In the West, the vast majority of annual precipitation falls during the winter months. Most of it falls as snow in the mountains, where it’s conveniently stored in snowpacks. In the Willamette Valley, these snowpacks, once melted, provide the majority of our water during our dry summers—anywhere from 40 to 80 percent. Winter snowpack irrigates millions of acres of crops, replenishes our reservoirs and aquifers, and provides habitat for innumerable creatures. It’s a critical resource.
In order to forecast how much water resources a certain region can expect to count on for a given year, we rely on Snow Water Equivalent (SWE) data. SWE is the amount of water contained within the snowpack. If all the snow in the Willamette basin melted instantly, the depth of the resulting water is the Willamette basin’s SWE.
April 1, give or take some days, represents the highest accumulation of snow—“peak SWE”—across the West. Up to April 1, it’s accumulating; after April 1, it’s melting. Thus April 1 SWE data are used as surrogates for the total seasonal accumulation and for the maximum seasonal snowpack on the ground.
April 1, 2013 SWE data is now in: 2013 is right on the median from 1981 to 2010 (see accompanying graph). The John Day, on the other hand, isn’t looking so good—they’re at 43 percent of average. But as for the Willamette, median historic SWE is good news. Enjoy it while you can. Projections for the future are grim.
Hell, the past is grim: because of both a warming climate and lower precipitation, most monitoring stations in the PNW have showed a significant decline in April 1 snowpack. Between 1950 and 1995 Cascades snowpack trended downward approximately 35 percent. Timing of the peak snowpack has moved earlier in the year—it’s historically shifted by as much as 40 days earlier. Earlier peak snowpack will increase March streamflows and reduce June, July, and August streamflows.
As for the future, it is almost certain that the April 1 snowpack will continue to decline in response to increasing temperatures. Peak SWE could come as early March, even February. Less snow and earlier snowmelt will have significant repercussions on, well, everything.
We know we’re changing our climate, but we’re unsure of what the results will be. SWE is a useful and critical gauge. Pay attention at the Natural Resources Conservation Service’s Oregon Snowpack Summary webpage: http://www.or.nrcs.usda.gov/snow/data/SWE_Maps&Graphs.html
by Nathaniel Brodie