Winter runoff concerns
Mitigation measures now need to keep in mind that nutrients move more in winter.
January 2, 2024 by James Careless
Anyone applying manure to a field knows that nutrient runoff is a concern, which is why many growers and applicators look at mitigation measures. However, for years it has been assumed that during the winter months, there is less opportunity for runoff due to the different types of weather events that occur during the cold months. However, rising winter temperatures in many areas could mean an increase in rain-on-snow runoff and melting snowpacks. In turn, this increased winter runoff will flush substantially higher levels of nutrients into groundwater, rivers, lakes and oceans on a large scale.
That’s the core argument in a recent publication entitled, “Winter runoff events pose an unquantified continental-scale risk of high wintertime nutrient export.” For farm fields applying manure, this could mean more nutrient runoff than landowners have traditionally come to expect.
Given the many forces that are increasing winter runoff, “We suggest that the assumption of low wintertime discharge and nutrient transport in historically snow-covered regions no longer holds,” the document reads. “Critically, however, we lack sufficient data to accurately measure and predict these episodic and potentially large wintertime nutrient export events at regional to continental scales.”
“We found that 40 percent of the U.S. has nutrient rich soils and large rain-on-snow events in the winter, so we’ve got at least 40 percent of the U.S. creating winter pollution when it rains and snow melts, but we’re not keeping track of how much, where it’s going, or what it’s impact on water quality is,” says Carol Adair, director, Aiken Forestry Science Lab and associate professor at the University of Vermont and one of the authors of the publication. “We also found in our analyses that winter discharge on the upper Mississippi and Missouri rivers has nearly doubled since 1925.”
To investigate their thesis on the impact of increased winter runoff, the writers looked into existing geospatial datasets covering rain-on-snow frequency overlain on nitrogen and phosphorus inventories to identify areas of the contiguous U.S. where water quality could be threatened by this change. To illustrate the potential impacts of these events, they examined flow and turbidity data from a large regional rain-on-snow event in the Mississippi River Basin.
“We estimated historical daily rain-on-snow frequency for the U.S. using output from the snow data assimilation model system (SNODAS) operated by the National Operational Hydrologic Remote Sensing Center, part of the National Weather Service and the National Oceanic and Atmospheric Administration,” the document says. A large rain-on-snow event is defined as at least 10 mm d1 of rain falling on a snowpack of at least 10 mm snow water equivalent. Next, “We calculated the average daily rain-on-snow frequency for 16 hydrologic years of SNODAS record (October 2003 to September 2019).”
Based on their assessment, the team found that “rain-on-snow, a major flood-generating mechanism for large areas of the globe … affects 53 percent of the contiguous U.S. and puts 50 percent of U.S. nitrogen and phosphorus pools (43 percent of the contiguous U.S.) at risk of export to groundwater and surface water. Further, the 2019 rain-on-snow event in the Mississippi River Basin demonstrates that these events could have large, cascading impacts on winter nutrient transport.”
Next, to formulate a map of total nitrogen pools across the contiguous U.S., the team combined maps of fertilizer and manure phosphorus application and topsoil phosphorus to create a map of total phosphorus pools across the contiguous U.S. “We estimated topsoil phosphorus (0-5 cm; kg P ha-1) using U.S. Geological Survey (USGS) total soil phosphorus concentration (mg P kg soil-1) and soil bulk density data.” Adair adds, “We did the same thing for nitrogen, but also included atmospheric nitrogen deposition.”
Factors at play
The driving force behind increased winter runoff is warmer winters, but the dynamics are more complicated. The cold and snow that historically reduced wintertime runoff and nutrient transport are now punctuated by runoff- and flood-producing snowmelt, rainfall and rain-on-snow events. “It’s especially complex as snow thins or disappears – so soils freeze and thaw, making it easier for them to be eroded by winter rains,” Adair notes.
While previously runoff from major midwinter “flushing” events were historically infrequent, the research indicates that these events are now more frequent.
Research found that humans add an estimated 22 to 26 Tg (teragrams) of phosphorus as fertilizer, 183 Tg of nitrogen as fertilizer and from nitrogen fixation by legumes, and 25 to 33 Tg of nitrogen via atmospheric deposition from fossil fuel combustion.
It could be argued that increased winter runoff is simply hastening the water-borne movement of nutrients that historically have happened in spring. If so, then why is the difference in timing a cause for concern?
The answer, according to the authors, is that the nutrient load in spring runoff is mitigated by certain natural factors.
A spring nutrient flux coincides with and is often tempered by springtime plant growth, resulting in plant nutrient uptake. Besides the typical springtime nutrient uptake, there are also environmental, seasonal factors. “Our initial analysis of the upper Mississippi and Missouri found that events in the winter carried more sediment (and therefore likely more nitrogen and phosphorous) than similar sized events in the spring, summer or fall,” says Adair.
This pulication asserts that more than 40 percent of the contiguous U.S. is at risk of nutrient export from rain-on-snow transmissions, and that half of the nitrogen and phosphorus pools in the contiguous U.S. are in areas with historically large, relatively frequent rain-on-snow events. The research team says there is a need for “a conceptual framework for winter nutrient transport with testable hypotheses, to serve as a starting point for developing a mechanistic, predictive understanding of winter nutrient transport and its impacts on water quality.
But despite some unknowns, there are still some ways to adjust one’s nutrient management approach and keep the team’s findings in mind.
“We do hope that our maps can be used by [nutrient] managers to pinpoint locations for investigation and management,” says Adair.
There is good news: most of the measures known to mitigate runoff still work; one simply can’t assume there will be minimal winter runoff.
“We think that we already know many management practices that may help to mitigate winter nutrient loss,” says Adair. “For example, winter cover crops or buffer strips.” •