Western US Sees Accelerated Winter Snowmelt, Report Reveals

New research has found that in the western parts of North America, more snow is melting during the winter months, with a third of 1,065 snow measurement stations stretching from the Mexican border to the Alaskan Arctic now showing increasing winter snowmelt.

Carried out by scientists from the University of Colorado Boulder, the study involved  analysis of 40 years’ worth of data, with significant melt increases seen predominantly in November and March, although melt is increasing in all cold season months, from October to March.

Published in the Nature Climate Change journal, the findings have important implications for water resource planning, affecting not just ski conditions (in the form of potentially fewer powder days and crustier snow conditions), but also agriculture and fire risks.

It was also found that melt taking place before April 1st has risen at nearly half of the 600 or so stations in the west, by an average of 3.5 per cent per decade.

In the western US, snow is the main source of water and streamflow, providing water resources to one billion people globally. Snowy mountains serve as water towers, reserving water high up the mountainscape until it melts, where it then becomes available lower down where it’s required during the summer months, rather like a natural drip irrigation system.

Because winter snowmelt is now accelerating, the timing of water entering this system is now shifting, so that there’s more water available in the winter and less during the summer.

This, of course, has big implications for water resource management and drought prediction for western regions, which depends on late winter snowpack levels for water. The change in delivery timings could also have an impact on agricultural irrigation and wildfire seasons.

There are also potential repercussions for ecology thanks to wetter soils in the winter. For example, wet soils have reduced capacity to absorb additional water during rainstorms or spring melt, which can increase the risk of flash flooding.

Wetter soil can also keep microbes awake during the winter at a time when they would otherwise likely be dormant, which can then have a knock-on effect for nutrient availability timings, water quality and carbon dioxide emissions.

Keith Musselman, lead author of the study and research associate at the university’s Institute of Arctic and Alpine Research (INSTAAR), said: “Particularly in cold mountain environments, snow accumulates over the winter- it grows and grows- and gets to a point where it reaches a maximum depth, before melt starts in the spring.

“That slow trickle of meltwater that reliably occurs over the dry season is something that we have built our entire water infrastructure on in the West. We rely very heavily on that water that comes down our rivers and streams in the warm season of July and August.”

Delving deeper

Further research from the university has found that not only is snow melting earlier, but more rain is now falling instead of snow across mountain ranges in the west.

More than 60 years of change in snowpack water storage was documented in the study, revealing that between 1950 and 2013, storage has fallen significantly in over 25 per cent of the mountain west, with the winter/spring seasonal boundary now starting to erode.

Lead author Kate Hale explained that every year on April 1st, state and regional water managers use the snow water equivalent metric (which is how much water is produced when a certain amount of snow melts) to predict and plan their annual water resources.

But what’s important to remember is that reviewing the situation on a single date each year doesn’t necessarily provide a full enough picture to make relevant decisions. For example, it doesn’t show whether the snow in question had already been melting, if it had all fallen at once on March 31st or if it had been building up slowly over a matter of months.

Ms Hale developed a new metric known as the snow storage index to cover both the timing and the amount of snowfall, as well as snowmelt, before and after April 1st. A high number on the scale was found in places where snowfall is very seasonal.

In the Cascades, for example, snow builds up during the autumn and winter months and this accumulation is stored for up to six months before it melts continuously during the spring and summer. But in the Rocky Mountains, the index number is lower, which means that snow accumulates and melts during the colder half of the year.

Noah Molotch, associate professor of geography and fellow at the INSTAAR, commented on the findings, saying: “From a hydrologic perspective, the only thing that’s unique about snow is that it delays the timing of water input to watersheds.

“And just looking at a snapshot of snow water equivalent doesn’t give you a sense as to how long that snow water equivalent has been on the ground.”

He went on to add that the snowpack is now “eroding and disappearing before our eyes”, which will naturally bring with it challenges for how infrastructure in the western US will be managed, infrastructure that has allowed this part of the country over the last 100 years.

There is light at the end of the tunnel, however, because the Rockies and the Front Range are both used to the alternating pattern of snowfall and snowmelt during the colder months. This may make it easier for it to adjust to similar patterns of decreased snowpack water storage being driven by climate change and global warming.

In contrast, mountain regions near the west coast (which are more reliant on snowmelt in the spring and summer), may find the adjustment harder when water melts earlier in the year and less is available later in the summer months.

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