There is a widespread public perception in India that the structural interventions in the Tibetan Plateau by China on the Yarlung-Tsangpo, a tributary of the Brahmaputra, will greatly reduce the flow of the Brahmaputra in downstream North-East (N-E) India.
Statements of Chinese interventions on the Yarlung being dangerous for India have the potential of generating new points of contention in China-India hydropolitics. The contentions have been further intensified by the recent operationalisation of the Zangmu hydro-power project on the Yarlung. This project is expected to produce 2.5 billion KWH of electricity annually. In 2016, five more hydroprojects are proposed to be completed on the river.
Undoubtedly, an estimated hydropower potential of 114,000 MW of the Yarlung is very lucrative for China in the context of mitigation of greenhouse gas (GHG) emissions. Despite a lot of clamour about such projects, hardly has there been an objective, scientific analysis of the issues. This has led to sensational myths and claims, which deserve to be reviewed.
The downstream impacts of hydro-power and water transfer projects are quite different and should be distinguished. The series of projects undertaken by China on the Yarlung and its tributaries is reportedly for hydro-power generation, which does not reduce the total flow but changes its hydrograph (flow pattern). The impacts on the hydrographs of the Brahmaputra will be generated not merely from the hydro-projects on Yarlung, but also from the downstream projects on the same river Siang/Dihang and the Dibang or the Luhit, all of which flow through India.
As a trans-Himalayan tributary, the Yarlung is substantially fed by snow and glacial melts, in addition to rainfall in semi-arid Tibet. A very large component of the total annual flow of the Brahmaputra is generated in the southern aspect of the Himalaya in Bhutan and India by tributaries from the Buri Dihing into the Teesta.
The total annual outflow of the Yarlung from China is estimated to be about 31billion cubic metres BCM), while the annual flow of the Brahmaputra at Bahadurabad, the gauging station near the end of the sub-basin in Bangladesh, is about 606 BCM. Thus, the contribution of Tibet in the total flow of the Brahmaputra can be taken as about 5%.
Further, while the peak flows at Nuxia and Tsela Dzong, a measuring station at the great bend of the Yarlung in the Tibetan plateau, are about 5,000 and 10,000 cumecs, the peak flow at Bahadurabad is approximately 50,000 cumecs. The lean season flow in Nuxia is to the tune of 500 cumecs, while the lean flow at Bahadurabad is about 5000 cumecs.
These figures do not support the linear algebraic thinking that the flow in a river is proportional to its length inside a country, as is perceived popularly — given that out of the Brahmaputra’s length of 2,880 km, 1,625 km in Tibet flows as the Yarlung Tsangpo.
Another point of concern relates to the impact of the projects on the sediment flow. The sediments offer immense ecosystem services for the downstream economies of India’s N-E states, and Bangladesh. The sediment flow to the downstream will be impacted by all the hundreds of hydro-power projects on the river and tributaries.
Most of these are in India. The projects around Zangmu in Tibet will also affect the flow of sediments, though sediment exclusion technology has improved manifold lately. Further, the actual sediment flow into the Brahmaputra will also be directly linked with hydro-power projects on the Dihang, the Dibang and the Luhit, in India upstream of the Sadiya. This is particularly due to the intense monsoon precipitation on the southern aspect of the Himalaya.
While Nuxia (Tibet) on the Yarlung receives around 350 mm of rainfall during monsoon, as the Yarlung crosses the Himalayan crest line and reaches the southern aspect with the names of Siang and Dihang, the annual precipitation in Pasighat goes up to about 4500 mm. The flow volume and discharge in the Yarlung is not sufficient to generate and transport a large sediment load.
Moreover, the annual suspended sediment load near Nuxia has been measured to be around 30 million metric tonnes, quite small as compared to the 735 million metric tonnes at Bahadurabad. However, in case of hydro-power projects in the southern aspect, sedimentexcluding technology can play a significant role.
With low water and sediment contribution as compared to the southern aspect, a water transfer project in the northern aspect –even if constructed at great economic and energy cost — cannot have serious impacts downstream. So, the distinction needs to be made not on the basis of whether a project is Chinese or Indian, but whether the project is on the northern or southern aspect of the Himalaya. The current working hypothesis does not reflect this scientific reality.
