Mumbai: Extremely heavy rainfall events over the last two monsoon periods may have led to a perception that India's summer monsoon rainfall has increased, but it has in fact decreased by 6% over the past 60 years, say Indian government and international climate change assessments. The summer monsoon in 2021, though 'normal', was marked by a number of such localised extreme rainfall events and displayed variability in patterns of rainfall dispersal, both of which will only increase in future, these assessments warn.
The summer, or south west, monsoon, accounts for 70% of India's annual rainfall and is crucial for its agricultural economy, which accounts for 11% of gross domestic product. In the 2021 monsoon season, India recorded 870 mm rainfall, against a long period average (LPA; 1961-2010) of 880 mm, the India Meteorological Department (IMD) said on October 1. The realised rainfall being 99% of LPA, this year's monsoon was categorised as normal.
The way this normal was achieved, however, was unprecedented, IMD data show. The monsoon started off at a healthy 110% of normal levels in June, dipped to 93% in July, dipped further to drought levels of 76% in August, then bounced back to a rare 135% of LPA in September. The incidence of localised extremely heavy rainfall events in July and September, especially on the western coast, were among the highest in the past five years. Hundreds died in floods triggered by extreme rain in Maharashtra in July, and in Gujarat in September. A cloud burst on October 9 flooded the twin cities of Hyderabad and Secunderabad in two hours. Cyclones like Tauktae and Gulab, which bookended the summer monsoon in 2021, have increased and are set to increase further, IndiaSpend had reported in May 2021.
In mid-August, a report by ratings agency CRISIL had flagged concerns about patterns of rainfall dispersion across the country, noting that the monsoon "went on a hiatus" during the important kharif (monsoon) sowing months of July and August. It also expressed concerns about reservoir levels falling below the long-term trend. Then came the excess September rain. By the first week of October, official data showed an increase in both reservoir levels and area sown under kharif crops.
But the September rain could not make up the overall seasonal deficit by end-August in the north and northeast, with the latter region particularly affected. As of October 8, the Indian Institute of Technology's Drought Monitor showed high levels of drought over most of the north east and parts of the Indo-Gangetic plain.
The abnormally high rain in September could also adversely impact yield for some short duration monsoon crops, experts told us. The paddy harvest in Punjab and Haryana has also been delayed due to untimely showers in late September, the central government said on October 1. Worse, October rain has become so unpredictable in recent years that it is becoming hard to advise farmers on what to do, experts said. Monsoon 2021 missed even the revised, extended dates for monsoon withdrawal, adding to the October uncertainty.
Such uncertainties in the monsoon will only increase, the climate change assessments say, which also project that climate change will lead to an increase in overall monsoon rainfall by the end of this century. Experts told us the recent monsoons show that these projected increases in the variability of rainfall patterns and extreme rainfall events have already begun.
Uneven rainfall over time and across regions
The countrywide total summer monsoon rainfall in 2021 was unevenly spread over both time and across four broad regions: South Peninsula (Karnataka, Telangana, Andhra Pradesh, Tamil Nadu, Kerala and the island union territories), Central India (Gujarat, Maharashtra, Goa, Madhya Pradesh, Chhattisgarh and Odisha), East and Northeast India (Bihar, Jharkhand, West Bengal and all north eastern states), and Northwest India (Rajasthan, Haryana, Delhi, Chandigarh, Punjab, Jammu and Kashmir, Ladakh, Himachal Pradesh, Uttarakhand and Uttar Pradesh).
All regions saw above-normal rainfall in June. The deficits started in July, with three out of the four regions seeing below normal rainfall. Conversely, the South Peninsula states and UTs saw rainfall 26% above the July normal. All regions except the east and north-east recorded below normal rainfall in August. At a 24% deficit, 2021 saw the first August drought since 2009, according to Climate Trends, a Delhi-based strategic communications initiative on climate change and energy transition. By the end of the month, India was staring at a deficit monsoon, with overall rain at -11%. Nearly a fifth of the country was facing a drought scare.
Recovering from a deficit as large as 24% on August 31 was unlikely, Climate Trends had said. In August 2009, 26% deficit rainfall was followed by rain deficiency of 19% in September. But September 2021 was different, recording surplus rainfall of 35 percentage points, the second highest rainfall in September in the last 28 years, after only 2019, according to IMD. With that, monsoon 2021's overall performance touched the normal range.
The geographical rainfall imbalances were reduced after the September rain, with only the east and north-east region showing a significant shortfall.
Even after the September rainfall recovery, however, reservoir levels in some northern states remain low, per Central Water Commission (CWC) data.
The CRISIL report in mid-August had noted concerns about reservoir levels below normal or lagging the 10-year average in 10 of 19 reservoir states, and cautioned that further deficient rainfall in the reservoir catchment areas in these states would not bode well for irrigation buffers. By October 7, reservoir levels across most of the country had recovered. Reservoirs in only Himachal Pradesh and Punjab remained far below their 10-year average, per CWC, at 26% and 41%, respectively. This is despite Punjab recording 77% excess rainfall in September.
Western coast bears the brunt
IMD data show that the number of heavy rain events in 2021 was the second lowest in the last five years, but further illustrate the unusual pattern of low August rainfall and very high September rainfall this year. Heavy rain events encompass very heavy (115.6 to 204.4 mm) and extremely heavy rainfall events (more than 204.4 mm of rain). August 2021 recorded the least extremely heavy rainfall events in the last five years and September 2021 the most. September 2021 also saw the second highest number of very heavy rainfall events since 2017.
Very heavy rainfall events were witnessed across the country, while extremely heavy rainfall events were seen mostly on the western Konkan coast spanning south Gujarat, Maharashtra, Goa and Karnataka.
"Maharashtra bore the brunt of extreme rain events this year," R.K. Jenamani, senior scientist with the IMD's National Centre for Medium Range Weather Forecasting in Noida, told IndiaSpend. "The Raigad floods caused a lot of damage. We have never seen Mahabaleshwar receiving more than 500 mm rain for two days in a row. That one week's rainfall in the Konkan-Goa belt [in July] made up a big part of our total seasonal rainfall."
The formation of a tropical storm in September, i.e. during the monsoon, was another rare event this season. Cyclone Gulab, only the third cyclone in the 21st century to form in September, had formed in the Bay of Bengal and made landfall near Srikakulam over north coastal Andhra Pradesh on September 26. Even more rare was the fact that after landfall, the remnants of Gulab crossed over peninsular India to the Arabian Sea, where it re-emerged as Severe Cyclonic Storm Shaheen.
Unseasonal rain affects short-term monsoon crops
Adding to the unseasonal rain in September, monsoon 2021 has not withdrawn in expected time frames. On May 15, 2020, the IMD had revised the monsoon arrival date for 58 of 64 cities and withdrawal dates for 52 of these cities, effective June 1, 2020, in line with changing arrival and withdrawal patterns over the past five decades. In some regions like north-western Rajasthan, north-western Maharashtra and north-western Gujarat, the monsoon withdrawal date was revised by as much as 10-14 days. In Jaisalmer, for instance, the monsoon could now be expected to withdraw by September 17 instead of September 1.
Even so, the monsoon in 2021 missed its renewed withdrawal date in Rajasthan and other parts of north-western India, sticking around for an additional three weeks, till October 6. 2021 is the third year in a row when India will see a delayed monsoon retreat. In 2020, withdrawal started on September 28 and in 2019, on October 9. All this unseasonal rain will adversely affect the yield of some short duration monsoon crops, say experts.
In the Indian subcontinent, the two main crop planting seasons are kharif (planted at the start of the monsoon) and rabi (planted at the end of the monsoon). The kharif season is thus entirely dependent on the monsoon. By September 17, 110.5 million hectares had been covered by kharif crops in India, a 4% increase over the average of the corresponding week in the past five years, and marginally less than last year, according to Ministry of Agriculture data. Among 21 major crops, seven showed a greater than 5% decrease in coverage area compared to the 5-year average.
By October 1, the overall area under kharif crops had increased by 0.21% over last year's levels, but the deficit for six of the 21 major crops had increased.
Putting this in context for farmers and the farm economy, Hetal Gandhi, director, CRISIL Research in Mumbai, said the overall kharif coverage data mask sharper decreases for some crops. "For kharif crops, we are expecting the sowing numbers to be flat or marginally down compared to last year, which would be less than a 0.5% drop. Even though overall acreage may show a flat trend, [sowing of] crops like cotton, jowar, bajra and groundnut are seeing a sharper drop," Gandhi told IndiaSpend on October 1.
Sowing, however, is only half the story; yield is the other. Maturation of the standing paddy crop in Punjab and Haryana was delayed due to unseasonal rain in September, the central government said on October 1. Noting high moisture content ranging from 18% to 22% in paddy samples from Punjab and Haryana, respectively, it advised agencies in both states to assist farmers with drying of paddy, to ensure that procurement could commence from October 11.
"Heavy rainfall in September is expected to impact yields of short duration kharif crops, which include groundnut, urad, soybean and maize," said Gandhi. These four crops are at the maturity stage, due to be harvested within the next couple of weeks. If the rains don't stop, the storage and transportation of these crops will be disrupted, and the rain is already affecting the quality of harvested and stored onions.
"Market placement is being done early and onions are being sold at a lesser price [by farmers, for fear of spoilage due to rain] which otherwise would have fetched better [prices] during the lean season of September to November," Gandhi said, adding that due to the rains, the farmers have sold much of their produce already, leading to prices crashing as supply has exceeded demand. "Overall, for farmers, we should hope there is no unseasonal rain now ahead of rabi sowing," she said.
September a 'new August', October an anomaly
Erratic rains may get worse and farmers will have to prepare, Sridhar Balasubramanian, associate professor of mechanical engineering and faculty at the IDP Climate Studies Centre at IIT Bombay, told IndiaSpend. "For example, often in June, an initial wet spell is followed by a lull or a bout of very heavy rain, neither of which are conducive to sowing."
"Looking at the past 10 years of data, September has become like a new August. October is also becoming unseasonably active but a clear pattern is not emerging. So it is very, very hard to advise the farmer on what to do," said Balasubramanian.
Monsoon rainfall data for the last 10 years show a trend of increasing rain in September, and widely varying patterns in October.
National and international climate change assessments have flagged these increasingly varying patterns, and IMD's seasonal rainfall data over the past 120 years show a decrease in the amount of monsoon rain.
The Ministry of Earth Sciences' first ever climate change assessment for the Indian region, released in 2020, said that monsoon rainfall had declined by around 6% from 1951 to 2015, with notable decreases over the Indo-Gangetic plains and the Western Ghats. At the same time, India was seeing more frequent dry spells and more intense wet spells during the summer monsoon season, said the assessment, a pattern witnessed in August and September 2021 as well. Over central India, the frequency of localised extreme single-day rain events exceeding 150 mm per day increased by about 75% between 1950 and 2015, the assessment said.
Climate change perpetuating monsoon uncertainty
What caused India's monsoon rainfall to decrease? Mean rainfall should have increased due to the effects of global warming induced by greenhouse gas emissions, per the government assessment, but the radiative effect of human aerosol emissions, which were at high levels in the 20th century, "considerably offset" the effects of warming, leading to a decrease in mean rainfall. With human-caused aerosol emissions expected to decrease in future, and with continued global warming, mean monsoon rainfall will increase by the end of the 21st century, as will variability in rainfall dispersal and frequency of localised extremely heavy rain events, the assessment cautioned.
Simply put, climate change means we will have more monsoon rain in future, but also more variability in rainfall dispersal over the monsoon period, as witnessed in 2021, and more localised extremely heavy rain in short spans of time, as seen in coastal Maharashtra, west Gujarat and Hyderabad this year.
The findings of the government assessment were repeated in the Intergovernmental Panel on Climate Change's Working Group I 6th Assessment report, published ahead of the Conference of Parties 26 on climate change, to be held in Glasgow, UK, later this month.
"There has been a noticeable declining trend in rainfall with monsoon deficits occurring with higher frequency in different regions in South Asia. Concurrently the frequency of heavy precipitation events has increased over India while the frequency of moderate rain events has decreased since 1950 (high confidence)," said the 6th Assessment.
The IPCC's assessment also projected increased monsoon rainfall and intensifying monsoon extremes in India and South Asia towards the end of the 21st century, based on models showing the dominance of 20th century human aerosol emissions giving way to the dominance of greenhouse gas emissions. Even a 0.5 degree centigrade increase in warming would imply increased precipitation of 3%, it said.
Pulak Guhathakurta, head of the Climate Research Division at IMD, Pune, said that the trend of increasing rain and increasing variability of monsoon precipitation is already being observed.
"The observed trend of rainfall all over India, whether during the monsoon or other months, is slightly showing a decreasing trend. We have seen multi-decadal variability, where 1901-1930/40 was a dry epoch, then there was a wet epoch up to 1970. Ever since, it has been a dry epoch, but slowly India is recovering from it. Last two years, we had a good monsoon. There is already a significant increasing trend in extreme rain," Guhathakurta told IndiaSpend. "The number of days with zero rain and very heavy rain is increasing, but the number of days with moderate rainfall, which is very good for agriculture, is decreasing," he added.
Balasubramanian of IIT Bombay also believes the increase in precipitation will be in the form of extreme rainfall. "If you look at India overall, IMD has actually reduced the long period average rainfall amount from 889 mm to 880 mm. But, if you look at individual places, Mahabaleshwar which used to get 4,000 mm rainfall is now getting 6,000 mm. So what IPCC is trying to say is that warming will increase the likelihood of precipitation by means of extreme events. We will still receive 880 mm annually, but we will see an increase in extreme rainfall events."
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