Hailing from a farming family in Himachal Pradesh, Tejasvi Ashish Chauhan has direct experience of changing climate. He has heard his family saying crop timings changed, rain faltered, snow declined. Those things intrigued him. Joining IIT Bombay for a masters gave him a chance to explore how sensitive the water cycle is to global warming. Now, a postdoctoral researcher with Max Planck Institute for Biogeochemistry, he continues researching the intersection of water and climate.
To understand how sensitive the water cycle is to global warming, Chauhan reckons we have to start with storage and fluxes. The atmosphere is storage, it holds water which comes down as rain eventually. Groundwater—as the name says, water stored in the ground—is another storage. The ocean is yet another storage.
Water moves from one storage site to the other through evaporation and precipitation. It goes from land to atmosphere through evaporation, from atmosphere to land through precipitation, from land to the ocean through subsurface and surface flows, from ocean to the atmosphere through evaporation.
This movement of water is known as the hydrological cycle or water cycle. There is, of course, another storage and that is the cryosphere, frozen water, in snow and ice, glaciers and icesheets.
Chauhan says, “We have to understand that the atmosphere as a storage has a separate sensitivity and fluxes like precipitation, evaporation have a separate sensitivity.”
The atmosphere’s storage capacity—“the bucket size, if you will”—increases at around 7 per cent per degree of global warming or per degree Celsius of temperature change. It simply means more water is piling up in the atmosphere.
While the atmosphere increases its bucket size by 7 per cent for every 1C rise in warming, fluxes like evaporation and precipitation, in their long-term average, are not that sensitive. They increase, maybe, around 2-3 per cent.
“What happens is that now the atmosphere’s capacity to store water increases much faster than the rate at which water cycles through it,” Chauhan tells Hot Rock. This imbalance has enormous consequences for people, crops, land, oceans; in fact, our whole life.
Since evaporation is a slow process, it takes longer to fill up the 7 per cent larger bucket. The now-larger bucket rains down harder. Not just that, Chauhan says, “There is a higher intermittency between rainfall events because the bucket is larger but the process that fills it, evaporation, does not increase by as much at the same rate.”
It means larger gaps between spells of rain, farmers’ cropping patterns going haywire, different stages of a plant’s life getting scrambled, food insecurity and food not being within your means.
The woes don’t stop there. Chauhan explains since the bucket is larger, extreme rainfall events increase as the bucket can hold 7 per cent more moisture in a warmer world.
“So, even though precipitation on average only increases by 2-3 per cent, extreme precipitation, which causes flooding, increases by 7 per cent or even more as recent studies have found. That is how we arrive at sensitivities.”
As it takes longer to fill the bucket and flip it down, the gap between spells increases. Which means that you may get drier periods. Yet another implication is that in a warmer world, land and ocean respond differently. Land warms faster than the ocean. Correspondingly, there is a stronger response from the land compared to the ocean. Because land heats up much faster than the ocean, where much evaporation has to happen, “land’s bucket size increases in the same time much more”. Now, the atmosphere over land is warmer and so it is more prone to extreme rainfall and drier periods.
Yet another vital player in the mix is soil moisture. While extreme rain increases by 7 per cent or more, according to Chauhan, “whether or not it translates into flooding depends on the state of soil moisture”.
“With global warming, there is huge uncertainty in what soil moisture would look like, complicating impacts of extreme rainfall and predicating floods.”
If you survive flood, there is drought. It comes in many types. Lack of rainfall leads to meteorological drought; reduced soil moisture is agricultural drought; dwindled runoff from rivers is hydrological drought; decreased groundwater is groundwater drought.
Chauhan says there is a lot of uncertainty about drought and their estimates in a warmed world. However, it is clear that “meteorological droughts are going to be quite frequent because rainfall becomes more intense, and more and more far apart”.
To tide over the increasing volatility in the operation of the water cycle, Chauhan suggests India has to design water resource systems around the question: “How do we buffer a water cycle that is becoming more and more intermittent?"
Given that there are extreme rainfall events and increasing gaps between spells of rain, he says, “We need more and more storage.” He doesn’t see dams as ideal solutions. Big surface reservoirs lose water through evaporation, take up much land, need suitable topography and are often designed around historical inflow statistics. Groundwater doesn’t have these losses of water but it’s difficult to recharge.
“I would put groundwater and a distributed storage building regionally, as one of the most important points of India’s adaptation strategy.
“So, I think we can aim for promoting decentralised storage systems everywhere through ponds, lakes, recharge wells, and check dams where geologically suitable. This may sound less technologically exciting, but I think it matters enormously,” he says.
India had a lot of these storage facilities which were later occupied or built upon. Increasing storage and harvesting rainwater will go a long way in addressing the crisis.
“The broader goal should be to improve storage in a distributed way and also ensure more and more ground water recharge,” Chauhan says.
“For solutions, I would prioritise: Manage groundwater extraction and recharge together and restore distributed storage like tanks and increase rainwater harvesting. Convert reservoirs to forecast-informed, basin-network operation. Build operational drought forecasting tied to agricultural and other water resources related decisions.”
