The Brahmaputra sub-basin is one of the longest and most critical, yet least understood river basins in the world. A highly complex drainage system of South Asia, draining parts of Southern Tibet in China, India’s northeast, all of Bhutan, and also large part of Bangladesh, the Brahmaputra flows across unique geo-environmental and bio-physical settings. Out of the total length of the Brahmaputra of 2,880 kilometres, 1,625 kilometres flows through the Tibetan plateau as the Yarlung Tsangpo, 918 kilometres is in India known as the Siang, Dihang, and Brahmaputra, and the rest of the 337 kilometres in Bangladesh is named the Jamuna until it merges into the Padma near Goalando. This geographical distribution of length apparently gives the impression that the geographical boundary around the headwaters carries the maximum flow of the river, which is a myth.

The various hypotheses about Chinese interventions

• There are various hypotheses about Chinese interventions in the Yarlung-Tsangpo stretch of the Brahmaputra. All these hypotheses emerge from China’s unique status as the source of transboundary river flows to the largest number of countries in the world. It started with the Chinese hydropower projects on the Yarlung-Tsangpo, which was initiated with the Zangmu gravity dam.
• Although China claimed that hydropower projects are essentially based on run-of-the-river technology, there has been an apprehension that water diversion is a hidden motive of such structures.
• Furthermore, there was the apprehension of sediment trapping in the construction, which would deprive those living downstream of sediments, the most prominent of being soil formation of the Brahmaputra floodplains in Assam, India (classified as a “supporting service”).
• Again, scholars like Brahma Chellaney also alluded of China’s mal-intentions towards India, when the former failed to comply with an existing Memorandum of Understanding (MOU) on Hydrological Data Sharing on the river Brahmaputra/Yarlung-Tsangpo between China and India.
• Under the direct exchange mechanism of the MOU, China is supposed to provide India with hydrological information (water level, discharge, and rainfall) for three stations, namely, Nugesha, Yangcun and Nuxia located in Tibet. This information is supposed to be provided twice a day at 08:00 hrs and 20:00 hrs (Beijing Time) during high flow season from May 15th to October 15thevery year.
• However, in 2017, China declined to share the data on the plea that the instruments and installations in the measuring stations were destroyed. In some corners in India there have been concerns that the Assam floods in Brahmaputra have been caused despite less than normal rainfall due to lack of data providing the necessary early warning.
• Lately, there has been a very strong contention on China’s 1,000-km long canal construction to divert the water – this has again been denied by China. However, ever since the news of the Siang  waters turning black has hit the media through the letter of a political leader from Arunachal Pradesh, the perception of Chinese conspiracy against Indian interests by polluting the Brahmaputra has again surfaced.
• It has been allegedly stated in large sections of Indian media and think-tanks that the tunnel construction by China is responsible for the blackening of the Brahmaputra, with the construction debris and effluents being released from the Tibetan boundaries. China, in an official statement, has again denied this.
• Until now, the Indian government has acted quite rationally rather than committing itself to the bandwagon of subscribing to the “Chinese conspiracy” hypothesis, and has asked for more scientific inquiry into the problem.

Understanding the flow, deciphering the myths

• Unfortunately, in most cases, political scientists, strategic thinkers, and international relation specialists have indulged in simple linear thinking without appreciating the complex hydrology and fluvial geomorphology of the Brahmaputra system. This lack of acknowledgement has caused some scientifically unfounded perceptions about potential impacts on India of probable upstream interventions in the river system.
• The Brahmaputra is identified as the flow downstream of the meeting of three tributaries, namely the Luhit, Dibang, and Dihang, near Sadiya in the Indian state of Assam.
• The link of Brahmaputra with the Yarlung river, which originates from the Angsi glacier near Mount Kailash, was discovered rather recently. As a trans-Himalayan tributary, the Yarlung is substantially fed by snow and glacial melts, in addition to rainfall.
• As such, the normalised melt index (defined as the volumetric snow and glacier upstream discharge divided by downstream natural discharge) of the Brahmaputra is merely in the range of 0.15-0.2, signifying that snow and glacial melt, the main source of run-off in the Tibetan region, contributes negligibly to the total flow.
• The Tibetan region lies in the rain shadow with the Himalayas acting as the barrier to the rain-laden monsoon. The annual precipitation in the trans-Himalaya averages about 300 mm per year. As the tributaries cross the Himalayan crest line, the annual average precipitation (mainly rainfall) reaches about 2,000 mm.
• Thus, a very large component of the total annual flow of Brahmaputra is generated in the southern aspect of the Himalaya in India by tributaries from Buri Dihing in the East to Teesta in the west. As per data published by the Chinese scholar Jiang and his colleagues, the total annual outflow of the Yarlung River from China is estimated to be about 31 billion cubic metres (BCM) while the annual flow of Brahmaputra at Bahadurabad, the gauging station near the end of the sub-basin in Bangladesh, is about 606 BCM. These figures do not support the linear algebraic thinking that the flow in a river is proportional to its length inside a country.
• The precipitation varies across the Brahmaputra sub-basin substantially. The sub-basin receives primarily two types of precipitation, rainfall and snowfall. As such, the Tibetan component of the basin i.e. the stretch of the Yarlung river, being located on the northern aspect of the Himalayas, receives much less rainfall as compared to the southern aspect of the basin – the stretches in India and Bangladesh.
• The Tibetan plateau and the higher reaches of the basin higher than 3,000 metres above sea level receive snowfall during the winter months from December to February. Furthermore, the southeastern part of Tibet receives monsoon rains during the months of July and August.
• Therefore, as a trans-Himalayan tributary, Yarlung is substantially fed by snow and glacial melts, in addition to rainfall. The annual precipitation in the trans-Himalaya averages about 300 mm per year. As the tributaries cross the Himalayan crest line, the annual average precipitation (mainly rainfall) reaches about 5,000 mm. Within the Assam valley, the average annual rainfall is more towards the northeast and gradually decreases towards the west.
• Table 1 shows the precipitation across the various stations in the basin, and reveals how Lhasa in the northern aspect of the Himalayas receives much less rainfall as compared to the regions in the southern aspect of the Himalayas in India, namely, Dibrugarh, Tezpur, and Guwahati, and in Bangladesh, at Bahadurabad.

Source: Bandyopadhyay et al (2016)

Further, while the peak flows at Nuxia and Tsela Dzong, measuring stations at the great bend  in the Tibetan plateau, are about 5,000 and 10,000 cubic metres per second (cumecs), the peak flow at Guwahati is approximately 55,000 cumecs (Fig. 1). In the peak flow periods, the Brahmaputra is fed by the summer monsoon but during the lean season, the share of its flow from the Yarlung river would be larger, the extent of which needs to be based on flow data. The lean season flow in Nuxia, as identified from a hydrograph given in Rivers and Lakes of Xizang (Tibet) (in Chinese), is to the tune of 300-500 cumecs, while the lean flow at Pasighat is to the tune of 2,000-plus cumecs, the one at Guwahati is around 4000-plus cumecs, and at Bahadurabad it is about 5,000 cumecs, all this based on peer-reviewed data.

Fig. 1: Relative hydrograph of the Brahmaputra

Source: Datta and Singh (2004)

• This reveals that the Brahmaputra gets fatter and mightier as it flows further downstream. This is more so because of the flow contribution of the various tributaries like Dibang, Luhit, Subansiri, Manas, Sankosh, Teesta to name a few.
• This can be noted from the fact that at Guwahati (Pandu), the percentage annual yield of the main river course from Pasighat is barely 34%, while the tributaries like Dibang, Luhit, Subansiri – as well as the tributaries joining between Pasighat and Guwahati – contribute the remaining 66%. Further downstream, the mainstream contribution diminishes further.
• Another point of concern relates to the impact of the projects on the sediment flow. Can water diversion affect sediment flow? The flow volume and discharge in the Yarlung river is not sufficient to generate and transport the very large sediment load as is prevalent in the downstream Brahmaputra.
• The annual suspended sediment load near Nuxia in Tibet is around 30 million metric tonnes, (as suggested in a 2016 volume titled River Morphodynamics and Stream Ecology of the Qinghai-Tibet Plateau by Wang and colleagues), which is miniscule as compared to the 735 million metric tonnes sediment load at Bahadurabad. Therefore, the large amount of suspended sediment load that gets deposited in the downstream to form a fertile Jamuna floodplain cannot be carried by the Yarlung-Tsangpo stretch. It is created further downstream in India, where precipitation is almost 12 times higher than the rain shadow Tibet.
• Given these data, let us have a relook at the perception-based hypotheses discussed earlier. It is clear that more than 80% of the flows of the Brahmaputra emerge within the Indian boundary for all the seasons. Moreover, as per data published by the Ministry of Water Resources (MoWR), Government of India, barely 25% of the renewable water resources is potentially utilisable, or has been harnessed.
• Therefore, any form of Chinese diversion does not really cause much change to the water availability in the Indian mainland, and especially to Assam. The same argument applies for sediment; sediment trapping by constructions in Tibet will not make India lose out on sediments, as most of the sediment flow happens in India.
• Thirdly, by looking at the hydrograph in Fig. 1 and the precipitation patterns given in Table 1, one may safely arrive at the position that the possibility of cloud-burst or flash floods in the rain shadow Tibetan boundary is remote.
• Therefore, the very contention that lack of early warning by not sharing high flow data with India by China has been responsible for Assam floods in 2017 despite less than usual rainfall is scientifically not tenable, and reasons need to be sought in hydro-meteorological variables in the Indian boundary.

The problem of classifying data

• The problem of turbidity in the Brahmaputra needs to be resolved as soon as possible. Till now, there is not a single scientific assessment that can point to the source of the turbidity, or the location from which the water becomes black. This could easily have been made available in public domain through remote sensing.
• Furthermore, the very idea of diverting Yarlung for 1,000 kilometres seems irrational, given that it might take many times more energy to lift the water as compared to the energy produced by the Zangmu hydropower project on the Yarlung. Again, one can never rule out the possibility of domestic construction activities within India further upstream, as well as excessive boulder mining in Arunachal Pradesh,– both of which are prone to change the colour of the water.
• However, without really blowing away the various possibilities, there needs to be an unbiased scientific inquiry into the problem, and placing the information in public domain for a more informed political discourse.
• It is now clear that the myths over the Chinese diversion of Brahmaputra spread in the media do not stand the test posed by scientific data and knowledge.
• Unfortunately, the jingoistic narrative in India has never been driven by science and real data, but by emotions. Public policy and basin governance cannot afford to be driven by populist jingoism, but by informed science, which should replace sensationalism based on linear and reductionist logic.