In the next three decades, Chennai may witness a marginal decrease in its annual rainfall. Rapid urbanisation, threat of sea-level rise and decrease in rainfall are likely to affect the fragile freshwater coastal aquifer along East Coast Road, finds a study by Department of Geology, Anna University.
The annual rainfall over the city may decrease at the rate of 6.4 mm per year until 2050 due to climate change. Similarly, the seasonal average rainfall too may see a decrease. For instance, the Northeast monsoon rainfall may reduce at a rate of 7.4 mm per year.
The projection of rainfall was done by downscaling the general circulation model (GCM) to a regional climate model and using PRECIS, a modelling system developed at the UK Met office.
The findings were part of the research paper on ‘ Impact of regional climate model projected rainfall, sea-level rise and urbanisation on coastal aquifer’, published recently in the journal, Environmental Science and Pollution Research.
L. Elango, professor, Department of Geology, Anna University, who also authored the study, noted that there were uncertainties and limitations associated with the downscaling of GCM to a regional climate and projections may not be accurate.
The decline in annual rainfall and sea-level rise may severely affect the thickness of freshwater source available in the fragile island-like coastal aquifer, which is bound by seawater along the East Coast Road, by 2050, the study said.
Sadhasivam Satish, one of the authors, said nearly 35 sq.km. area between Thiruvanmiyur and Muttukadu backwaters and with Buckingham canal in the west was taken up for the study. Groundwater remains a vital source in the residential areas along ECR and the groundwater pumping rate is a minimum of 9 million litres a day due to rapid population growth and changes in land use.
While the decline in rainfall may reduce groundwater recharge, the area is vulnerable to sea-level rise and seawater intrusion would affect the groundwater quality, he said.
According to the study, the sea level is likely to rise by 10 cm by 2050 due to various factors, including global warming. This would impact the coastal freshwater aquifer. Given the present level of groundwater pumping and dip in recharge, the water level in these areas is expected to decline by 1.77 metre if the seawater rise is 2 mm per year.
However, when the groundwater drawal is reduced by 5%, the groundwater level would improve by 0.15 metre per year and the drop in water level is estimated to be 1.62 metre per year.
The increase in sea level will prevent a significant drop in groundwater level. But, seawater will occupy the space of freshwater and reduce the volume of freshwater available in the localities, said Razi Sadath, research fellow, Anna University.
As an effort to alleviate seawater intrusion, the government must control development of high-rise buildings along ECR and reduce groundwater extraction and maintain the freshwater aquifer. The proposed desalination plants along ECR may help decrease groundwater dependence in the localities, Prof. Elango added.
