North East monsoon: Will poor rains leave Chennai city parched ?

CHENNAI: From mild showers to very heavy rains with thunderstorms – monsoon season in Tamil Nadu seems to have left a deficit in some districts while surplus rainfall is a reality in others.

The State has received only 5% excess rainfall during the northeast monsoon season, and among those, 16 districts have recorded a deficit so far.

Rainfall over Chennai, Kancheepuram and Tiruvallur has reduced thereby reducing the source of water in city reservoirs. However, even if these reservoirs don’t have enough water by December, water managers point out that there are six reservoirs with 6 TMC of water, which means that there wouldn’t be water shortage in Chennai till June.

Less rainfall

Unlike the previous years, the northeast monsoon began 5 days early on October 15. And as soon as it began, the north coastal districts recorded excess rainfall. However, later, it reduced, to the extent that coastal districts, including Chennai, have reported deficit spells till date.

From October 1 to November 20, Tamil Nadu has received 315 mm of rainfall against 301 mm (5% excess). Of which, Chennai reported 550 mm (an increase by just 1 %), and Kancheepuram, Tiruvallur and Chengalpattu reported a deficit by 37%, 9% and 26% respectively. These are the main sources of water to the reservoirs in the city and suburbs including Red Hills, Poondi, and Chembarambakkam dams.

In the last two years, Chennai and neighbouring districts have recorded excess rainfall during the southwest and northeast monsoon seasons. So, storage in the reservoirs was adequate through the catchment areas of the waterbodies. This had negated the threat of drought during summer.

However, this year, Chennai, Kancheepuram, Tiruvallur and Chengalpattu did not receive excess rainfall during the southwest monsoon. Though the northeast monsoon had begun early, north coastal districts have not recorded excessive rainfall levels till November 20.

“During the first monsoon spell on October 15, Chennai received extremely heavy rainfall and recorded over 20 cm. The highest amount of rainfall was reported in north Chennai – New Manali Town, Kathivakkam, Perambur and Kolathur (21-23 cm). “Though intense spells were expected last week due to a low-pressure area over the southwest Bay of Bengal, only moderate showers were recorded in the city and neighbouring districts,” said a senior official with Regional Meteorological Centre (RMC).

Storage in reservoirs

The RMC had predicted heavy rainfall over the coastal districts from November 23 under the influence of a low-pressure area over Bay of Bengal, which is likely to intensify into a depression.

The current storage in six reservoirs in the city and suburbs is 6 TMC against the total capacity of 13 TMC. Among these, Poondi reservoir does not have sufficient water at present compared to the other water bodies.

“Usually, in December, the water level increases in the catchment areas of the dams. So, this year, we’re expecting excessive rainfall from next month. In case the monsoon fails this year, we’ll be able to manage the drinking water supply for the next six months, as we have 6 TMC of water in the reservoirs. There will not be a drought situation during the summer next year,” a senior official in Chennai Metropolitan Water Supply and Sewerage Board (CMWSSB) told DT Next. “Even if the Veeranam reservoir dries up, it will be refilled with water from Mettur dam.”

Desal plants

The Metro Water department has also assured that the Minjur desalination plant with a capacity of 100 MLD would ensure continuous drinking water supply to north Chennai. And, the two desalination plants in Nemmeli, with a capacity of 100 MLD and 150 MLD each, would distribute drinking water to the southern parts of the city.

The CMWSSB is constructing the fourth desalination plant in Perur with 400 MLD capacity, which would become an additional source of water for Chennai.

Meanwhile, Poondi reservoir has received 1.35 TMC of Krishna water from Andhra Pradesh till date. Since Chennai is yet to receive excessive rainfall, the discharge from Kandeluru dam will continue until further notice from the Water Resources Department (WRD).

A senior WRD official stated that the neighbouring state has released 800 cusecs of water, and at least 278 cusecs reached Poondi reservoir on Wednesday, which has also been receiving 150-280 cusecs of Krishna water in the last two months. “We’re expecting to receive 3 TMC from Andhra during the first spell and 8 TMC of Krishna water, as per the agreement between the two states. If there is intense rainfall, which increases water level in the catchment in Chennai and outskirts, we may request the states to stop the discharge,” added the official.