SUMMER’S set in earlier in Sindh, so the irrigation supply to meet the agronomic demands of the province should come earlier too — as should our national water distribution strategy.

Early summer crops are vital for the agricultural economy of Sindh — from mangoes and vegetables to cotton and rice. Early produce from Sindh fetches good market prices. If, however, early summer sowings in Sindh are deprived of water, not only do the farmers suffer, the overall agricultural economy of Pakistan shrinks as well.

Despite April and May being months of little rainfall, the Indus river and its tributaries start swelling during this time due to snowmelt in the mountains. As the snowmelt wanes by mid-April, glacial melt kicks in. The rivers keep swelling as glacial melt keeps increasing with rising temperatures.

From July to September, the monsoon rains add to the glacial melt and the rivers start overflowing their banks, inundating active floodplains and recharging riverine groundwater. By the end of September, when both the rains and glacial melt are over, the groundwater from the active floodplains starts seeping back into the river and keeps the river flowing until the winter rains kick in between January to March.

In its natural rhythm, the Indus never runs dry. Anthropogenic agronomical 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 grandest of all works resulted as a consequence of the Indus Waters Treaty through which it was ‘agreed’ as to which tributaries of the Indus would be allowed to flow and which ones would not.

Read: Revisiting the Indus Waters Treaty

Rivers were classified as ‘surplus’ and ‘deficient’ and inter-river transfers were built to ‘balance’ them out. Waters flowing out to the sea was declared ‘escapages’, and consultants were called in to advise how much escapage should be ‘allowed’. Five large dams were allowed under IWT to ‘control’ the major tributaries of the Indus river system. Two of these dams in Pakistan, Mangla and Tarbela, are touted today as the backbone of our irrigation system.

To assess the vitality of large dams in our irrigation system, Wapda’s data for Tarbela Dam was assessed from 1975 to 2010, the years when the dam’s performance was optimal. The data shows that on average, the dam had stored 6.92 million acre feet of water in the summers (Kharif) and had released 6.77 MAF to supplement irrigation in winters (Rabi). However, not all the water released from the dam actually reaches a farmer’s field. Established conveyance losses of 30 per cent in the rivers, 30pc in canals, and 30pc in watercourses means that only about 2.32 MAF of irrigation is supplemented at the farm-gate by the Tarbela reservoir. Comparing it with the 104 MAF used in the irrigation sector in an average year, Tarbela’s contribution is insignificant. And so is Mangla’s.

Mangla and Tarbela are touted as the backbone of our irrigation system. Are they?

A paper by Kharal and Ali, published by the International Centre 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 historical data from 1940 to 2003. The study found that post-Tarbela losses in the Indus, between Tarbela and Kotri, increased from 10.86 MAF to 18.22 MAF, a net additional loss of 7.36 MAF — which is already higher than the 6.77 MAF of water that Tarbela releases in the Rabi season. In other words, 7.36 MAF more water would reach Kotri every year if there was no Tarbela Dam. The same study has also reported that during pre-Tarbela winters, the river would gain about 2.5 MAF of additional water between Tarbela and Kotri, primarily due to groundwater seepage. However, in the post-Tarbela era, the river loses about 2.3 MAF of water between the same reach — or about 4.8 MAF net loss in water in the winter months at Kotri. This loss alone more than nullifies the 2.32 MAF of contribution of the Indus in the Rabi season.

So much for dams as the backbone of irrigation.

Now let’s analyse the water situation of Sindh in early summer each year. There’s been a history of Sindh-Punjab water disputes since 1856, but post-independence events are naturally of more relevance.

Read: Sindh, Punjab move closer to resolving water disputes

According to the Indus River System Authority’s Chronological Exposé of the 1991 apportionment accord, several committees/commissions were made to resolve Sindh-Punjab water issues. 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 allocations to provinces was around 2 MAF. Even today, in the light of the 1991 accord, the disputed allocation between the provinces is around the same.

Data from Tarbela between 1975 and 2010 shows that from April to June, the dam fills up its reservoir with 1.9 MAF on average. This is the summer flow from snow and glacial melt, which is the critical agronomic requirement for the Kharif sowing season, especially in Sindh, but is held up by the dam. The Dam Manual, which is the dam manager’s bible, says that the filling of the reservoir should begin with the early summer flows without waiting for the monsoons, despite the fact that monsoon flows alone are enough to fill the reservoir. Tarbela and Mangla combined hold back between 3.5 to 4 MAF of early summer flows. These flows, if not held back, would by themselves ease tensions between the federating units.

This year, winter rains and snowmelt were below average, but record April temperatures have set off ‘bridge-breaking’ glacial melt up in the mountains. Why don’t we see that water flowing down to Kotri?

In its natural state, the Indus is an amazing river. It never fails to deliver. Let the rivers flow.

The writers are experts on hydrology and water resources.

Published in Dawn, May 19th, 2022