ISLAMABAD: The Indus River System Authority (Irsa) on Thursday allowed 33-month closure of a Tarbela dam tunnel for alarming safety reasons and estimated water shortage of 18 per cent during Rabi cropping season, starting Oct 1.

The decisions were taken at an advisory committee meeting of the regulator presided over by Zahid Hussain Junejo and attended by its members, provincial representatives and heads of Wapda and Met Office.

Sources told Dawn the closure of Tunnel-5 of Tarbela dam was demanded by the Water and Power Development Authority (Wapda) responsible for operations and maintenance of the country’s major reservoirs because of unavoidable dam safety issues arising out of the movement of a four-mile sedimentation delta that is close to dam tunnels.

This would, however, deprive provinces to the extent of 40,000-50,000 cusecs of irrigation water supplies in early kharif months during dry years and about 25,000-30,000 cusecs in wet years. However, in the case of carryover water quantities of one million acre-feet and above, the impact on irrigation supplies would be negligible. The T-5 closure is scheduled to begin immediately and conclude in May 2025.

Irsa allows closure of Tarbela dam tunnel for 33 months

The sources said the sedimentation delta of about four miles had been moving over the years and was so close to the tunnel off-take points that its cone-like structures could block tunnels with a minor tremor. There is no way it could be stopped but as a remedial measure, the tunnel’s water off-take levels could be raised so that even in the case of blockade of tunnel low levels, water could be discharged from higher off-take points.

The increase in tunnel levels has already been completed early this year in the case of tunnel-3 and 4 from elevation levels of 1,160 to 1,365 feet. Under a similar effort, the T-5 minimum elevation level has to be increased from 1,190 feet at present to 1,148 feet. As consequence, the minimum conservation of the overall dam has already been increased from 1,395 to 1398 feet. Wapda would launch a fresh survey to determine if this should be further raised to 1,403 feet. Unfortunately, the dam’s maximum conservation, which currently stands at 1,550 feet, could not be increased.

Because of the normal silting process, Tarbela’s live storage capacity has already dropped from 9.6 million acre-feet (MAF) to 5.8MAF at present, with a 40pc water storage capacity loss.

The advisory committee approved 18pc water shortage estimates albeit with Punjab’s dissenting opinion over allowing water conveyance losses at 8.6pc in the Indus reach. It was, therefore, decided to make it conditional with actual review by Oct 31.

Irsa rejected a demand of the Sindh government to end the existing three-tier formula for distribution of water shares among the provinces and instead application of shortages to all provinces, including Balochistan and Khyber Pakhtunkhwa. The majority view was that the Council of Common Interests had already taken cognisance of the matter for a decision and Irsa’s stakeholders should wait for the outcome.

The advisory committee approved the likely anticipated water availability at the four Rim-Stations of 21.67 MAF for Rabi 2022-23 worked out by its technical committee early this week. This quantity is about 3pc more than last year and about 4pc less than the 10-year average.

Based on the above Rim-Station Inflows and 9.5 MAF of carryover in dams, the balance of distributable water between Sindh and Punjab was worked out at 28.47MAF. As a result, these two provinces would get 18pc water shortage given the fact that KP and Balochistan are exempt from shortfall because of their inherent infrastructure constraints.

The provincial shares were thus worked at 16.23MAF for Punjab, followed by 12.24MAF for Sindh, 1.022MAF for Balochistan and 0.7MAF for KP. This likely availability at the canal heads of 30.19 MAF is about 10pc higher than last year’s availability of 27.43MAF and 2pc higher than the 10-year average of 29.68MAF.

Published in Dawn, September 30th, 2022