"Snow is important because it forms its own reservoir," said Justin Mankin of Columbia University, US. "But the consequences of reduced snowpack are not the same for all places – it is also a function of where and when people demand water. Water managers in a lot of places may need to prepare for a world where the snow reservoir no longer exists."

The basins at risk lie in the American West, southern Europe, the Middle East and central Asia, Mankin and colleagues from the US, Switzerland and the Netherlands found. The team examined 421 drainage basins across the northern hemisphere, using climate models, water-use patterns and population data. In total, 97 of the basins had at least a two-thirds chance of experiencing water declines if current demand continues.

"We did not know where snow was critical to supplying water for people and thus where people are most likely to be sensitive to projected snow changes," said Mankin. "Many of the basins that are currently snow-dependent have large risks of declines in snow's future potential to supply current demands given both changes in rainfall and snowpack. But despite these risks, innate variations in year-to-year or decade-to-decade climate mean that these same basins have the potential for increases as well, and both of these possibilities are entirely consistent with a warming world."

In many regions, the models showed that changing climate is about as likely to increase water supply, by boosting rainfall, as to decrease it. The Indus and Ganges basins in India, where about 1 billion people live, could see similar levels of water supply, as could the Huai basin in China, home to 130 million. Increased glacier melting in the Himalayas could temporarily boost supplies to Uzbekistan and Kazakhstan.

"We only consider changes in climate, but an even larger source of uncertainty is people and how their consumptive patterns will change," said Mankin. "We know populations will increase and that demand will as well, but that technological and management changes could offset at least some of these increases. So there is some capacity to adapt, but that capacity is going to be different for different places. Water managers need to reconcile these changes against their basin's other vulnerabilities."

 The team examined the 32 basins with the largest populations – 1.45 billion people altogether – in more detail. These included basins in northern and central California, the basins of the Colorado and Rio Grande rivers, the Atlas basin in Morocco, the Ebro-Duero basin, which feeds water to Portugal, Spain and southern France, the Shatt al Arab basin, which serves Iraq, Syria, eastern Turkey, northern Saudi Arabia and eastern Iran, and basins in eastern Italy, the southern Balkans, several Caucasus nations and northern Turkey.

The snowpack changes could also affect ecosystems, for example by enabling undamped forests to catch fire earlier in the year, leading food sources for birds to run out in late summer, or harming trout that rely on a steady flow of cold water during summer.

Now the team will examine some of the sites identified as at greatest risk, for example the western US, and investigate the potential for adaptation to offset some of the damage. Daniel Viviroli of the University of Zurich, who participated in this study, and his colleague Tracy Ewen will lead a project searching for "particular patterns of climate variability that govern snowfall, accumulation and melt, which could lend greater predictability to the short-term snow changes that have this potential to confound long-term decision-making". Other projects, meanwhile, will look at the implications of snowmelt changes for ecosystems, including differences in seasonal soil moisture and drought.

Mankin and colleagues reported their findings in Environmental Research Letters (ERL).

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