These events are unpredictable and difficult to forecast. Yet they will become more common as the planet warms and more winter precipitation falls as rain rather than snow.

91̽»¨ mountain hydrology experts are using the physics behind these events to better predict the risks.

“One of the main misconceptions is that either the rain falls and washes the snow away, or that heat from the rain is melting the snow,” said , a 91̽»¨doctoral student in civil and environmental engineering. He will present his research Dec. 18 at the annual meeting of the .

Most of the largest floods on record in the western U.S. are associated with rain falling on snow. But it’s not that the rain is melting or washing away the snow.

Instead, it’s the warm, humid air surrounding the drops that is most to blame for the melting, Wayand said. Moisture in the air condenses on the cold snow just like water droplets form on a cold drink can. The energy released when the humid air condenses is absorbed by the snow. The other main reason is that rainstorms bring warmer air, and this air blows across the snow to melt its surface. His work support previous research showing that these processes provide 60 to 90 percent of the energy for melting.

Places that experience rain-on-snow flooding are cities on rivers that begin in the mountains, such as Sacramento, California, and Centralia, Washington. In the 1997 New Year’s Day in Northern California, melting snow exacerbated flooding, which broke levees and caused millions of dollars in damage. The biggest recent rain-on-snow event in Washington was the in the Snoqualmie basin. And the in summer of 2013 included snow from the Canadian Rockies that caused rivers to overflow their banks.

The 91̽»¨researchers developed a model by recreating the 10 worst rain-on-snow flooding events between 1980 and 2008 in three regions: the Snoqualmie basin in Washington state, the upper San Joaquin basin in central California and the East North Fork of the Feather River basin in southern California.

Their results allow them to gauge the risks for any basin and any incoming storm. The three factors that matter most, they found, are the shape of the basin, the elevation of the rain-to-snow transition before and during the storm, and the amount of tree cover. Basins most vulnerable to snowmelt are treeless basins with a lot of area within the rain-snow transition zone, where the precipitation can fall as snow and then rain.

Trees reduce the risk of flooding because they slow the storm’s winds.

“If you’ve ever been in a forest on a windy day, it’s a lot calmer,” Wayand said. That slows the energy transferred from condensation and from contact with warm air to the snowpack.

Simulations also show that meltwater accounted for up to about a quarter of the total flooding. That supports earlier research showing that snow is not the main contributor to rain-on-snow floods, but cannot be neglected since it adds water to an already heavy winter rainstorm.

The complexity of mountain weather also plays a role.

“The increase in precipitation with elevation is much greater than usual for some of these storms,” said , a 91̽»¨associate professor of civil and environmental engineering. “Higher flows can result from heavier rainfall rates at higher elevations, rather than from snowmelt.”

In related work, Lundquist’s group has developed a and is in the foothills east of Seattle. The scientists aim to better understand how changes in climate and forestry practices might affect municipal water supplies and flood risks.

Wayand and another student in the group have developed a high school for Seattle teachers to explain rain-on-snow events and the physics behind why they occur. They hope to begin teaching the curriculum sometime next year.

The other collaborator on the work being presented in San Francisco is at the National Center for Atmospheric Research in Colorado.

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For more information, contact Wayand at 360-265-7720 and nicway@uw.edu or Lundquist at 206-685-7594 and jdlund@uw.edu.

Instead, the hardy Norsemen and early inhabitants of Russia and Canada have microorganisms called cyanobacteria to mostly thank for abundant grasses that attracted game to hunt and then provided fodder once cattle were domesticated. The process is still underway in the region’s pristine floodplains.

The new findings are surprising because it’s long been assumed that nitrogen crucial to plant growth mainly arrived with floods of river water each spring, according to , a 91̽»¨ professor of and lead author of a in the Nov. 6, 2013 issue of the journal PLOS ONE.

Discovering that cyanobacteria in the floodplains were responsible for nitrogen fixation – that is taking it from the atmosphere and “fixing” it into a form plants can use – partially resolves the scientific debate of how humans harvested grasses there for hundreds of years without fertilizing, DeLuca said. It raises the question of whether farmers today might reduce fertilizer use by taking advantage of cyanobacteria that occur, not just in the floodplains studied, but in soils around the world, he said.

It also might lead to more accurate models of nitrogen in river systems because none of the prominent models consider nitrogen being fixed in floodplains, DeLuca said. Scientists model nitrogen loading of rivers, especially where industrial fertilizers and effluent from wastewater-treatment plants cause dead zones and other problems in the lower reaches and mouths of rivers.

Ten rivers and 71 flood plains were studied in northern Fennoscandia, a region that includes parts of Scandinavia and Finland. The rivers were chosen because their upper reaches are pristine, haven’t been dammed and are not subject to sources of human-caused nitrogen enrichment – much like river systems humans encountered there hundreds of years ago, as agriculture emerged in such “boreal” habitats. Boreal habitat – found at 60 degrees latitude and north all the way into the Arctic Circle, where it meets tundra habitat – is the second largest biome or habitat type on Earth.

In the northern regions of the boreal, the surrounding hillsides have thin, infertile soils and lack shrubs or herbs that can fix nitrogen. In these uplands, feather mosses create a microhabitat for cyanobacteria, which fix a modest amount of nitrogen that mostly stays on site in soils, trees and shrubs. Little of it reaches waterways. On the floodplains, high rates of nitrogen fixation occur in thick slimy black mats of cyanobacteria growing in seasonably submerged sediments and coating the exposed roots and stems of willows and sedges.

“We joke and call the floodplains the ‘mangroves of the North’ because there are almost impenetrable tangles of willow tree roots in places, like a micro version of the tropical and subtropical mangroves that are known to harbor highly active colonies of cyanobacteria,” DeLuca said.

“It turns out there’s a lot of nitrogen fixation going on in both,” he said. For example, the

scientists discovered that in spite of the dark, cold, snowy winters of Northern Sweden, the cyanobacteria there fix nitrogen at rates similar to those living the life in the toasty, sun-warmed Florida Everglades.

The amount of nitrogen provided by the cyanobacteria to unharvested willows and sedges is perhaps a quarter of what U.S. farmers in the Midwest apply in industrial fertilizers to grain crops and as little as a sixth of what they apply to corn.

Human-made fertilizers can be fuel-intensive to produce and use, for example, it takes the energy of about a gallon of diesel to produce 4 pounds of nitrogen fertilizer. In developing countries in particular, nitrogen fertilization rates are spiraling upward, driving up fossil-fuel consumption, DeLuca said. Meanwhile, cyanobacteria naturally occurring in farm soils aren’t fixing nitrogen at all in the presence of all that fertilizer, they just don’t expend the energy when nitrogen is so readily available, he said.

“Although modest in comparison to modern fertilization, the observation that cyanobacteria could drive the productivity of these boreal floodplain systems so effectively for so long makes one question whether cyanobacteria could be used to maintain the productivity of agricultural systems, without large synthetic nitrogen fertilizer inputs,” he said.

Co-authors on the paper are with the Institute for Subarctic Landscape Research, Sweden, with Pontificia Universidad Católica, Chile, and with Stockholm University.

Funding for the work came from the European Regional Development Fund and the Bank of Sweden Tercentenary Foundation.

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For more information:
DeLuca, 206-685-1928, deluca@uw.edu