Scientists seek to unlock mysteries of Sierra snow droughts

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See a comparison between this year's and last year's snow pack. Jason Bean

What’s the difference between one snow drought and the next?

More than most people realize.

And those differences have implications for everything from skiing conditions to water supply in the western United States.

That’s according to a paper by two Nevada researchers in the journal Earth Interactions.

The researchers took a deep dive into the history of Sierra Nevada snow from 1951 until 2017 to understand the forces that shape the region’s snowpack.

The results are important because they show how conditions associated with human-induced climate change can thwart the formation of a robust snowpack.

“We kind of wanted to look at what are the flavors of snow droughts,” said Ben Hatchett, a climate scientist at Desert Research Institute in Reno and co-author of the paper. “How we get (them) could have some major implications for either water resources, or recreation or ecological impacts.”

Hatchett and co-author Dan McEvoy, also a climate scientist at DRI, hatched the idea for the study during ski and snowboard trips in the backcountry of the Sierra Nevada and Cascade ranges.

In the past four years alone the ranges have seen historic dryness and record precipitation. There have also been numerous snow droughts, including during a portion of the winter of 2016-17 which was the wettest on record in some spots.

“There really hasn’t been much done looking specifically at the mechanisms that drive these snow droughts,” McEvoy said. “Ben and I both spend a lot of time in the mountains. It is really sort of personal to understand how this drought affects not only ecology and hydrology but also recreation.”

What’s a snow drought?

Snow drought is a term that’s been getting more attention in recent years, particularly in the western U.S.

It refers to a situation during winter when there is less than a normal amount of snow.

It’s a scenario that can happen for a variety of reasons which is why scientists have defined two major types of snow drought.

The first is a dry snow drought which, like the name suggests, occurs when there is a lack of winter precipitation.

The second is a warm snow drought which can occur even when precipitation is normal or above normal. That’s because warmer temperatures lead a greater percentage of precipitation falling as rain instead of snow.

Understanding the different types of snow drought is important because the occurrence of one can have major ramifications for western communities.

But, for the most part, the methods people use to define and measure snow droughts lack the nuance required to fully understand how they affect communities and the natural world, especially as the planet warms.

“As it warms we are likely going to see less snow in the mountains in the west,” McEvoy said. “We’re really trying to let people know they should be monitoring this throughout the season.”

What did they learn?

Earlier research focused on April 1 as the date used to determine whether a mountain area endured a snow drought, then researchers would define whether it was a dry or warm drought.

Hatchett and McEvoy dug deeper by looking at eight different seasons with snow drought and examining how those droughts progressed. They used monthly, daily and even hourly data.

They found snow droughts vary in time and location. Some snow droughts start early and dissipate and some don’t emerge until late in the season.

Also, they found snow droughts vary by elevation with lower elevation areas more susceptible to snow drought.

“(Snow droughts) really evolve differently every year,” McEvoy said.

They also found some of the years associated with heavy precipitation and even flooding were also years of snow drought.

The winter of 1997, for example, is widely remembered for extensive January flooding in Reno. That year was also a snow drought as much of the moisture came as rain and degraded the snowpack.

“As the climate grows warmer and more precipitation falls as rain instead of snow, we are seeing we can have an average or above-average precipitation year and still have a below-average snowpack.”

By showing that snow droughts come in many forms the researchers hope to give other researchers, policy makers and the public more tools to examine how snow droughts affect the environment and communities.

For example, early season snow droughts might be more detrimental to ski resorts because they can depress traffic during the holidays. For water managers, a late season snow drought can be damaging if it means reservoir operators miss opportunities to capture melt.

Hatchett and McEvoy said they hope to build on the research by developing a snow drought tracking index that would allow people to assess conditions at a glance, similar to the way avalanche forecasters track and communicate avalanche risk.

“The big question we have been discussing is how can we condense all these into an index,” Hatchett said. “That is going to be a big challenge.”