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This indicator measures trends in the amount and timing of snowpack in the western United States.
Figure 1. Trends in April Snowpack in the Western United States, 1955–2020
From 1955 to 2020, April snowpack declined at 86 percent of the sites measured (see Figure 1). The average change across all sites amounts to about a 19 percent decline. Large and consistent decreases in April snowpack have been observed throughout the western United States (see Figure 1). Decreases have been especially prominent in Washington, Oregon, northern California, and the northern Rockies. While some stations have experienced increases in April snowpack, all 12 states included in this indicator experienced a decrease in snowpack on average from 1955 to 2020 (see Figure 1). In the Northwest (Idaho, Oregon, Washington), all but four stations saw decreases in snowpack over the period of record. About 81 percent of sites have experienced a shift toward earlier peak snowpack (see Figure 2). This earlier trend is especially pronounced in southwestern states like Colorado, New Mexico, and Utah. Across all stations, peak snowpack has shifted earlier by an average of nearly eight days since 1982 (see Figure 3), based on the long-term average rate of change.
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. Long-term observations across the contiguous 48 states show that nearly 80 percent of weather stations examined have experienced a decrease in the proportion of precipitation falling as snow (see the Snowfall indicator). In addition, with warmer winters and springs (see the Seasonal Temperature indicator), the seasonality of snowpack is also changing. Higher temperatures cause snow to melt earlier, which in turn affects the timing and availability of water.
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 hydropower, irrigation, and drinking water. In most western river basins, snowpack is a larger component of water storage than human-constructed reservoirs.1 Continued reductions in snowpack and shifts in snowmelt are expected in the future, which will reduce hydropower production in the Southwest and the Northwest.
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. (For a look at long-term trends in the timing of spring snowmelt runoff in United States, see the Streamflow indicator.) Additionally, warming and earlier snowmelt accelerate the start of the wildfire season and promote more wildfire activity in the western United States and Alaska (see the Wildfires indicator). Altogether, snowpack’s sensitivity to climate and its many related effects make this a valuable indicator to track.
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. Figure 1 shows long-term rates of change for April 1, the most frequent observation date, because it could reflect changes in precipitation, and it is extensively used for spring streamflow and water supply forecasting. Figures 2 and 3 focus on the day of each year when snowpack is at its deepest (that is, peak snowpack) to determine if it is occurring earlier or later.
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 as part of the snow telemetry (SNOTEL) network. The long-term analysis in Figure 1 is based on data from nearly 700 permanent measurement sites in the western United States. The peak snowpack analysis shown in Figures 2 and 3 requires daily measurements, so it uses a smaller set of 328 SNOTEL sites that have data since 1982, which is when a large number of SNOTEL devices were deployed and came online.