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Warmer winters, rain threaten snowpack

October 10, 2022

Increased runoff poses risks to water quality.

A new study published in the journal Environmental Research Letters warns that large swaths of the continental U.S. are experiencing “rain-on-snow” events in winter. When rain falls on snowpack, it threatens to cause snowmelt and runoff.

As rainfall melts snow and thaws frozen soil, nitrogen, phosphorus and other runoff carries nutrients into lakes and rivers. The phenomenon is common in spring and summer seasons. However, as winter temperatures rise, the occurrence may become more common in cold-climate places.

Later in the year, pollutant loads, air temperature and a lack of wind can cause algae blooms, sometimes toxic, to flourish.

According to lead author Erin Seybold of the Kansas Geological Survey, about 43% of the contiguous U.S. contains nutrient pools in areas at risk of being affected by rain-on-snow events. Most are located in the Northeast, the north-central region and the mountainous West.

The study identifies places of concern around the U.S. It doesn’t describe the frequency or exact consequences.

Researchers assembled maps of nitrogen and phosphorus pools across the U.S. from the U.S. Geological Survey (USGS) and the Nutrient Use Geographic Information System, a database of crops, livestock and fertilizer use across the country. Places at risk of large rain-on-snow events, compiled using data from the National Oceanic and Atmospheric Administration (NOAA) were overlaid the assembled maps.

“We’re used to thinking about winter as a time when nothing happens. Everything’s frozen. Nutrients are locked up in snow and not moving,” said senior study author and biogeochemist Carol Adair of the University of Vermont. “And this is a shift, it’s a change.”

Seybold commented that large-scale monitoring efforts across the country have conventionally been designed for spring and summer months. Data collection project plans have yet to be adapted for winter months and adverse conditions for field work.