ISLAMABAD, Sept 3: In a major shift in the water distribution mechanism being followed since early 1990s, the Indus River System Authority (Irsa) is working on a new mechanism which is likely to restrict Punjab’s water share to the Jhelum zone and that of three other provinces to the Indus zone because of the upgradation of Mangla Dam as the country’s largest water reservoir.

A senior official told Dawn that Irsa, the country’s irrigation water regulator, had started consultations with all stakeholders, including four provinces, Wapda and dam managements, to change water distribution plan ahead of the Rabi season beginning on Oct 1.

Irsa has asked relevant agencies to report if there could be any operational difficulties or technical constraints in changing the decades-old irrigation pattern that has been there since a landmark water apportionment accord was signed by the federation and the federating units in 1991.

On Aug 23 this year, Mangla dam became the country’s largest reservoir overtaking Tarbela’s top position when its storage exceeded 6.5 million acre feet (MAF).

On Tuesday, Mangla’s water level attained about 1,238 feet with storage of a little over 7MAF — only four feet below its maximum conservation level of 1,242 feet. The 30-foot raised Mangla dam will now have a maximum storage of 7.3MAF when filled to capacity — an addition of about 2.5MAF.

But Wapda’s dam experts have warned the irrigation and government authorities against further conservation at Mangla in view of another impending rainy spell on Sept 5-7 and advised sparing some space for the upcoming inflows to avoid flood situation downstream.

As a consequence, Wapda authorities have been authorised to let river waters flow down the dam instead of being conserved and remain vigilant if more discharges are required to keep space for more water at the dam as a result of the fresh rainy spell. It has, however, advised that it should be ensured that the remaining four-foot capacity at the dam is also filled during the current month.

With Tarbela dam at its maximum conservation level of 1,550 feet with storage of 6.5MAF, the country’s cumulative storage increased to about 13.88MAF, including 7.1MAF of Mangla and the remaining at Chashma barrage against last year’s storage of 9.68MAF.

It is against this background that Irsa is considering to change the water distribution plan for the coming Rabi season. Since Mangla’s storage has gone much higher than Tarbela’s, Punjab’s irrigation requirement would be met through Mangla dam and of the three other provinces through the Indus zone.

Earlier, all provinces were provided supplies under a combined storage pool and link canals like Chashma-Jhelum Canal and Taunsa-Punjnad Link canals used to be utilised for diverting water from the Indus zone to the Jhelum zone.

The change in the distribution plan will be made by closing down the two link canals during the Rabi season. Officials said the flows in Indus and Kabul rivers had started coming down and if a situation emerged necessitating drawdown from Tarbela dam, the first step over the next 10 days would be to close down the link canals by stopping 15,000 cusecs currently flowing through the CJ-Link canal.

For the Rabi season, the water share of Punjab will be met strictly through Mangla dam and the requirement of three other provinces through the Indus zone. Irsa has directed provinces, Wapda and dam managements to give their opinion on the planned change and also report any technical constraints so that the process for Rabi water availability and distribution plan may be formulated at the earliest.

When asked if there was any plan to release water from Tarbela to create space for storage of rainwater and avoid flood situation during the expected rainy spell later this week, the official said the matter was looked into by Wapda and Irsa authorities who were of the opinion that even though Tarbela was now full, there was no need to open up spillways for aggressive discharges because the Indus zone had the operational capacity to take care of additional flows from expected rains.

He said the meetings of Irsa’s technical and advisory committees would be convened in the second and last weeks of September to finalise availability and distribution plans and formally change the irrigation pattern.