Winter rains increasing soil erosion
Extreme weather events, farming practices wear away topsoil
“When you have frozen soils, you get runoff immediately.”
Dennis Todey
Director of the U.S. Department of Agriculture’s Midwest Climate Hub in Ames, Iowa
Many southeastern Wisconsin residents woke to a downpour early last week.
More than two inches fell before 1 p.m. in Milwaukee, which broke a record last set in 1913 for rainfall in a single February day. The city's sewerage district allowed overflows of untreated wastewater into local waterways to prevent sewage backup into homes and businesses. Some roads flooded.
Heavy rainfall on top of bare, frozen ground can cause another problem, especially on farmland: It erodes soil, causing dirt and other pollutants to run off into streams and rivers. It's a particular concern in the Midwest, where researchers estimate topsoil is eroding 100 times faster than it is forming.
Here's what to know about how it happens.
How does rain cause soil erosion?
When soil warms up and thaws, rain soaks in until the ground becomes saturated. Only at that point, does it run over the ground, carrying loose soil particles with it as it heads to the nearest body of water.
When the ground is cold and hard, however, no rain can seep into it.
“When you have frozen soils, you get runoff immediately,” said Dennis Todey, director of the U.S. Department of Agriculture's Midwest Climate Hub in Ames, Iowa.
Farm fields also are more likely to be bare in the wintertime. When a few inches of rain falls during growing season, the crops can help block some of the raindrops from falling directly onto the soil, and their roots help hold soil in place.
Todey stressed that it's not just torrential rain that can be damaging — even a half-inch of rain in the winter can cause problems.
Is climate change making it worse?
Anecdotally, Todey said, winter rain and soil erosion seems to be happening more frequently.
A 2022 study from the University of Vermont found that the upper Midwest is particularly at risk of increased wa
ter pollution from runoff because warming winters mean more heavy rainfall on top of snow. The study looked specifically at flooding that occurred on the Mississippi River in winter and spring of 2019, finding that “rain-on-snow” events released massive amount of nutrients and sediment into the river.
Climatologists and farmers alike are noticing bigger rainfalls occurring during times of the year when fields are bare. Wisconsin is expected to grow warmer and wetter through the mid-21st century, and experience more extreme precipitation events, according to the Wisconsin Initiative on Climate Change Impacts.
Why is soil erosion a concern in the Midwest?
The Midwest is known for rich, fertile topsoil good for growing crops, making agriculture a star industry in the region.
But that surface layer is disappearing, worn away both by extreme weather events and farming practices that routinely disturb it. Tilling, for example, which turns the soil to prepare it for seeding, can increase the likelihood of soil erosion and runoff.
Researchers at the University of Massachusetts Amherst estimated that the rate of soil erosion in the Midwest is between 10 and 1,000 times greater than it was prior to the rise of modern agricultural practices, and that the region has lost more than 57 billion metric tons of topsoil since farmers began tilling a century and a half ago.
If it continues, all types of agriculture will suffer a drop in productivity, which will have an impact on local economies that depend on farming and on the food we see at our dinner table.
How can erosion be reduced?
Because soil erosion tends to be a bigger problem for farm fields, Todey said it’s largely up to landowners to manage their land as best they can, generally striving not to leave fields bare when rain and wind could move topsoil around.
One way to do that is with cover crops, which are planted for the specific purpose of covering the soil between seasons when other crops, like corn, are grown for harvest. These crops — like alfalfa, rye, red clover and mustard — shield the ground from the elements and also root down into the soil, helping it stick together.
Another practice is no-till farming, in which farmers plant seeds directly into undisturbed soil instead of churning it before planting, locking in nutrients and strengthening the soil. A 2021 study from the University of Illinois UrbanaChampaign found that switching to notill in vulnerable areas can reduce loss by more than 70%.
More farmers are employing practices like these. In the latest U.S. Census of Agriculture, released in 2017, 37% of field acres used no-till, and the use of cover crops on Midwest farms increased from about 2% in 2011 to 7% in 2021.