The researchers, from ETH, Zurich in Switzerland and Utrecht University used measurement data and climate models to closely examine water balance in both of these mountain ranges. Both have peaks that rise to over 6,000 metres and also glaciers. Climate models for the remainder of the century indicate that both regions will experience similar increases in annual mean temperatures - the milder scenarios predict a rise of one to three degrees, the more extreme as much as four to six degrees. The study is published in the journal PNAS.

To calculate and compare each region's water balance, the researchers created a new and very extensive model of the upper Langtang valley in Nepal and the Juncal region of the central Andes in Chile. Both are important water catchment areas for the millions of people who live in the surrounding lowlands.

In the upper Langtang valley, water discharge will increase in the first half of this century, in the extreme scenario, by as much as 70 percent. Maximum discharge could be reached by around 2050 to 2060, after which discharge is expected to remain the same or drop steadily as we move towards the year 2100.

The Juncal region is set to become drier in the future. During the dry season, which is already quite long, river water will become scarce. According to their calculations, the researchers still expect water discharge to remain at current levels in the period from 2010 to 2030. However, after that, there will be a steady decline in the amount of water available. So in an extreme scenario, water discharge from the entire catchment area in the Juncal region could shrink to a third of the current level by the year 2100.

Just how much water is carried by streams and rivers ultimately depends on the fates of the local glaciers. Glaciers in both areas are going to shrink, with those in the Juncal region diminishing by up to 70 percent while in Langtang, the more extreme predictions indicate a loss of "only" 55 percent.

In turn, the different degrees of glacial melt will also lead to differing scenarios in terms of water discharge: in Langtang, the proportion of glacial melt in the discharge will increase and peak by 2050 before receding. In Juncal, glacial melt already peaked before 2010, and now the proportion of discharge from glacial melt is dropping steadily. This is explained by the fact that the Langtang glaciers are situated at very high altitudes. Once they melt at some point in the future, this meltwater will then replace that which today comes from glaciers lower down. What's more, the tongues of many glaciers in Langtang are covered in massive layers of rock debris; these insulate the glaciers, which then recede less quickly. In Juncal, however, the highest glaciers are already melting, as they are situated lower than those in the Himalayas.

The new model also indicates that Langtang could be in for increased rainfall in the future, which will exacerbate the effect the increasing glacier melt is having on water discharge. But this is not the case in Chile, where the drought in the summer months of December to March will get worse. Today, farmers in Chile's most fertile regions use meltwater to irrigate their fields. "If the upper basins of rivers deliver less water in the future, it will be essential to take action to encourage conservation of the country's water reserves," says corresponding author Silvan Ragettli, post-doctoral student at the Institute of Environmental Engineering at ETH Zurich.

In contrast, efforts in the Nepalese Himalayas will have to focus on flood management. Climate change will mean higher snow lines. This will increase the amount of rain that falls in high elevations. "Increased precipitation in the form of rain means that water runs off immediately, which in turn can lead to massive flooding," says Ragettli.