Snowpack, Hydrology & Drought
As with much of the American West, rising temperatures in the Pacific Northwest are changing our region’s hydrology, causing precipitation to fall more as rain and less as snow. This might not sound like a big deal, but it most definitely is.
Picture mountain snow as a natural reservoir that—in slowly melting over the summer months—provides water during our region’s warmest, driest time. Losing this snow to rising temperatures is bad news for everyone from farmers who rely on snowmelt-fed irrigation to water their crops to American Indian tribes who depend on a steady flow of snow to keep stream temperatures in a range that salmon species and other traditionally-harvested fish have evolved to tolerate. A lack of snow also means droughts can occur even if precipitation stays stable.
Consider the drought years of 2011 to 2015. The drought hit the states of California, Oregon, and Washington hard. However, during the drought precipitation measurements for California—which endured the worst of the drought—were low, but anomalously so when compared both to historical metrological measurements and reconstructed paleoclimate records from proxies, such as tree rings. Meanwhile, in Oregon and Washington precipitation levels remained nearly normal throughout the drought years, although during some years the timing of precipitation was atypical for the two states.
So what made the drought particularly droughty? A major factor was temperature. The years 2011 to 2015 were exceptionally warm for the region, with 2015 being the warmest on record for all three Pacific states, according to NOAA records. This led to very low snowpack levels across the region, with 2015 being the worst year on record for snowpack at some 80 percent of high-elevation monitoring sites in California, Oregon, and Washington, according to an analysis performed by CIRC researchers. Not surprisingly, 2015 became known as a “snow drought.”
CIRC’s climate model projections strongly suggest that the Pacific Northwest will experience more of these snow droughts in the future, as many mountain ecosystems transition from being snow-dominated to being rain-dominated. As we note on our Precipitation page, precipitation projections for our region have a low degree of confidence to them and show annual precipitation remaining largely within the natural variability already observed. But as the recent drought has shown, temperature by itself can produce a drought.
There is another complication to this transition from snow to rain that is less intuitive: flooding. Warming temperatures are expected to produce earlier snowmelts that in turn are expected to produce earlier and larger stream flows in the winter and spring. This is projected to lead to more intense and possibly more destructive winter flooding.
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