Chinese scientists have completed a first of its kind study to pinpoint the sources of the Brahmaputra and Indus rivers using satellite images, and have found that the length and drainage areas of both rivers exceeded earlier estimates.

Researchers from the Chinese Academy of Sciences (CAS), an official think-tank in Beijing, used remote-sensing satellite images and data from several expeditions to the Tibetan plateau to map the sources of the Brahmaputra, Indus, Salween and Irrawaddy rivers.

They located the source of the Brahmaputra, or Yarlung Tsangpo as it is known in Tibet, on the Angsi glacier on the northern side of the Himalayas, in the Tibetan country of Burang. The source of the river was earlier thought to be on the Chemayungdung glacier, further south.

The CAS study has mapped the river's length at 3,848 km, while earlier studies had estimated its length at 2,900-3,350 km. It also measured its drainage area at 712,035 sq km, with earlier estimates ranging from 520,000 sq km to 1.73 million sq km.

“Previously, the sources of the four rivers were never clearly designated, and differing accounts regarding their lengths and drainage areas confused researchers for many years due to restrictions of natural conditions and surveying and mapping technologies,” Liu Shaochuang, a researcher with the Institute of Remote Sensing Applications at CAS, was quoted as saying by the State-run Xinhua news agency.

Mr. Liu said the Indus river's headstream was mapped near Mount Kailash in Tibet, 30 km away from where its source was earlier thought to be. He said his study calculated the river's length at 3,600 km, exceeding earlier estimates of 3,200 km.

Mr. Liu's team used remote-sensing images provided by the U.S. Landsat satellite and the French SPOT satellite to map the Qinghai-Tibet plateau. He said the study would be “of great significance” to environmental researchers.

It, however, remains unclear if CAS will make the studies available to Indian researchers. Chinese institutes have been reluctant to share glacial and hydrological data citing strategic reasons, and have, in the past, balked at entering into agreements with Indian institutes.

India and China will, in coming months, hold talks to renew an agreement on sharing flood-related hydrological data on the Brahmaputra. Last year, the two countries signed an agreement to share data from June 1 to October 15 until 2012, based on an earlier MoU signed in 2008.

Indian officials have also called on China to release more information about its plans to develop hydropower projects on the river's upper and middle reaches. Prime Minister Manmohan Singh said earlier this month India “trusts” China's statements that “nothing will be done that affects India's interest”, but would also “verify” Chinese claims.

In November, China started damming the middle reaches of the river to begin construction of a 510 MW run-of-the-river hydropower project.

The dam at Zangmu is the only one of 28 proposed hydropower projects on the river that has been approved by the Chinese government. Chinese hydropower groups have been lobbying the government to give the go-ahead for other dams.

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