SUMMER has set in earlier in Sindh, so irrigation supply to meet the province’s agronomic demands is also expected to arrive earlier, as will our national water delivery strategy.
Early summer crops are vital to Sindh’s agricultural economy, from mangoes and vegetables to cotton and rice. Early Sindh products fetch good prices in the market. However, if early summer plantings in Sindh are deprived of water, not only farmers suffer, but Pakistan’s overall agricultural economy also shrinks.
Although April and May are low rainfall months, the Indus River and its tributaries begin to swell during this time due to melting snow in the mountains. As snowmelt decreases in mid-April, glacial melting kicks in. Rivers continue to swell as glacial melting continues to increase with rising temperatures.
From July to September, monsoon rains combine with melting ice and rivers begin to overflow, flooding active floodplains and recharging fluvial groundwater. In late September, when the rains and ice melt are over, groundwater from active floodplains begins to seep into the river and keep it flowing until the winter rains kick in between January and March.
In its natural rhythm, the Indus never dries up. Anthropogenic agronomic practices in the Indus basin have evolved around the rhythms of the river over millennia.
But with the so-called development of modern irrigation, human beings decided to regulate the rivers themselves through heroic engineering. The greatest of all works resulted from the Indus Waters Treaty by which it was “agreed” to determine which tributaries of the Indus would be allowed to flow and which would not.
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The rivers were classified as “surplus” and “deficit” and inter-river transfers were constructed to “balance” them. Waters flowing out to sea were declared “leakage”, and consultants were called in to advise on the amount of escape to be “allowed”. Five large dams were authorized under the IWT to “control” the main tributaries of the Indus river system. Two such dams in Pakistan, Mangla and Tarbela, are presented today as the backbone of our irrigation system.
To assess the vitality of large dams in our irrigation system, Wapda data for the Tarbela dam was assessed from 1975 to 2010, the years when the performance of the dam was optimal. The data shows that on average, the dam had stored 6.92 million acre-feet of water in summer (Kharif) and released 6.77 MAF to supplement irrigation in winter (Rabi). However, not all the water released from the dam reaches a farmer’s field. The established transport losses of 30% in rivers, 30% in canals and 30% in streams means that only about 2.32 MAF of irrigation is supplemented on the farm by the Tarbela reservoir. Compared to the 104 MAFs used in the irrigation sector in an average year, Tarbela’s contribution is insignificant. And Mangla too.
Mangla and Tarbela are presented as the backbone of our irrigation system. Are they?
A paper by Kharal and Ali, published by the International Center for Advanced Mediterranean Agronomic Studies in 2007, assessed the losses and gains in the Indus river system before and after the construction of the Tarbela reservoir, in the context of the data records from 1940 to 2003 The study found that post-Tarbela losses in the Indus between Tarbela and Kotri fell from 10.86 MAF to 18.22 MAF, an additional net loss of 7.36 MAF – this which is already higher than the 6.77 MAF of water that Tarbela releases in the Rabi. season. In other words, 7.36 more MAF would reach Kotri every year if there was no Tarbela dam. The same study also reported that during pre-Tarbela winters, the river would gain about 2.5 MAF of additional water between Tarbela and Kotri, mainly due to groundwater infiltration. However, in the post-Tarbela era, the river loses around 2.3 MAF of water between the same stretch – equivalent to around 4.8 MAF of net water loss during the winter months at Kotri. This loss alone more than negates the 2.32 MAF contribution from Indus in the Rabi season.
So much for dams as the backbone of irrigation.
Now let us analyze the water situation of Sindh at the beginning of summer every year. There has been a history of water disputes between Sindh and Punjab since 1856, but post-independence events are naturally more relevant.
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According to the chronological statement of the 1991 allocation agreement of Indus River System Authority, several committees/commissions have been established to resolve the water issues between Sindh and Punjab. These included the Akhtar Hussain Committee of 1968, the Fazl-e-Akbar Committee of 1970, the Anwar-ul-Haq Commission of 1976, the Justice Haleem Committee of 1983, etc. The recommendations of these committees/commissions would show that the difference between the allocations to the provinces was around 2 MAF. Even today, in light of the 1991 accord, the contested distribution among the provinces is roughly the same.
Data from Tarbela between 1975 and 2010 show that from April to June, the dam fills its reservoir with an average of 1.9 MAF. This is the summer flow due to melting snow and glaciers, which is the critical agronomic need for the Kharif planting season, especially in Sindh, but is held back by the dam. The Dam Manual, which is the dam manager’s bible, says that the filling of the reservoir should start with the early summer flows without waiting for the monsoons, when the monsoon flows alone are enough to fill the reservoir. Tarbela and Mangla combined retain between 3.5 and 4 MAF of early summer flows. These flows, if they were not contained, would by themselves calm the tensions between the federating units.
This year, winter rains and snowmelt have been below average, but April’s record high temperatures have triggered “bridge-breaking” glacial melt in the mountains. Why don’t we see this water flowing towards Kotri?
In its natural state, the Indus is an amazing river. He never fails to deliver. Let the rivers flow.
The authors are experts in hydrology and water resources.
Posted in Dawn, May 19, 2022