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 Climate Change Indicators: Snowpack
This indicator measures trends in mountain snowpack in the
western United States.
Key Points
From 1955 to 2016, April snowpack declined at more than 90
percent of the sites measured (see Figure 1). The average
change across all sites amounts to about a 23-percent
decline.
Large and consistent decreases have been observed throughout
the western United States. Decreases have been especially
prominent in Washington, Oregon, and the northern Rockies.
While some stations have seen increases in snowpack, all 11
states included in this indicator have experienced a
decrease in snowpack on average over the time period. In the
Northwest (Idaho, Oregon, Washington), all but three
stations saw decreases in snowpack over the period of
record. |
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Background
Temperature and precipitation are key factors affecting
snowpack, which is the amount or thickness of snow that
accumulates on the ground. In a warming climate, more
precipitation will be expected to fall as rain rather than
snow in most areas—reducing the extent and depth of
snowpack. Higher temperatures in the spring can cause snow
to melt earlier.
Mountain snowpack plays a key role in the water cycle in
western North America, storing water in the winter when the
snow falls and releasing it as runoff in spring and summer
when the snow melts. Millions of people in the West depend
on the melting of mountain snowpack for power, irrigation,
and drinking water. In most western river basins, snowpack
is a larger component of water storage than
human-constructed reservoirs.
Changes in mountain snowpack can affect agriculture, winter
recreation, and tourism in some areas, as well as plants and
wildlife. For example, certain types of trees rely on snow
for insulation from freezing temperatures, as do some animal
species. In addition, fish spawning could be disrupted if
changes in snowpack or snowmelt alter the timing and
abundance of streamflows. |
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About the Indicator
This indicator uses a measurement called snow water
equivalent to determine trends in snowpack. Snow water
equivalent is the amount of water contained within the
snowpack at a particular location. It can be thought of as
the depth of water that would result if the entire snowpack
were to melt.
The U.S. Department of Agriculture and other collaborators
have measured snowpack since the early 1900s. In the early
years of data collection, researchers measured snow water
equivalent manually, but since 1980, measurements at some
locations have been collected with automated instruments.
This indicator is based on data from approximately 700
permanent measurement sites in the western United States.
The indicator shows long-term rates of change for April 1,
the most frequent observation date, because it could reflect
changes in snowfall, and it is extensively used for spring
streamflow forecasting.
Indicator Notes
EPA selected 1955 as a starting point for this analysis
because many measurement sites in the Southwest were
established in the early 1950s. Natural variability in the
Earth’s climate means that snowpack trends may vary slightly
when measured over different time periods. For example, the
period from 1945 to 1955 was unusually snowy in the
Northwest, so if this indicator were to start in 1945 or
1950, the Northwest would appear to show larger decreases
over time. However, the general direction of the trend is
the same regardless of the start date.
Although most parts of the West have seen reductions in
snowpack―consistent with overall warming trends shown in the
U.S. and Global Temperature indicator―snowfall trends may be
partially influenced by non-climatic factors such as
observation methods, land-use changes, and forest canopy
changes.
Data Sources
Data for this indicator came from the U.S. Department of
Agriculture’s Natural Resources Conservation Service Water
and Climate Center and the California Department of Water
Resources. The map was constructed using methods described
by Mote et al. (2005). The U.S. Department of Agriculture
data are available at:
www.wcc.nrcs.usda.gov.
The California Department of Water Resources data are
available at:
https://cdec.water.ca.gov/snow/current/snow/index.html.
Technical Documentation
Download related technical information PDF |
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 Figure
1. Trends in April Snowpack in the Western United
States, 1955–2016
This map shows trends in April snowpack in the western
United States, measured in terms of snow water equivalent.
Blue circles represent increased snowpack; red circles
represent a decrease.
Data source: Mote and Sharp, 20162 |
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EPA Page |
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This is the
EPA page for this topic. To see if the Trump
administration has changed the EPA page, simply click the
link and compare the information with this page. If you
notice changes were made to the EPA page, please post a
comment. Thanks. |
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Additional Climate Change Information |
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(Beginner - Listening,
reading)
A video lesson to
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The English is
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The English is
spoken at 75% of normal speed.
Great English listening and reading practice.
Researchers believe Africa may lose as much as 30 percent of its animal and
plant species by the end of this century. |
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