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[KATHMANDU] Water levels in Asia’s two major river systems, fed by Himalayan glaciers, are unlikely to drop over the next century, says a new study.
Led by Walter Immerzeel, post-doctoral researcher at the Utrecht University, Netherlands, the study predicts that the shrinking of glaciers that feed the Ganges and Indus river basins will be compensated by increased monsoon rains.  
The study published this month (4 August) in Nature Geoscience assessed the hydrological impact of climate change on two climatically contrasting watersheds — the Baltoro in Pakistan and the Langtang in Nepal that drain into the Indus and Ganges rivers respectively. 
"The most significant outcome of the study is that, under the projected climate change, the glaciers in the Himalaya will reduce in size, but the runoff from glacier catchments will increase," says Marc Bierkens, a co-author from Utrecht University.
Bierkens told SciDev.Net that melt water will increase until 2070 while precipitation will increase throughout the 21st century.
The researchers used computer models of glacier movements and water balance in the two watersheds, which vary significantly. The Langtang’s relatively smaller glaciers melt quite quickly with increased water discharge from increased monsoon rains. The Baltoro watershed is drier and colder but has larger glaciers. 
The new study contradicts Immerzeel’s earlier study of the same rivers published in the Science journal three years ago where he predicted a drop in water levels in both by 2050.
"We used a better, more detailed glacier model that takes into account the slow response of glaciers to climate change and also considers the latest suite of climate models that predict a wetter monsoon during the 21st century," explains Bierkens.
Local water experts feel more work needs to be done to understand changes in monsoon patterns and snowmelt. "The results very much depend on the climate scenarios used for the analysis and available scenarios have high uncertainties in projecting monsoon precipitation," says Arun Shrestha, water resource expert at the International Centre for Integrated Mountain Development, Kathmandu.
The authors accept the limitations of the study. "We only looked at average discharge, not at high and low flows or timing of discharge, which may change to a less suitable regime under climate change," Bierkens said. 

Link to article in Nature Geoscience