Mettur dam the lifeline of Delta districts shares a few common things with the very famous Hoover Dam. Both these dams were constructed in similar periods during the 20th century. Hoover was built between 1931 and 1936 while Mettur was constructed between 1925 and 1934. Both have been built on rivers that have seen some pitched battles among states on water rights. Stanley Reservoir was named after Colonel Sir George Frederick Stanley the Governor of Madras from 1929 to 1934. Hoover was named after Herbert Hoover the 31st President of United States who played a key role in the dam construction. Not exactly similar but both Colorado and Cauvery have periods of excess and deficit cycles. The deficit cycles in Colorado are much longer and more intense compared to Cauvery. Nevertheless there is a lot of learning to be derived from the basin management practices of Colorado for Cauvery.
In an earlier post we had talked of how June 12th the traditional date of opening for Mettur is a tough ask for Tamil Nadu. 2024 was the first time in five years Mettur missed its traditional opening date. But between 2009 and 2019 except for 2011 all other years the dame was not opened before 19th July. 2024 seems complete the loop back to 2019 by sharing similar traits during the early monsoon season. Just 7 days flipped the storage position of the Cauvery Basin during the year 2019. Between 7th August and 13th August the storage doubled in the Karnataka dams increasing from 49 TMC to 110.3 TMC. Similarly Mettur storage tripled in just 5 days between 10th August and 14th August. The storage increased from 26.2 TMC to 75.3 TMC during this period.
The current storage at Mettur is at 13 TMC while on 31st July 2019 just before the start of the monsoon surge storage was 16.4 TMC. There is a very high chance like 2019 this year also we may see a week flip the storage position over the Cauvery Basin. Just like 2019 back to back circulations may ensure strong monsoon dynamics prevail over Peninsular India and parts of Central India to bring some much needed vigorous monsoon conditions.
Compared to 2019 this year the storage in the Karnataka basin dams is nearly 25 TMC more giving a better chance for Mettur to benefit more from this upcoming spell of rains. Kabini is almost at FRL while the other two bigger dams, Hemavathi and KRS, are already above 50%. In comparison both these dams were at nearly 1/3rd of the total capacity just before the start of the surge during 2019. With MJO expected to remain favourable for the most of the remaining days of July this phase becomes extremely critical for the Cauvery Basin.
Sub seasonal outlooks indicate the upcoming week may bring very heavy to extremely heavy rains over most parts of Western Ghats. This is also reflecting in the GLOFAS flood forecast charts where for the first time we are seeing probable high flows in the Cauvery Basin. Interestingly like 2019 it appears once again Hemavathi may be a big player in the inflows into Cauvery. In 2019 Hemavathi saw a cumulative inflow of nearly 3.5 lakh cusecs between 9th to 12th August. This in turn swelled the inflows into KRS to a massive 2 lakh cusecs on 12th August. Weather models indicate good rains across most parts of the Cauvery Basin including Wayanad which brings inflow into Kabini. But it is essential to mention looking at the charts the catchment areas of Hemavathi seem to have more chance of sustained heavy rainfall compared to Wayanad and Kodagu areas.
Going by ensemble rainfall estimates there is a high probability a cumulative inflow of 50 to 75 TMC can be expected in Mettur before month end. The current storage available at Karnataka dam as mentioned above gives us a 25 TMC bonus compared to 2019. Taking this into account it is right time for the Tamil Nadu government to prepare for the opening of Mettur dam around 30th July when the storage may reach a comfortable 60 TMC at the least. With Karnataka dams likely to be in FRL there is a high chance during the remaining period of Southwest Monsoon the daily inflows may be comfortable enough to sustain Delta irrigation.
On the weather front starting from today west coast is likely to come under widespread heavy rains gradually with Sunday to Wednesday likely to be the peak of this spell of rains. Consequent to picking up of the monsoon we can expect a gradual reduction in thunderstorms over leeward areas of Tamil Nadu form today. Today we may see isolated thunderstorms in a few places which will reduce further from tomorrow.