrigation. Photo by Ajay Tallam/Flickr.