Harnessing runoff to cope with droughts in South India

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?

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 'Unprecedented drought in South India and recent water scarcity' published in the journal Environmental Research Letters.  

Monsoon in India

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.

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.

The southwest monsoon

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.

Watch this video to know more on how the monsoon clouds form over India

The northeast monsoon

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. 

The NEM and droughts

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:

There were five episodes of drought (with >29 percent deficits) in the period with the  2016–2018 being the recent one while others included the ones during 2001–03, 1949–a 1951, 2002–04, and the Great Drought of 1876–78), which was associated with the Great Madras Famine.

The study found that the recent drought of 2016-18 was even worse than the Great Drought of 1876-78 with a precipitation deficit of 45 percent as compared to the Great Drought that had a deficit of 37 percent.

The 2016–2018 drought had severe implications for water availability. Satellite data showed that this drought led to a total water loss of 79 km3 in December 2016 while the total water loss in June 2017 and 2019 was 46.5 and 41.7 km3, respectively. The 2016–2018 drought caused a significant loss in total water storage (TWS), which also led to a severe depletion in groundwater across South India.

This drought coupled with unsustainable groundwater extraction practices led to massive levels of groundwater depletion and the combined effect of depletion in surface water and groundwater led to a severe water crisis in South India.

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 2016 and 2017, cooler sea surface temperature (SST) anomalies prevailed in the tropical Indo-Pacific ocean and were associated with La Niña conditions in the central Pacific ocean. (La Niña is a climate pattern that occurs irregularly every two to seven years when the surface waters over the equatorial Pacific Ocean are cool and this affects global weather patterns). At the same time cooling was observed over the Indian Ocean. Both years saw cooler SSTs in the eastern tropical Indian Ocean and western tropical Pacific, and warmer SSTs in the western Indian Ocean and central Pacific.

These SST patterns, along with sea level pressure changes and surface air temperature changes gave rise to anomalous westerlies in the equatorial Indian Ocean, which weakened moisture transport from the Bay of Bengal during the northeast monsoon.

Of the five major droughts that occurred in South India in 1876, 2016, 1938, 1988, and 1974 in order of severity, four occurred during La Niña conditions while the drought of 1876 was linked with El Niño.

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.

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.

What can we as communities do?

Another paper titled 'The Chennai Water Crisis: Insufficient rainwater or suboptimal harnessing of runoff?' in the journal Current Science by Sumant Nigam, Alfredo Ruiz-Barradas and Agniv Sengupta analyses 116 years (1901–2016) of rainfall in Chennai Sub-basin.

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.

The crisis that Chennai faced has not been because of insufficient rainfall, but with what happens to the rainwater once it falls on the ground. The paper argues that Chennai’s water woes arise not from insufficient rainfall, but rather from the suboptimal harnessing of runoff.

Analysis of runoff that includes rainwater leftover after its hydrologic and atmospheric processing generated in Chennai shows that 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.

Storing this runoff will not be difficult as the four reservoirs currently supplying water to Chennai city can store 42 percent of the winter runoff at 80 percent full capacity.

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.

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.