Earlier snowpack melt in Western US could bring summer water scarcity


Date:
May 22, 2023
Source:
University of Colorado at Boulder
Summary:
Mountain snowpack, typically seen as the water tower of the Western
United States and Canada, is in decline, according to a new study.

Researchers created the Snow Storage Index to assess snow water
storage from 1950-2013 and found that storage has significantly
declined in more than 25% of the Mountain West, in part because
more snow is melting during winter and spring.


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Snow is melting earlier, and more rain is falling instead of snow in
the mountain ranges of the Western U.S. and Canada, leading to a leaner snowpack that could impact agriculture, wildfire risk and municipal water supplies come summer, according to a new study from the University of
Colorado Boulder.

Published in Nature Communications Earth & Environment, the study
documents more than 60 years of change in snowpack water storage across
Western North America. It found that from 1950 to 2013, snowpack water
storage has significantly declined in more than 25% of the Mountain
West, in part because more snow is melting during winter and spring,
eroding this seasonal boundary.

"On average and in every mountainous region that we looked at, snow
melt is occurring closer in time to when it fell," said Kate Hale,
lead author of the study and a 2022 geography graduate. "The timing
of water availability is shifting toward earlier in the springtime,
with less snow melt and water availability later in the summertime,
suggesting that there will be water scarcity later in the year."
Timing is everything The Western U.S. and Canada depend on snow for most
of their water. The Rocky Mountains, Sierra Nevadas and other mountain
ranges have long served as, essentially, water towers for the region:
They store snow throughout the winter, which then melts and becomes
available as water in spring and summer, when demand is greatest.

Every year on April 1, state and regional water managers use a metric
known as snow water equivalent (SWE) -- how much water will be produced
when an amount of snow melts -- to predict and plan for water resources
that year, said Hale, now a postdoctoral researcher at University
of Vermont.

But that April 1 snapshot is exactly that: one moment in time. It doesn't reveal if that snow slowly accumulated over the past six months, if it
all fell in one giant heap on March 31, or if it was already melting.

"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,"
said Noah Molotch, associate professor of geography and fellow at the
Institute of Arctic and Alpine Research (INSTAAR) at CU Boulder.

So Hale used two publicly available data sources to develop a new
measurement known as Snow Storage Index (SSI) that incorporates the
timingand amount of snowfall, as well as snowmelt, before and after
April 1. In contrast to the singular moment in time captured by SWE,
Hale's SSI shows a metaphorical video: incorporating into one number,
the time between when rain or snow falls on a landscape in the winter
season and when it becomes available to that area as surface water.

"The snow storage index allows us to look at snow water storage, not just
in the context of how much is there at any given time, but the duration
of that storage on the ground," said Molotch.

This allowed the researchers to analyze how well each mountainous region
of the West has acted as a water tower over the past 60 years and discover
that their performance has been declining across the board.

Managing water now and for the future A "high" SSI -- a number as close to
1.0 as possible -- was found in places where snowfall is very seasonal. In
the Cascades, for example, snow accumulates in the fall and winter season,
and is stored up to six months before melting somewhat continuously
in the spring and summer. Here in Colorado's Rocky Mountains, however,
the SSI is lower -- somewhere between 0 and 0.5 -- which means that snow
both accumulates and melts throughout the colder half of the year.

But because the Rockies and the Front Range are already used to this alternating pattern of snowfall and snowmelt during winter and spring
seasons, as a region it may adjust easier to similar patterns of
decreased snowpack water storage associated with global warming. The
mountain regions near the West Coast that are highly reliant on snowpack meltwater in the spring and summer, however, may be in for a painful
adjustment when that water melts earlier in the year -- and is simply
no longer available come late summer.

The researchers hope that this new measurement can serve as a tool for scientists and water resource managers to make better predictions and,
when necessary, plan ahead for less.

Half a century ago, an era of dam building in the Western United States
allowed the region to flourish in terms of access to water for cities
and for agriculture, said Molotch. But as these "water towers" melt away,
so too may the reservoirs they filled.

"The snowpack is eroding and disappearing before our eyes. That's going
to present challenges in terms of managing the infrastructure that's
allowed the Western United States to flourish over the last 100 years,"
said Molotch.

Additional authors on this publication include: Keith Jennings,
Lynker, Boulder, Colorado; Keith Musselman, Department of Geography and theInstitute of Arctic and Alpine Research (INSTAAR), CU Boulder; and
Ben Livneh, Cooperative Institute for Research in Environmental Sciences (CIRES) and the Department of Civil, Environmental, and Architectural Engineering, CU Boulder.

* RELATED_TOPICS
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========================================================================== Story Source: Materials provided by
University_of_Colorado_at_Boulder. Original written by Kelsey
Simpkins. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Katherine E. Hale, Keith S. Jennings, Keith N. Musselman,
Ben Livneh,
Noah P. Molotch. Recent decreases in snow water storage in western
North America. Communications Earth & Environment, 2023; 4 (1)
DOI: 10.1038/ s43247-023-00751-3 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2023/05/230522131337.htm

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