The tillage of so-called ephemeral gullies can significantly increase soil erosion on cropland fields, according to studies by the National Sedimentation Laboratory in Oxford, Miss., and the Natural Resources Conservation Service.
According to Ron Bingner, agricultural engineer with USDA’s Agricultural Research Service, and Carlos Alonso, hydraulic engineer, both with the National Sedimentation Laboratory, ephemeral gullies occur in agricultural fields when concentrated water flows erode cropland soils and carve out small drainage ditches, which then transport field runoff laden with eroded sediments into nearby streams. These gullies may lead to soil losses that exceed soil losses from sheet or rill erosion.
Bingner says that ephemeral gullies are typically filled in throughout the year by agricultural tillage practices. These tillage practices can remove or hide gullies, but the channels often reappear in the same location after subsequent rainstorms. These new channels easily erode the recently tilled fields and start another cycle of gully development and topsoil reduction that can expand across production fields.
The gullies can form on almost any field, according to Bingner. “Ephemeral gullies are normally easily visible on steeper fields. On flatland areas they are usually fairly wide and shallow, in places where water will tend to concentrate, form its own channel and erode the channel.
“Even these wide flat areas are a big source of erosion. We’ve seen some areas in Ohio where over 50 percent of the erosion can occur from flatland areas.”
How wide and deep ephemeral gullies become depends on rainfall, topography, soil type and tillage practice, noted Bingner. “Usually the gullies can be taken out by an implement, but sometimes they get so wide, a tractor can’t drive over them. At that point, they can become permanent gullies that the producer has to work around or bring in heavy equipment to remove.”
Bingner says gullies can form anytime there is a disturbance of the soil followed by rainfall or irrigation that produces significant runoff. “Once the headcut starts forming in the field, then any subsequent precipitation can move the headcut upstream.”
A headcut is a place where there are several inches of vertical discontinuity in the soil, which creates a small waterfall during rainfall. Head cuts generally move upstream.
“An ephemeral gully is a very specific kind of gully,” said NRCS agricultural engineer Fred Theurer, who participated in the study. “It’s caused by tillage in cropland because the gully is only as deep as the tillage layer. There are other kinds of gullies such as the classical gully caused by highly erodible soil.”
While ephemeral gullies have always occurred, it wasn’t until the last 20 years that scientists gave them a name and started studying their impact.
Recently, Alonso and Bingner teamed up with University at Buffalo scientists Lee Gordon and Sean Bennett and Theurer to evaluate the effects of ephemeral gullies on erosion.
The team developed a model to evaluate how tillage practices can affect the formation and evolution of ephemeral gullies and subsequent soil erosion rates. They used historical precipitation data, on-site field observations, and recently developed watershed modeling technology to simulate the effect of tillage practices on long-term ephemeral gully growth and evolution.
During a five-month growing season, tillage activities were simulated using two alternatives: once-a-year conventional tillage and no-till management practice. The collaborators applied the model to replicate a 10-year production span.
Their findings suggest that on average, tillage in areas prone to ephemeral gully erosion can produce significantly higher soil erosion rates compared to those same regions under no-till management practices. Simulated cumulative ephemeral gully soil erosion rates for the tilled fields were anywhere from 240 percent to 460 percent higher than soil erosion rates from the no-tilled fields.
The negative effects of tillage simulated in these watershed models reinforce the advantages of using soil conservation technologies such as no-till planting and other reduced tillage management practices, the researchers say.
The latter includes grass waterways, strip cropping or barriers such as stiff grass hedges. The most predominant approach to dealing with ephemeral gullies are the thick root systems of a grass waterway, according to Bingner. “If you’re seeing this type of gully formation in your fields, go to your local district conservation office. They can help you find seed for a grass waterway and firm the area up.”
Theurer added that more research is needed because no-till in some crops doesn’t always reduce the formation of ephemeral gullies. For example, in no-till soybeans, gullies will form in between the soybean plants. “I’m speculating here but soybeans do not have a spread out root mass. It takes a root mass to prevent the erosion. There is a lot we don’t know about what is causing them.”
Another way to prevent ephemeral gullies are diversions. Noted Theurer, “What causes the ephemeral gullies are concentrated flow of water. If you can route the water around the field, it’s not going to be able to concentrate in areas prone to ephemeral gullies.”
Downstream impacts from sediment leaving the field due to ephemeral gullies “could lead to some water quality problems,” according to Bingner.
e-mail: [email protected]