Chennai

Northeast monsoon bows out, bestowing copious water but sparing people from damage caused by cyclones

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Residents of Chennai heave a sigh of relief because they wouldn't have to scrounge for water as several dams received plentiful inflows, the water table improved substantially and average rainfall target was met

When the northeast monsoon officially withdrew on January 10, it had left the State with far more water than when it blew into Tamil Nadu. It also allowed the residents of Chennai to heave a sigh of relief because they wouldn’t have to scrounge for water in the summer of 2020, as they did in 2019.

Known to manifest in myriad forms every season, the northeast monsoon surprised this time too. No cyclones, no devastation on account of rains, and yet, several dams received copious inflows, the water table improved substantially and average rainfall target was met.

Considered vitalas it provides 48% of the State’s annual rainfall (44 cm), the season is eagerly awaited. Coming after a year that sawa 24% rain deficit, expectations were high for 2019 and the monsoon managed to live up to expectations, delivering normal rainfall in the State.

Rains were mostly influenced by easterly wave activity and the State received 45.3 cm of rainfall between October and December, a2% excess (excess rainfall up to 19% of the long period average is termed normal by the Meteorological Department).

The season was memorable for its weather dynamics and new records set — be it the above average rains with no major weather systems, several major dams brimming with water at the end of the monsoon and an Arabian Sea that unusually brewed up cyclones even during December.

A subdued Bay

“Every northeast monsoon has a unique pattern and the weather keeps evolving. It is not unusual for the monsoon to spill over to early January. But, this time, the Arabian Sea churned up three cyclones — Kyarr, Maha and Pawa. That is a rare event. The Bay of Bengal remained subdued,” said N. Puviarasan, Area Cyclone Warning Centre, Regional Meteorological Centre, Chennai.

One of the most arid districts — Ramanathapuram — received 45% excess rain as the easterly wave triggered favourable conditions, he added.

Heavy rains during northeast monsoon months preceded by bountiful rains during the southwest monsoon have pushed up the availability of water for drinking and agrarian needs in the State this year. Groundwater levels too saw a sustained rise across the districts, with Salem witnessing the highest increase in water table by 5.42 metres in December 2019. “In Ramanathapuram, rainfall suited the agricultural season. About 500 acres of ayacut faced flooding as rains continued till January,” said L. Sornamanickam, Joint Director of Agriculture, Ramanathapuram.

Uneven spread

Tamil Nadu received 453.5 mm rainfall between October 1, 2019 and December 31, 2019, resulting in a 2% excess compared to the long-period average during the north-east monsoon in the State. However, the rainfall was not evenly spread. While some distrcits in the west received excess rainfall, most central and northern districts ended deficient. The Nilgiris district registered 64% more rainfall than the long period average- the highest excess recorded among districts while Vellore recorded the highest deficit in rainfall - 27% lesser rain than usual.

The map shows the district-wise distribution of excess or deficient rainfall compared to long period average. The darker the red, the more the deficiency and the darker the blue, the higher the excess.

Northeast monsoon bows out, bestowing copious water but sparing people from damage caused by cyclones
 

Thick and fast

Heavy rains upstream of Mettur dam during October and November helped increase the discharge into the Cauvery delta region, noted officials of the Water Resources Department (WRD).

The inflow in Biligundulu, which touched three lakh cusecs from excess rainfall in the catchment areas of Cauvery in Karnataka, added to the run-off into the Mettur dam. The Stanley reservoir at Mettur filled up to the brim four times this season. The water level in the dam attained its full reservoir level of 120ft on September 7, 24, October 23 and November 11 and was maintained at that level for nearly 100 days. The Ellis Saddle surplus gates of the dam were first raised on September 6 for samba cultivation.

Officials noted that the Bhavanisagar dam, another major resource, also filled up fast and its level was being maintained at 105 ft. “It went surplus last week. Several of the major dams now have good storage,” said an official. Dams in Tirunelveli district, including Papanasam dam, which had only dead storage till May, overflowed in December.

After 40 years, the Noyyal river had a flow of 5,500 cusecs this season and Coimbatore district also witnessed the Noyyal’s waters reaching the Cauvery. The rains increased storage in 28 major tanks, which enriched the water table in Coimbatore and Tiruppur.

Even in districts that ended up with deficit rain like Madurai and Salem, farmers and residents rejoiced as the areas recorded better rainfall compared to the previous year and dry lakes were rejuvenated. Waterbodies in the urban areas too filled up, raising hopes of drinking water availability throughout the year. In Mettur division, five out of 18 lakes have reached their capacity.

Marked improvement

As of December 2019, Nagappatinnam had an average ground water level of 1.23 m - the most in the State. Districts along the central and western regions of the State recorded the healthiest ground water levels. The average ground water levels registered an improvement in all districts as on December 2019 compared to the levels recorded in December 2018. This increase was most pronounced among the northern and central districts while the improvement in the western districts was relatively muted. The average level in Salem rose by 5.42m, the most in the State.

Northeast monsoon bows out, bestowing copious water but sparing people from damage caused by cyclones
 

The Vaigai river running through Theni, Dindigul and Madurai districts witnessed water flow unusually for more days though these districts received below normal rainfall. WRD Executive Engineer of Periyar Vaigai basin, T. Subramanian, said irrigation was possible not only in the 1.7 lakh acres of land in Madurai, Theni and Dindigul districts but also in areas in Sivagangai and Nilayur regions.

Describing the retreating monsoon to be volatile, weather expert Y.E.A. Raj said the region between Pamban and Thiruchendur is not conducive for clouding. But, this time, the entire belt from Cuddalore to Kanniyakumari received good rainfall due to easterly waves. Thoothukudi too was a rain surplus district and flooding paralysed normal life for 20 days.

Several weather stations had exceeded 100 cm of rainfall, including Coonoor (137 cm) and Pamban (129 cm) this season. The forecast of South Asian Climate Outlook Forum, which is supported by the World Meteorological Organisation, has been almost spot on. It had predicted normal rainfall for coastal Tamil Nadu and slight shortfall in western districts, he said.

Improved storage

One-thirds of all reservoirs in the State were filled over 90% of their capacities as on January 9, 2020 and nearly half of them had water levels filled upto at least 70% of their capacity. The Bhavanisagar reservoir was filled 100% - the highest. Majority of them are better stocked as on January 9, 2020 compared to the same day in 2019.

Reservoir% filled in 2019% filled in 2020Change
Bhavanisagar70.5310029.47
Papanasam69.0497.5328.49
Parambikulam82.7992.199.4
Manimuthar72.8291.5418.72
Mettur44.1290.6346.51
Pechiparai8.9274.1165.19
Aliyar57.2773.2115.94
Perunchanai75.5769.2-6.37
Thirumurthy71.7357.28-14.45
Amravathi50.4856.075.59
Vaigai49.3555.716.36
Sholayar62.0952.6-9.49
Sathanur44.1946.912.72
Periyar26.2728.612.34
Krishnagiri30.4313.81-16.62

 

Dharmapuri, as predicted, ended up with below normal rainfall. While the average rains were seemingly higher than previous years, the district was affected by patchy distribution. There were instances of heavy rains filling up a waterbody in a village and a dry spell rocked the neighbouring village. The inflow into Biligundulu did not have much impact in Dharmapuri.

According to Mr. Raj, past studies of data since 1916 have shown that whenever the State experiences an abundant southwest monsoon, it ends up with a below normal or just normal northeast monsoon.

After a good southwest monsoon that brought 16% surplus rains, expectations were high that an excess northeast monsoon would follow to create a new record and a rare climatic feature. “This year too, it scraped through with sufficient rains, conforming to the trend,” he said.

What worked

Meteorologists and weather bloggers noted that some of the global parameters like negative southern oscillation and positive Indian Ocean dipole favoured the northeast monsoon and brought rains over arid areas. Ramanathapuram was one of the districts that received more rainfall than Chennai district.

“Early arrival of easterly winds around October first week gave a good start to the monsoon. Unlike cyclones, easterly waves influenced rainfall over specific regions and were not strong enough to cover a wide region. For instance, Cuddalore recorded good rainfall whereas the neighbouring Puducherry had to manage with a deficit season. It defied the commonly held belief that the northeast monsoon is driven by strong weather systems,” said K. Srikanth, a weather blogger with Chennaiyil Oru Mazhaikalam.

The unseasonal inflow into Mettur dam this year benefited the region in the tail end. Chennai indirectly benefited through these weather disturbances that developed at a lower latitude, he added.

How Chennai fared

Chennai was one of the districts that constantly missed rains this monsoon, pulling down the rainfall to 64 cm, which is a 16% shortfall. Though it was a second consecutive monsoon-deficit year, Chennaiites are largely relieved that they are out of the spectre of another drought that was looming large.

The inadequacies of the stormwater drain network were once again laid bare as city roads were inundated even after short spells of rain.

Rising levels

A simiar analysis for average ground water levels in Chennai shows that the levels improved in all zones of the city except Kodambakkam, Madhavaram, and Royapuram which saw a drop in ground water levels. Ground water level in Perungudi saw the most improvement as of December 2019 compared to that as of December 2018 while they worsened the most in Kodambakkam. Ground water level in Sholinganallur was the highest in the city as of December 2019.

ZoneDecember 18December 19Change
Sholinganallur3.482.68-0.8
Perungudi4.623.15-1.47
Manali4.033.45-0.58
Anna Nagar4.253.71-0.54
Thiruvotriyur4.193.9-0.29
Alandur4.934.05-0.88
Ambathur4.884.54-0.34
Thiruvika Nagar4.944.6-0.34
Madhavaram4.364.60.24
Teynampet4.774.66-0.11
valasaravakkam5.135-0.13
Tondiarpet6.375.48-0.89
Kodambakkam4.885.660.78
Royapuram6.236.270.04

 

A comfortable storage in the city reservoirs and a recharged water table gave Chennai Metrowater the confidence to announce that the city was officially out of a shortage.

Just as in the past, an extended and surplus southwest monsoon saved Chennai from yet another drought year. Severe water crisis and demand for water tankers turned people’s attention towards harnessing rainwater. Mani, a resident of Bishop Garden, in R.A. Puram, said that it had been a few months since their apartment complex bought a tanker load of water. “Our complex has become self-sufficient with shallow wells charged with rooftop water. Our own resources yield about 40,000 litres a day,” he added.

People along OMR were the first to wake up to the crisis and many gated communities adopted innovative rainwater harvesting measures. Intermittent rainfall spread through the months improved the water table across the city and put an end to the scramble for water. Rain Centre’s director Sekar Raghavan said this year more people opted for rainwater harvesting to escape a repeat of the 2003 and 2018-19 droughts.

Chennai's key sources

While water levels in reservoirs in and around Chennai were not as high as some others in the State as on January 9, 2020, most showed significant improvement in their levels compared to the same day in 2019. Veeranam and Redhills were filled the most as the levels in the latter showed an improvement of 52.57 percentage points - the most in the city.

Reservoir% filled in 2019% filled in 2020Change (% points)
Veeranam87.579.81-7.69
Redhills26.7979.3652.57
Poondi8.2946.5238.23
Chembarambakkam2.3345.7643.43
Cholavaram4.446.662.22

 

“In clayey soil areas like Ambattur, Mogappair and Nanganallur, water percolation would be slow. But the groundwater table would increase rapidly unlike sandy areas like Triplicane, Besant Nagar or Tiruvottiyur where water table is distributed over a larger area. It takes a long time to build up an aquifer,” he said.

More public awareness on recharge wells in public spaces like road corners in R.A. Puram and Gandhi Nagar also paved the way for improved groundwater levels. Instead of indiscriminate construction of stormwater drains, the civic body must explore possibilities of sinking recharge wells in keeping with soil conditions in an area. Excess rainwater collected could be diverted to neighbouring tanks and then to the waterways through drains, Mr. Raghavan added.

Desilting of waterbodies, be it through government or people’s initiatives, has yielded better storage. Abundant inflow into 2,000 rain-fed tanks has led to cultivation of paddy, vegetables and minor foodgrains in 90,000 hectares of land in Tirunelveli and Tenkasi districts. The desilting of supply channels and waterbodies ahead of the monsoon under the Kudimaramath scheme helped many districts, including Kancheepuram and Tirunelveli, to store water and increase cultivation.

Officials of the WRD noted that 65% of the 924 tanks in Kancheepuram district saw a surplus this year.

The initial rainfall recharged the water table that had plummeted during prolonged drought and then filled up the tanks. The two check dams built across the Palar river also helped conserve 1,500 million cubic feet of rainwater.

Weather experts noted that Chennai’s annual rainfall had crossed the 100-cm mark this year. The weather stations in Tambaram, Poonamallee, Meenambakkam and south Chennai received more rainfall than Nungambakkam.

Mr. Raj said it is wrong to declare Chennai and Tamil Nadu to be rain-deficit. “We have had epochs of poor rainfall like a few years in 1981-1990 and 2011-2019. But, we cannot conclude that there has been a decrease in northeast monsoon rainfall. But the southwest monsoon is growing to be more reliable for places like Chennai in the past three decades,” he said.

Experts call for a denser network of observation stations to study the spatial variations of rainfall over Chennai district. Every northeast monsoon is different and studies are limited. There is something to learn about the evolving weather parameters of the incredibly unstable monsoon. Residents and government agencies must be prepared to tackle the vagaries of the weather every year as the monsoon could spring surprises, experts added.

(With inputs from S. Sundar in Madurai, P. Sudhakar in Tirunelveli, Vignesh Vijayakumar in Salem, P.V. Srividya in Dharmapuri and Karthik Madhavan in Coimbatore)

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Printable version | Jan 25, 2020 11:06:21 PM | https://www.thehindu.com/news/cities/chennai/a-bountiful-monsoon-bows-out/article30548207.ece

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