With Karnataka refusing to release Tamil Nadu's share of Cauvery water this year, the focus has been on the unavailability of water for crops in Tamil Nadu and the resultant stress on farmers. However, there’s another aspect of the crisis that has gone largely unreported – a long-term adverse trend of Tamil Nadu's own making, which is also threatening agriculture in the Cauvery delta.
Since 1971, the three major Cauvery delta districts in Tamil Nadu – Thanjavur, Tiruvarur and Nagapattinam – have seen a massive diversion of farmland to other purposes, an ongoing study at the Madras Institute of Development Studies has found.
Using data from the Indian Space Research Organisation, the study, being authored by water-management expert S Janakarajan, said in its interim report in September that hundreds of square kilometres of agricultural land has been diverted and built up in the three districts over the last four decades.
Further, Janakarajan said, with the Mettur dam – the entry point of the Cauvery in Tamil Nadu – blocking the river's natural flow and vital sediment deposits, the coastal delta district of Nagapattinam has become highly vulnerable to invasion by the sea. Parts of the district could start slipping under the sea in the coming decades if mitigation efforts are not implemented, he added.
This data also assumes significance since a significant shrinkage in area of cultivation could adversely affect food production in Tamil Nadu.
The study titled Combating Climate Change: Vulnerability of Cauvery Delta, Wet Lands, Food Security and Livelihood Resilience, funded by the Indian Council of Social Science Research, said the built-up area (areas in the delta where construction has taken place) in Thanjavur, the largest agricultural delta district, has increased from 381 sq km in 1971 to 535 sq km in 2014. In Tiruvarur and Nagapattinam, the built-up area went up from 370 sq km and 237 sq km in 1971 to 580 sq km and 334 sq km in 2014.
In other words, in the last 43 years, the built-up area in Thanjavur, Tiruvarur and Nagapattinam has increased by 40.5%, 41% and 56% respectively.
"At the moment, we do not have a better data source than satellite maps to study this change in land use," said Janakarajan, adding that the reduction in cultivation area and unavailability of water was a deadly concoction that could hurt farm output.
The corresponding data on area of wasteland (farmland that has become fallow due to agriculture inactivity) is also worrying as such land is susceptible to non-agricultural development in the future.
For instance, in Thanjavur, wasteland area increased from 44 sq km in 1971 to 186 sq km in 2014. There has been a massive increase in wasteland in Nagapattinam, with the study identifying 297 sq km of wasteland in 2014 compared with just 12 sq km in 1971. In Tiruvarur, wasteland has increased from 5.58 sq km to 52.7 sq km during this 43-year period.
The case of Nagapattinam
The massive land use changes in Nagapattinam have a lot to do with the invasion of the sea. Janakarajan said that during the study he found that seawater had contaminated aquifers as much as 50 km into the coast.
Being the eastern end of the Cauvery delta, Nagapattinam is the last to receive Cauvery water after it is released from the Mettur dam, which is located in Salem, in the western end of Tamil Nadu. In years when Karnataka refused to release Tamil Nadu's share of water fully, like this year, many areas in Nagapattinam went dry.
As a consequence, farmers in the delta are forced to depend on groundwater. "But since groundwater is turning saline, farming has become even more difficult," added Janakarajan.
A major contributing factor to the invasion of the sea is the Mettur dam. Since its construction in 1934, said Janakarajan, silt deposits along the Cauvery river's course have decreased considerably.
A delta is a sheet of land that is either at sea level or at a slight elevation. Since the Cauvery delta is low, silt accumulation from the river was a crucial element in maintaining equilibrium and stopping the sea from invading the coast.
When it was first constructed, the Mettur dam's total capacity was 93.5 thousand million cubic feet of water, and its maximum water level was 120 feet. If one went by the study, the actual capacity of the dam could be much lower now due to silt deposits. Janakarajan has calculated that the Mettur dam has lost an average of 0.45% of its capacity every year since its construction.
"We do not have data on the actual amount of silt deposits in Mettur," said Janakarajan. But all the sediments accumulated in the dam would have, in normal course, spread across the delta, he pointed out.
Adding to this was the 2004 tsunami when the sea invaded the coast and deposited large amounts of salt on the land. Nagapattinam was the worst-affected area in India at that time. Over 6,000 people died in the district when the tsunami struck.
Janakarajan added that if measures to halt the invasion of the sea were not taken up seriously, parts of Nagapattinam could be run over by the sea in a few decades. Climate change and rising sea levels could accelerate this trend.
The full study will be published by December.