Both seasonal snow melt and glacier melt can contribute to stream flow, yet these two separate factors are often studied together. "We suspected that, if you just look at glaciers, the numbers [at risk of water shortage] are substantially smaller but people tend to mix up this seasonal snow cover and glaciers and lump them together," Dennis Lettenmaier of the University of Washington, US, told environmentalresearchweb. "We felt it was time to get a quantitative handle on how big an issue this was."

Along with colleagues from the University of Washington and the Pacific Northwest National Laboratory, Lettenmaier found that 8.9% of the global population – 600 million people – rely on rivers that receive at least 5% of their maximum flow from glacier melt. And 1.8% of the world’s population (120 million people) is potentially affected by rivers that receive a maximum of half their stream flow from glacier melt. The largest at-risk populations are in Asia but these numbers are still comparatively small.

"We were a little surprised that the numbers were as small as they are, although we suspected that the case was being overstated in terms of the glacier contribution," said Lettenmaier.

Lettenmaier notes that the study only examines the month when there is maximum glacier melt and makes approximations based on computations of net radiation without dealing with many other technical issues. The researchers are now focusing on specific sites where substantial populations' water supplies may be at risk to try to understand the details better on a case-by-case basis.

"This is a screening study to try to scope out the global magnitude of the problem," Lettenmaier said. "It shows that seasonal snow cover is a much bigger issue and probably deserves a bit more attention. Changes to this will affect a lot more people than glacier retreat."

The researchers used a global hydrology model that encompassed all river basins with glaciers in their headwaters. First they ran the model for areas without glacier cover, and then repeated the run for areas containing glaciers, adding in calculations of the amount of energy available to melt the glaciers. By comparing stream flow between areas with and without glaciers the team was able to estimate glacier contribution to river discharge.

"We took the month where the available energy was the highest, and we estimated how much melt would end up in the stream," said Lettenmaier. "We then calculated the ratio of this component compared with the component of the stream flow that comes from other sources such as direct run-off, rainfall, and non-glacier snowmelt."

The study was published in Environmental Research Letters (ERL).