By Deepa Padmanabhan
- Flash droughts are droughts that intensify more rapidly than normal, posing a risk to agriculture, ecosystems and water availability.
- A new study predicts a 7-8 fold increase in the frequency of flash droughts in India due to concurrent occurrence of extreme dry and hot periods during the monsoon season and greenhouse emissions.
- An increased frequency of flash drought pose a major risk to crop production due to soil moisture depletion and intraseasonal monsoon variation.
A new study predicts an increase in the frequency of flash droughts in India towards the end of this century. Intraseasonal variability of the summer monsoon rainfall and anthropogenic warming have been found to amplify the risk of future flash droughts and this can have negative impacts on crop production, irrigation demands, and groundwater abstraction in India, according to the study.
Flash droughts are droughts that intensify more rapidly than normal, posing a risk to agriculture, ecosystems and water availability. Conventional droughts take months and sometimes even years to develop to full intensity. Flash droughts on the other hand develop at an unusually fast rate due to extreme weather conditions and persist from a few weeks to some months. Such droughts can be localised to a specific region or can become widespread and affect a large part of the country.
The new study by researchers from the Indian Institute of Technology (IIT), Gandhinagar, investigated the causes of past flash droughts in India between 1951 and 2016. Based on the findings, the researchers predict an increase in the frequency of flash droughts in the future.
In India in recent years, flash droughts occurred in 1986, 2001 and 2015. The 2001 flash drought-affected north and central India while the 1986 and 2015 flash droughts were more widespread, impacting crop production. A 2020 study found that 10%-15% of rice and maize crop areas are affected by the flash droughts each year in India.
Ulka Kelkar, Director of Climate Program at the World Resource Institute (WRI) India, who was not involved in the study, said that as the climate warms, evaporation is likely to increase, and summer rainfall is expected to fall in a few intense spells. “We tend to think of drought as a slow-onset disaster. But many cropping practices are very tightly dependent on the timely arrival of the monsoon,” she said.
Though the impact may be lesser than a more severe conventional drought, frequent flash droughts can have a cumulative drastic effect on agriculture. The rapid onset of drought and lack of early warning does not give enough time for preparation and drought mitigation and could lead to extensive damage. For instance, in 2012, a flash drought in the United States of America impacted a large part of the country causing agricultural loss of over $30 billion.
Identifying flash droughts
Flash droughts can be identified either by monitoring changes in precipitation, evapotranspiration, or soil moisture. In the current study, the researchers used a hydrology model to simulate soil moisture patterns across the country to identify the duration and extent of flash droughts. A defining feature of a flash drought is the rapid depletion of soil moisture in the topsoil layer which then moves deeper affecting the ‘root-zone’ of crops and vegetation.
Wind speed, daily precipitation, and maximum and minimum temperature data of the last 60 years were fed into the model to estimate soil moisture over 5-day periods. The onset of flash droughts was identified as the time when soil moisture dropped below a particular level (the 20th percentile) over the 5-day period.
With these criteria, the researchers considered flash droughts with a minimum duration of 15 days and a maximum duration of 90 days and identified 15 flash drought events in the past. Of these, the flash drought in the year 1979 that affected 40% of the country, was observed to be the most severe.
The 1979 drought in India has been described as “the worst in this century nationwide” in a New York Times article. The hardest-hit areas received 75% less than normal rainfall and some states had reached ‘near-famine’ conditions. The reasons for the drought were not clear at that time but were thought to be due to odd-weather patterns and unusually hot temperatures.
This study offers some clues. On investigating the primary drivers of flash droughts, the researchers found that a simultaneous occurrence of extreme hot and dry periods resulting in depletion of soil moisture were the main causes.
Vimal Mishra, Associate Professor of Civil Engineering, IIT Gandhinagar and lead author of the study explains that in 1979, around 125 days after the onset of the monsoon season, the soil moisture was very high. But within 10 days, the soil moisture dropped very quickly as there was a dry spell and temperatures were high, with an average increase of more than 2 degrees centigrade.
He said, “Normally, conventional droughts take months to build up. But flash droughts can occur during the monsoon season when there are long breaks in the monsoon and the temperature is high. Soil moisture dries within 1-2 weeks, causing flash droughts.”
To gain an understanding of how flash droughts would occur in future climates, they used the same model but fed data from a global climate model, Community Earth System Model that provides temperature and precipitation data for the year 1900-2100, to estimate soil moisture.
The model predicts a 7-8 fold increase in the frequency of flash droughts in India similar to the one in 1979. Mishra said that although during the initial phase of the monsoon, there is a projected increase in rainfall, during the later season, rainfall is projected to decline. But temperatures are projected to increase throughout resulting in extreme hot and dry periods. This intraseasonal variation in monsoon along with warmer temperatures will increase the risk of flash droughts.
The influence of anthropogenic events such as greenhouse emissions, land use-land cover change and industrial aerosols were also investigated. The study found that greenhouse emissions will significantly increase the frequency of extreme hot and dry periods, which are the main drivers of flash droughts.
Raghu Murtugudde, professor of Atmospheric and Oceanic Science at the University of Maryland, USA, who was not involved in the study said, “The study looks at the past history of flash droughts in terms of the dynamic patterns before getting into future projections. This is a good way to do things. However, the models remain unreliable.”
He said that most models are unable to produce the historic trends in the monsoon and the projections of regional patterns for rain, for e.g. which month will have maximum rain, are different for each model.
Since flash droughts are a very regional phenomena, he added that it is critical to understand reliable regional patterns within the overall dynamic patterns.
“India is like a popcorn kettle that is getting hotter. But where the kernels will pop or not is a difficult process to model or project. So I tend to be very skeptical about projections beyond a decade or two,” he said.
But he expects that warming of the Indian Ocean and weather influences from the Atlantic and Pacific Oceans may produce large-scale extreme changes in monsoon which could cause flash droughts.
Impact on agriculture
The Indian summer monsoon varies across different time scales such as interannual or diurnal, but the intraseasonal variations are the most prominent and crucial ones that impact agriculture production, according to Arindam Chakraborty, Professor, Centre for Atmospheric and Oceanic Sciences, IISc, Bangalore.
Chakraborty who was not involved in the research said, “this study is essential to show how atmospheric forcings such as longer breaks (in monsoon) can impact the soil moisture and thus the agriculture production.”
Murtugudde added, “the impact on agriculture is even more complicated because just the warming will do a lot of damage even if rainfall doesn’t change. Droughts themselves get exacerbated because of the warming in terms of soil moisture loss.”
Kelkar’s work in the Jalna district of Maharashtra has revealed that poorer farmers, usually from low caste or small farm holders are most vulnerable to drought. When droughts become severe or widespread, they impact agriculture as labour work is hard to find and even factories can shut down due to water shortage, said Kelkar.
“With climate change, we need to prepare for drought just as we would for a sudden flood or storm – with early warning systems, drought-tolerant crop varieties, hardy cattle breeds, cattle shelters, and crop insurance,” she warned.