The first month of Southwest Monsoon season 2026 has come to an end. The cumulative inflow in the Cauvery Basin dams of Karnataka for the month was just 4.1 TMC. This is the 5th lowest ever inflow during the month of June in more than half a century. One look at the district wise rainfall data for the month will show how bad things have been so far. All the districts that are catchment areas for rivers like Cauvery, Krishna, Godavari are reeling under deficit. On an average during Southwest Monsoon the inflow into Karnataka dams in the Cauvery basin is about 250 TMC
Since 1974 the cumulative inflow into Karnataka dams during the month of June has been less than 10 TMC on 10 occasions. Of these 5 years have been El Nino Years and it would be 6 if we take into account 2026 is expected to be a strong El Nino year. Interestingly we have had Nino years like 1991, 1994, 2004, 2006, 2015, 2018 that saw very good inflows during June. All of these years except 2015 ended as a normal or above normal year for Cauvery basin overall during SWM.
It is also essential to look at Nino years with low inflow during June. 8 Nino years saw less than 20 TMC inflow during the month of June. Eventually only 1997, 2009 and 2014 ended above normal. This highlights the difficulty 2026 may face considering the extremely low inflow we have seen during the month of June. The evolution of El Nino and IOD this year seems to be closer to 2023. This is a worry because the year 2023 saw overall cumulative inflow of just 119 TMC during the entire Southwest Monsoon season.
Inflow into Cauvery during La Nina years is more stable while Nino and Neutral years seem to oscillate a lot. El Nino years that started well have most ended well going by historical trends. Overall there is a 41% chance for a below normal inflow during El Nino years. As mentioned above the probability of below normal seasonal inflow becomes higher with a poor June. 2026 unfortunately sits in the lower end of this table.
While inflows are always a factor of how the rains happen historical inflows could give an idea of what to expect. Overall there is a high possibility for the seasonal inflow to end between 175 and 200 TMC this year. This may effectively end as a below normal year considering an average inflow of 250 TMC during SWM season. It is also essential to point out the Supreme court judgement has awarded about 125 TMC during the period June to September. A further 42 TMC is the quota for TN during the NEM season. If the seasonal inflow ends up around 200 TMC it is unlikely TN may get its full share for the rest of the year. This once again highlights the need for a robust distress sharing formula that takes into account inflow.
On the weather front Monsoon is expected to remain active for the next week or two. Models indicate Konkan and adjoining Sahyadri Ghats may be the biggest beneficiary of this spell of active monsoon conditions. Cauvery basin may get some much needed inflows though it remains to be seen how much eventually translates. But there is hope though. 2019 saw just 3.2 TMC during the month of June. On the back of a strong monsoon surge during late July and August Mettur reached FRL by 7th September.
Chennai could see some passing showers like yesterday with temperatures expected to remain below normal for the next couple of days.