The years 2016 to 2018 saw a major drought in the South of India due to low winter rainfall from the northeast monsoon – rainfall crucial for water availability, agriculture and livelihoods for the southern states of Tamil Nadu and Andhra Pradesh.
The impact was so severe that in Chennai, India’s sixth largest city, four of the city’s major reservoirs went bone-dry and groundwater levels reached dangerously low levels. As if this was not enough, a “Day Zero” was declared in 2019 and residents in the city scrambled to obtain water from tankers.
Why did this occur? Could these extreme events repeat themselves again? Would it be possible to predict them in advance to manage them better and reduce their impacts in the future?
<p><em>Researchers Vimal Mishra, Kaustubh Thirumalai, Sahil Jain and Saran Aadhar analysed rainfall patterns over the last 150 years to answer these questions in their study titled '<a href="https://iopscience.iop.org/article/10.1088/1748-9326/abf289/pdf">Unprecedented drought in South India and recent water scarcity</a>' published in the journal </em><a href="https://iopscience.iop.org"><em>Environmental Research Letters. </em> </a></p>
India experiences two major monsoon seasons—the Indian summer monsoon (ISM), also known as the south western monsoon (SWM) and the north eastern monsoon (NEM) or the winter monsoon.
<p><em>While the SWM is the major source of rainfall for India over the period of June to September, NEM is more important for parts of South India that receive a majority of their annual rainfall during October to December. Although rainfall from the NEM is lesser as compared to SWM, it is critically important for water availability, agriculture, and livelihoods of millions of people residing in peninsular India</em>.</p>
The movement of the sun to the north during summer heats up the Indian landmass to create an area of low pressure that attracts moisture-laden winds from the cooler ocean to form the monsoons. Low pressure near the equator also pulls together moist winds to form a cloud belt around the earth. Heat pushes this cloud belt forward as it merges with the monsoon winds to form a trough. The advancing monsoon is helped by jet streams blowing from the west to east. Western Ghats, the Eastern Ghats and the Himalayas offer enough highlands to make the moist winds yield rains while the Himalayas confine the monsoons within the Indian subcontinent.
<p><em>Watch <a href="https://www.indiawaterportal.org/articles/mysterious-and-moody-monsoon">this video</a> to know more on how the monsoon clouds form over India</em></p>
The reverse happens during the winter, when the land is colder than the sea, establishing a pressure gradient from land to sea. This causes the winds to blow over the Indian subcontinent toward the Indian Ocean in a northeasterly direction, causing the northeast monsoon.
The NEM occurs in the months of October–December (OND) and while it contributes only about 11 percent of the all-India annual rainfall as compared to the SWM, it accounts for 30 percent and 60 percent of the annual mean rainfall in Tamil Nadu, coastal Andhra Pradesh, and Rayalseema thus greatly affecting rice and maize production in the states.
However, the monsoon season over India is changing rapidly over the past few decades and NEM has also being showing changing patterns leading to unprecedented droughts in the south of India. The mechanisms of why and how these monsoon seasons are shifting in the context of global warming can be very important for improving predictions of drought conditions in India.
The analysis of NEM rainfall patterns over the last one hundred and fifty years reveals that:
<p><em>The study found that <h3>the recent drought of 2016-18 was even worse than the Great Drought of 1876-78 </h3>with a precipitation deficit of 45 percent as compared to the Great Drought that had a deficit of 37 percent</em>.</p>
The team looked at sea surface temperatures (SST), sea-level pressure (SLP) and winds generated during the winter monsoon to understand how they affected the northeast monsoon rainfall.
The study found that in
<p><em>Thus, negative IOD and La Niña conditions led to deficits in NEM rainfall and prevented moisture transport from the Bay of Bengal into peninsular India. The prevalence of La Niña throughout 2016 and 2017 further worsened this drought that started in 2016</em>.</p>
Such rainfall deficits over consecutive years can result in repeated droughts, which can negatively impact surface and groundwater storage, and affect water availability and agriculture in the densely populated South Indian region. Prediction of La Nina can be crucial to plan strategies to cope with impending drought like conditions earlier on, argues the study.
<p><em>Another paper titled '<a href="https%3A%2F%2Fwww.currentscience.ac.in%2FVolumes%2F120%2F01%2F0043.pdf">The Chennai Water Crisis: Insufficient rainwater or suboptimal harnessing of runoff?</a>' in the journal <a href="https://www.currentscience.ac.in">Current Science</a> by Sumant Nigam, Alfredo Ruiz-Barradas and Agniv Sengupta analyses 116 years (1901–2016) of rainfall in Chennai Sub-basin</em>.</p>
The paper informs that late summer (September) rainfall and NEM in the Cauvery Basin shows a decline in recent years (1987–2016) and this decline, as well as the mid-20th century increase, can be attributed to natural multidecadal climate variability (Atlantic Multidecadal Oscillation) and are not necessarily due to climate change.
<p><em>Analysis of runoff that includes rainwater leftover after its hydrologic and atmospheric processing generated in Chennai shows that <h3>harnessing and utilising even half of the winter monsoon runoff in the Chennai Sub-basin can help to meet the water demand of the city for about seven months</h3></em>.</p>
While more work needs to be done to find out how the runoff can be saved and stored, harvesting 42 percent of the winter runoff no longer seems beyond the realm of possibilities, states the paper.
<p><em>Chennai city’s water woes are not due to insufficient rainwater in the regional sub-basin, but rather due to suboptimal harnessing of related runoff, confirming that ‘Metrocities are in runoff-rich regions but water-deprived’, argues the paper</em>.</p>