Snowpack, Hydrology, & Drought
Mountain snow, or snowpack, acts as a natural water reservoir. By slowly melting over the summer months, snowpack provides water during what is typically the Pacific Northwest’s warmest and driest time of year. As with much of the American West, rising temperatures in the Pacific Northwest are making it far more likely that precipitation will fall more as rain and less as snow. This trend is expected to continue as temperatures continue to rise under human-caused climate change.
Snow Declines Across the American West
This figure depicts a calculation made by CIRC researchers of snowpack declines from 1955 to 2016.
The calculation is based on data from in-the-field measurements of mountain snowpack taken from sites run by the U.S. Department of Agriculture’s Natural Resources Conservation Service and the California Department of Water Resources.
Each circle represents a site with at least 40 years of snowpack measurements. What’s being measured is snow water equivalent (SWE), or the amount of water you would get if you melted a given amount of snow. Red circles represent decreases in SWE. Blue circles represent increases. The size of any given circle denotes by how much SWE has either decreased or increased. Depending on the color, the bigger the circle the bigger the decrease or increase in SWE. As you have probably noticed, there are significantly more red circles than blue ones on the map and a fair number of large red circles at that. In fact, over 90% of the circles on this map are red, meaning over 90% of snow monitoring sites with long records in the American West show declines (Mote et al. 2018). This Image is licensed under Creative Commons Attribution 4.0 International (CC BY 4.0).
How Much Snow have We lost?
This figure shows simulated snowpack declines. As with the previous figure, reds denote decreases in SWE and blues increases. The figure was created using the Variable Infiltration Capacity Hydrologic Model (VIC), the brainchild of former CIRC researcher Dennis Lettenmaier. Using the Hamlet and Lettenmaier dataset, CIRC researchers calculated SWE via a VIC simulation for the years 1955–2014. The VIC simulations took historic climate data—how warm it was and how much precipitation fell—and then deduced that if you had these conditions you would get such and such amounts of SWE at the different elevation bands modeled. The hydrologic model was used in part to corroborate the data-crunching that produced the figure above. It was also used to determine how much SWE has been lost in the American West from 1955 to 2014. That number, as you might imagine, is large. From 1955 to 2014, the snowpack equivalent of 25 to 50 cubic kilometers of water (roughly 6–12 cubic miles) have dissapeared from the American West, according to CIRC research. This is comparable to the water volume of a full Lake Mead, which can hold up to 32 km3 (not quite 8 cubic miles) of water (Mote et al. 2018). This Image is licensed under Creative Commons Attribution 4.0 International (CC BY 4.0).
Why is April 1st used in Snow Monitoring?
April 1st marks the point of maximum snowpack accumulation for much of the American West.
April 1st also marks the turning point after which snowpack begins to melt. Although the exact date of peak snowpack may occur earlier or later than this—depending on factors, such as location, elevation, and year-to-year variability—streamflow forecasts using SWE numbers on April 1st have long been used for predicting summer streamflow. So much so that April 1st snowpack numbers act as a kind of bellwether for summer water supplies.
Years with low April 1st snowpack tend to be followed by low flows in rivers and streams during the summers that follow. This can sometimes lead to or exacerbate water scarcities, especially in snowmelt-dominated basins.
Over 90% of snow monitoring sites with long records (more than 40 years worth) in the American West show declines, according to CIRC research (Mote et al. 2018).
Using a hydrologic model to corroborate these results, CIRC researchers concluded that this translates to the snowpack equivalent of 25 to 50 cubic kilometers of water (roughly 6–12 cubic miles). To make that really big number make sense for their readers, the researchers compare it to Lake Mead, which can hold up to 32 km3 (not quite 8 cubic miles) of water (Mote el al. 2018).
Watersheds in the Pacific Northwest that receive a mix of rain and snow and derive a substantial portion of streamflow from spring snowmelt are most sensitive to future warming expected during the winter months (Vano et al. 2015).
The Cascade Mountains in Oregon and Washington are expected
to be particularly hard hit by declines in snowpack with a projected decrease of 65%—or 37.5 km3—in April 1 snow water equivalent (SWE) storage—by the 2080s under the high emissions scenario (RCP 8.5) (Gergel et al. 2017).
By the mid-21st century (2040–2069) under the high emissions scenario (RCP 8.5), every SNOTEL site in the West is likely to see less snowfall and that snowfall is more likely to come in extreme snowfall events (Lute et al. 2015). (SNOTEL sites are automated snow- observing stations.)
Sites that currently experience average winter temperatures that hover just above freezing are projected to see the largest decreases in the amount of snow that falls during extreme snowfall events, declining 20–50% from historical records. These include most of the SNOTEL sites in Oregon and Washington (Lute et al. 2015).
In the Cascade Mountains are some of the hardest hit SNOTEL sites, which are projected to experience a 35–70% reduction in snowfall from historical levels (Lute et al 2015).
Low snowfall years will become common in the Cascades by the mid- 21st century, whereas high snowfall years will become exceedingly rare (Lute et al. 2015).
The water year 2014–2015 was dubbed a “snow drought” because precipitation was near-normal while abnormally warm temperatures led to record low snowpack. Record low spring snowpack measurements were set at 80% of mountain recording sites (or SNOTEL sites) in the Western United States (Mote et al. 2016).
Spring snowpack in 2015 was the lowest on record for Oregon—89% below normal—and tied for lowest on record for Washington (Mote et al. 2016).
Anthropogenic forcing added about 1° C (1.8° F) of extra warming to the water year 2014–2015 exacerbating the “snow drought” (Mote et al. 2016).
- Snow Drought: A snow drought occurs when a region receives a less-than-adequate amount of snow. This can happen when above-normal temperatures force precipitation to fall as rain instead of as snow, when not enough precipitation has fallen to create an adequate amount of snow, or through a combination of warm temperatures and low precipitation levels.
- Snow water equivalent (SWE): The amount of water you would get if you melted a given amount of snow. It is a commonly used measure for calculating snowpack.
Gergel, Diana R., Bart Nijssen, John T. Abatzoglou, Dennis P. Lettenmaier, and Matt R. Stumbaugh. “Effects of Climate Change on Snowpack and Fire Potential in the Western USA.”
Climatic Change 141, no. 2 (2017): 287-299.
Lute, A. C., John T. Abatzoglou, and Katherine C. Hegewisch. “Projected changes in snowfall extremes and interannual variability of snowfall in the western United States.”
Water Resources Research 51, no. 2 (2015): 960-972.
Mote, Philip W., Sihan Li, Dennis P. Lettenmaier, Mu Xiao, and Ruth Engel. “Dramatic declines in snowpack in the western US.”
Nature Partner Journals: Climate and Atmospheric Science volume 1, 2. (2018).
Mote, Philip W., David E. Rupp, Sihan Li, Darrin J. Sharp, Friederike Otto, Peter F. Uhe, Mu Xiao, Dennis P. Lettenmaier, Heidi Cullen, and Myles R. Allen. “Perspectives on the Causes of Exceptionally Low 2015 Snowpack in the Western United States.”
Geophysical Research Letters 43, no. 20 (2016).
Vano, Julie A., Bart Nijssen, and Dennis P. Lettenmaier. “Seasonal Hydrologic Responses to Climate Change in the Pacific Northwest.”
Water Resources Research 51, no. 4 (2015): 1959-1976.
The 2015 Snow Drought
The year 2015 was the warmest year on record for both Oregon and Washington, according to NOAA. Warm temperatures meant that much of the precipitation that fell during the cool months did so as rain instead of as snow. As a result, in 2015 record low April 1st snowpack measurements were set at 80% of SNOTEL sites across the American West. Here in the Pacific Northwest, snowpack conditions for April 1st were well below normal (3–58% of normal) across the region. Averaged over the region, April 1st SWE in 2015 was 37% of normal. In Oregon, 2015’s April 1st snowpack was the lowest on record for Oregon—89% below normal. In Washington, 2015’s April 1st snowpack tied for the lowest on record.
Visualizing the 2015 Snow Drought
This image shows the Pacific Northwest's April 1st, 2015 median snow water equivalent compared to historical baseline 1981–2010. Cool colors (greens and blues) represent SWE levels above normal (i.e. what was normal for the recorded years 1981–2010). Warmer colors (the oranges and reds) represent lower-than-normal SWE levels. Obviously, there is a lot of red and orange on this map. (Image Credit: Natural Resources Conservation Service.
CIRC Researcher Kathie Dello talks Snow Droughts
This video was made in 2015 in conjunction with Oregon Sea Grant. The video describes the Pacific Northwest's recent historic drought and role low snowpack played in it. It also highlights how people and organizations found ways to adapt.