Whither rain? Chennai stares at a water crisis

Water managers in Chennai are a worried lot, as the northeast monsoon has so far failed to replenish water resources.

If the rains remain elusive over the catchment areas of major lakes that the city relies on for its drinking water needs, this will be the third consecutive year that their levels fall to precariously low levels.

The city is now pinning its hopes on a weather system building in the Bay of Bengal to bridge the rain deficit and defuse the looming water crisis.

Managing water needs in the fast-expanding coastal city is a struggle every summer and the expectation is always for a bountiful northeast monsoon in the last three months of the year to fill up the lakes. The northeast monsoon replenishes water sources, including groundwater, till the next monsoon.

There is, technically, still a month to go before the monsoon winds up. But, Chennai residents are already dreading the thought of scrambling for water in the coming year. The city has been on an alternate day, piped water supply scheme for nearly two years now, at the rate of 675 million litres a day (mld).

Rain ahead?

The Meteorological Department has forecast that a low pressure area is likely to form over the southeast Bay of Bengal by December 9. S. Balachandran, Deputy Director General of Meteorology, Chennai, said: “Rainfall over the north coastal areas will depend on how the system evolves, and the direction it takes. The city is currently 50% short of its seasonal average rainfall. We still have another 20 more days for the monsoon to bridge the gap if not wipe it out entirely.” Chennai district has so far received 34 cm against its seasonal share of 69 cm.

Though November is considered the wettest month of the year, weather experts note that there have been years when December rain has compensated for the deficit. While there is hope for more rain, it remains to be seen if end-of-the-year showers will be up to the Herculean task of filling up reservoirs. But, rain in December will certainly help recharge groundwater, another key source of water for the city.

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Hoping December doesn’t disappoint

Weather expert Y.E.A. Raj cited the example of 2016, when October and November together recorded only 7 cm of rainfall. Cyclone Vardah in December pushed the monthly rainfall to 25 cm. “It was a poor northeast monsoon. But December rain saved us from a disaster scenario and a good southwest monsoon helped take the annual rainfall across the benchmark of 100 cm,” he said.

Conceding that weather models forecast rains well into January, he said that the accuracy of such predictions decrease after three days. Pointing out that people often confuse water availability with abundant rainfall, he said 2017 was one of the classic examples when the city experienced a heavy northeast monsoon, yet the reservoirs did not touch full storage.

Weather bloggers note that December is an erratic month and cannot really be banked upon. K. Srikanth, who blogs with Chennaiyil Oru Mazhaikalam, said: “We will need intense weather disturbances like Cyclone Thane (2011), Cyclone Vardah (2016) and Cyclone Ockhi for good rainfall in December. Even if we get intense rain to cover the deficit, there is a possibility that it may be lost as run-off.”

Noting that there is a link between the number of rainy days and performance of the northeast monsoon, he said Chennai has had good rains when it experienced more than 25 days of rain as in 2011 and 2017. This year, Nungambakkam has recorded 20 rainy days so far.

The previous rain spell brought heavy rain in the suburbs, particularly in around the reservoirs. But, it did not translate into inflows, as catchment areas remained dry. For instance, a good rain spell in areas like Sriperumbudur would have brought more water into the Chembarambakkam lake.

Comparatively, storage in reservoirs was better last year because of rains spread over November and December. An average rain spell should last three to four days for groundwater recharge. This would alsohelp speed up inflows into reservoirs, said officials of the Water Resources Department.

“Short spells followed by a long dry period will only dry land and slow down the process of water travelling from catchment areas to reservoirs. We need rains of 10-15 cm spread over three or four days to recharge groundwater levels and boost storage,” said an official.

Urbanisation around the reservoirs and poor maintenance of channels bringing inflows into lakes were cited as other reasons for reservoirs not benefiting from the monsoon.

Contingency plans

Having to deal with a drinking water crunch for three consecutive years, Chennai Metrowater has chalked out plans to offset the decline in drinking water sources this year. While Krishna water from Andhra Pradesh and water from Veeranam would act as lifelines, Metrowater has planned to tap resources in abandoned quarries in Erumaiyur, Thirumudivakkam and Sikkarayapuram. Like in the past, the agency may turn to agricultural wells in Tiruvallur district for up to 100 mld of supply.

Metrowater’s project to revitalise small waterbodies, including Retteri, Ayanambakkam and Perumbakkam, as drinking water sources is appreciated as a need-of-the-hour initiative. To sustain water supply to the metropolis, the water agency is also exploring new aquifers around the city.

The road ahead

In these days of short, intense spells of rainfall, restoration of lakes and recycling grey water may hold the answer to a recurring water crisis, said water experts.

Balaji Narasimhan, associate professor, Department of Civil Engineering, IIT Madras, said more tanks in places such as Manimangalam and Sembakkam must be rejuvenated as buffer drinking water sources. It would also protect them despite urbanisation. Decentralised supply, as is being done in Porur lake, must be adopted.

“The loop has to be closed with grey water recycling for better water conservation. Shallow groundwater aquifers must be recharged for such crisis management,” he explained.

Groundwater is like a fixed deposit that can be depended on during drought. But it is often misused and not adequately replenished, experts have argued. The decrease in rain has already taken a toll on the water table that has dipped to a depth of 7 metres this November.

Sekar Raghavan, Director, Rain Centre, said: “Houses in Besant Nagar can survive on water from their wells for two more months while other city residents are sinking deeper bore wells. That's the impact of harnessing rainwater and a good rain water harvesting (RWH) system would help tide over water shortage. Open wells must be dug wherever possible.”

RWH is more important during less rainy months as recharge wells could trap whatever little water that comes as precipitation. Instead of draining floodwater into sea through stormwater drains, recharge wells could be built along the roads and inside SWD to increase water table, he added.

Sometimes, even a defunct RWH system could help recharge groundwater by at least 15%. L. Elango, professor, Department of Geology, Anna University said large apartment complexes could have dedicated RWH sumps.

Recycled water key

Citing a study by his department, he said grey water recycling would cut down fresh water consumption by at least 40%. Recycled water could contribute to at least 30% of water availability in the next decade. “It needs proper planning and implementation to reduce water stress. Households could go for smaller sewage treatment plants that would help in self-sufficiency. Recycled water could contribute up to 300 mld of available resources by 2030,” he said.

While the city waits with bated breath for the evolving system to quench its thirst, adopting nature-based and innovative solutions could be the key to sustaining water supply, unmindful of the vagaries of weather.

City reservoir levels as on December 8

Some of the years when December rain helped reduce deficit

Average northeast monsoon rainfall: 84 cm  

Number of rainy days in Chennai that led to better NE monsoon performance

*Source: Weather expert Y.E.A. Raj and Chennaiyil Oru Mazhaikalam