Frozen soil, high stream flows and melting snow near Pullman, Wash., and Moscow, Idaho, in February all combined into making the perfect conditions for one of the largest extreme runoff and erosion event in 30 years, Pacific Northwest soil scientists say. (Courtesy Erin Brooks/University of Idaho)

Flooding caused by frozen soil on the Palouse in February was one of the largest events in 30 years, Pacific Northwest soil scientists say.

Historically high stream flows across the Palouse at that time, on top of the frozen soil, made “ideal conditions for a powerful erosion event.”

“It has been a while since we have experienced erosion rates this high or widespread,” Carol McFarland, Washington State University senior extension coordinator, told the Capital Press.

The event was not confined to the immediate Palouse. The researchers indicate that areas of Whitman, Lincoln and Adams counties were affected, as well as the Pendleton, Ore., area.

Financial loss is tough to determine, McFarland said, as technological advances have historically masked losses in productivity caused by soil erosion.

“But long-term consequences of soil loss are readily apparent within many fields that show eroded knobs with exposed subsoil, where everything from crop establishment to harvest return on investment are a challenge,” McFarland said.

How it happened

The event, which occurred Feb. 23-24, was set up by cold, dry conditions in January, with little to no snow cover to insulate the soil across the region, as nighttime temperatures dropped below 20 degrees Fahrenheit over a two-week period.

Overnight temperatures dropped down to -14 degrees Fahrenheit in Moscow, Idaho, on Feb. 12. By mid-February, the soil was frozen down to 6 inches in Moscow.

In the weeks prior to the snow melt, seven to 10 inches of snow accumulated over the frozen soil in the Moscow-Pullman, Wash., area.

This was followed by warming temperatures beginning Feb. 22 and more than 1.5 inches of rainfall, with 1 to 2 inches of water melted from the snowpack over frozen soil which was largely impermeable to water, making the perfect conditions for an extreme runoff and erosion event, said McFarland said in a March Timely Topic on Washington State University’s Small Grains website.

Vulnerability

The fields that were most vulnerable were those planted to winter wheat following a crop that produces relatively low amounts of residue, such as cool season legumes like chickpeas, peas and lentils.

About 40% of total acreage in the annual cropping zone is planted to winter wheat, and about 80% of that is planted following cool season legumes, she said.

“Actually a field that has a lot of surface roughness, such as a rough moldboard plow, has been documented by long-term ARS research to have less erosion than a smooth fall-seeded field,” said Erin Brooks, University of Idaho professor and agricultural engineer.

The worst erosion would be after summer fallow, when the wheat is fall-planted and going into a winter erosion period, said Dave Huggins, USDA Agricultural Research Service soil scientist.

Next would be winter wheat following a grain legume, with both under a conventional tillage regime, and then fall-tilled but unplanted fields. These are “usually pretty rough,” and don’t see erosion, but “tillage erosion” could be significant over time, Huggins said.

How much is topsoil worth?

There’s a wide spectrum of how much erosion is occurring on fields, from less than 5 tons per acre to well over 50 tons per acre, McFarland said.

“About 5 tons per acre per year is considered tolerable,” McFarland said.

How much is a ton of topsoil worth?

“Some say it is priceless,” she said. “Fifteen tons per acre is about the thickness of a dime, or one-tenth of an inch. So 50 tons per acre is about one-third of an inch of soil loss. In general, it takes about 500 years for an inch of topsoil to form so 50 tons of topsoil loss would take about 170 years to recover.”

What to do

Long-term practices which include low soil disturbance, such as continuous no-tillage and return of crop residues to the soil, can make a “substantial difference” as to how vulnerable a field is, McFarland said.

For example, WSU’s Cook Agronomy Farm, where no-till has been practiced continuously for more than 25 years, stood in direct contrast to neighboring fields with a history of tillage where soil erosion was excessive.

Much progress has been made since the historic days on the Palouse, due to research on mitigating erosion and farmers who have adopted those practices, McFarland said.

“Despite the impact on the soil seen in this event — and it reminds us that there are still gains to be made in terms of farming practices to increase resilience to more extreme weather events — things could have been worse without the gains that have already been made,” she said.