Summer Drought in the Willamette Basin?

droughtcollageIn the West, most of the annual precipitation falls during the winter months. Most of it falls as snow in the mountains, where it’s 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, 2014 SWE data is now in. And it’s not good. Willamette Basin SWE is just 55 percent of normal as of March 29. Though the precipitation of the last week has brought a final gasp of snow to the Cascades, it’s not enough to bring us close to average. It’s also come too late—the snowpack that is near 5,000 feet in elevation and below has begun melting since the first of March, about a month earlier than average.

Unfortunately, this may well be the new normal. 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–just as it has this year. Earlier peak snowpack will increase March streamflows and reduce June, July, and August streamflows.

On the bright side, for this year at least, the Willamette River Basin has not been nearly so stricken by drought as have the Klamath and Rogue River basins in Southern Oregon, where the governor has declared a state of emergency.

By Nathaniel Brodie

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