This spring, a muddy lake on the Mississippi River sparked a flurry of controversy about agricultural drainage. Lake Pepin, on the Minnesota-Wisconsin border, is filling up with silt. Most of the sediment is carried into the lake by the Minnesota River, a turbid prairie river that meanders through some of the most productive – and heavily tiled – cropland in the world.

Who is to blame for the sediment clogging Lake Pepin and other waterways that flow through farm country? It seems that lots of fingers point at farmers and their drainage practices.

Midwest soils are fertile but often have poor natural drainage. The Corn Belt’s network of surface-drainage ditches and subsurface tile helps farmers meet the world’s growing demand for food.

Artificial drainage also reduces soil erosion, promotes timely field operations, reduces soil compaction and allows farmers to use less tillage, says Minnesota farmer Bill Gordon, vice-president of the Minnesota Soybean Growers Association.

But artificial drainage also carries nutrients, sediment and other pollutants along with drainage water into streams and rivers, contributingto poor water quality.

“People have very high expectations for water quality, and they are paying more attention to this issue,” says Warren Formo, director of the Minnesota Agricultural Water Resources Coalition, which works on behalf of farmers. “Questions about the effects of drainage on water quality have really burst onto the scene in the last few years.”

Conservation practices have slashed the amount of soil moving from farm fields into rivers and streams, but “every farm can find one or two things to do better. I talk to farmers about their responsibilities to manage and maintain their drainage systems. To non-farmers, I talk about the conservation benefits of good drainage management,” Formo says.

The Corn & Soybean Digest asked Midwest experts to answer some common questions about farm-field erosion, clarify some misconceptions about drainage and suggest some practical remedies that can immediately cut sediment losses and improve drainage water quality.

Q & A: Understanding farm-field erosion and sediment loss

What is the source of sediment in streams?

It can come from upland fields, from the stream channel itself or from stream banks and bluffs, says Greg McIsaac,a retired University of Illinois associate professor of natural resources and a soil-erosion expert with Decatur-based Agricultural Watershed Institute, a conservation group.

“Stream-bank and -bed erosion are natural processes,” he says, but they can be accelerated by land-use changes, such as row-crop production and urbanization, that increase runoff.

“More intense rainfalls also contribute to more erosion in fields and stream banks.”

Why is sediment in surface waters harmful?

High sediment loadsimpair water clarity, disrupt aquatic plant growth and destroy fish and wildlife habitat, McIsaac says. Sediment deposition also reduces the capacity of stream channels to convey water, contributing to more frequent flooding. Phosphorus (P) and other pollutants can move along with soil, too, further degrading water quality.

What factors influence field erosion?

Field erosion is determined by the slope of the land, the amount of vegetation or residue covering the soil and rainfall intensity, McIsaac says. Soil moisture content, hydraulic conductivity and subsurface drainage also affect erosion.

How well are farmers controlling soil erosion from their fields today?

“Erosion rates have come down dramatically” since the 1980s, says Gary Sands, a University of Minnesota Extension biosystems engineer.

The USDA’s 2007 National Resources Inventory found that wind and water erosion on cropland decreased 43% between 1982 and 2007. Sheet and rill erosion dropped from 1.68 billion tons/year to 960 million tons/year.

That’s confirmed by a June 2010 report from the Natural Resources Conservation Service (NRCS). The Cropland Effects Assessment Project in the Upper Mississippi River Basin, or CEAP, estimates that conservation practices have reduced surface-water flow from cultivated farm fields by 16%, and cut sediment loss by 69%.

Reducing soil erosion and sediment loss has made a significant difference in water quality, says Don Baloun, the NRCS Minnesota state conservationist. Conservation practices have reduced in-stream sediment loads by 37%, P loads by 40% and atrazine loads by 51%, CEAP models show at the outlet of the Upper Mississippi River Basin at Grafton, IL.

 One of the main reasons for these improvements is the CRP, McIsaac notes. “A lot of those contracts are expiring, and if crop prices stay high, there will be an incentive for farmers to plow up grasslands, which will likely contribute to more erosion.”

How does tile drainage affect soil erosion?

Tile installation “must be paired with careful nutrient-management practices for rate, form, timing and application method,” says the NRCS’s Don Baloun.

According to the NRCS CEAP report, two-thirds of Upper-Mississippi cropland requires better nutrient-management practices to reduce N or P loss.

Tiling greatly reduces surface runoff and soil erosion from fields, Sands says.

“A large percentage of Midwest soils have poor natural drainage; they’re more prone to surface runoff. Subsurface drainage allows a greater portion of water to infiltrate the soil.” Tiling also increases crop yields, allows timely field operations, and makes it more practical to reduce tillage, retaining more protective residue on the soil, he says.

But, tiling also increases the risk of N leaching, which the NRCS calls “the most critical conservation concern.” Dried-out soils are more susceptible to wind erosion than wet soils, too, says Baloun. And many wet soils being tiled to improve yields don’t have water-erosion problems, he adds.

It’s easy to demonize tile drainage, Sands says. “We’ve traded some environmental issues for others.”

Does tile drainage increase stream-flow volumes?

Critics charge that tiling increases stream flows, causing fragile riverbanks to drop more sediment, Baloun says.

However, the effects of tile drainage on stream hydrology are far from clear, Sands says. “There are so many things going on that it’s difficult to understand and tease apart all the individual effects. It’s very hard to say what annual row cropping or drainage do to water balance, hydrology and stream flow.”

Matt Helmers, Extension agricultural engineer at Iowa State University (ISU), underscores that uncertainty. “We have seen changes in annual stream flow, but there are many reasons for that. It’s hard to point to one thing.”

It also depends on what else is going on in the field, Greg McIsaac adds. “Are the tiles draining a pond or wetland? Are you tiling an area that would otherwise produce a lot of surface runoff? Are you draining terraces, which hold soil in the field? The impact of tile on stream flow is not straightforward.”

That’s why farmers and society “need more research on how tile systems of different types affect hydrology,” says Warren Formo, director of the Minnesota Agricultural Water Resources Coalition.

How to reduce sediment loss

Farmers have made real gains in reducing cropland erosion and sediment loss, says Iowa State University Extension Agricultural Engineer Matt Helmers. “But there’s a lot more we can do.”

Start with residue and tillage management. “Residue is the single most important factor influencing soil loss,” says Jodi DeJong-Hughes, a University of Minnesota Extension tillage expert. Residue protects the soil from raindrop impact, slows down water velocity over the field, decreases soil detachment and soil sealing and increases water infiltration.

“All tillage decreases residue cover,” Helmers says. “Think about how you can increase residue and reduce tillage.” Iowa Learning Farms Project modeling shows that reduced-tillage systems cut soil loss by up to 90% in erodible fields, he adds.

Stop disking ephemeral gullies. “We have to stop doing that!” says Minnesota NRCS State Conservationist Don Baloun.

Rainstorms carve out these small ditches in fields, which then transport field runoff laden with sediment. Tillage can temporarily hide them, but they usually reappear and can lead to significant sediment loss, Baloun says.

In areas prone to ephemeral gullies, he says, farmers must reduce tillage to keep soil covered and establish grass waterways or strips to slow water flow.

Install a buffer. These grassy strips between fields and water bodies filter out sediments and nutrients from surface runoff and shallow groundwater. A correctly designed and placed buffer can trap 90% of sediment from the drainage area, Helmers says. “In the future we may consider designing buffers that vary in width, according to water-flow patterns at the site.”

Protect tile outlets. Water coming out of drainage pipes has energy and can contribute to ditch or stream-bank erosion, says Gary Sands, a University of Minnesota Extension biosystems engineer. There are many ways to protect tile outlets. “Often, it’s just a simple matter of placing some rocks or ‘rip-rap’ at the outlet.”

Fix eroded side inlets. “This is another area that needs more attention,” Sands says. Eroded ditch inlets widen and deepen over time. There are many good, inexpensive ways to stabilize them, he says.

Improve open surface tile intakes. Runoff entering old-style open tile intakes carries sediment, phosphorus, and other pollutants through the drainage system and into surface waters, Sands says.

Alternative intakes such as rock or blind intakes and slotted intake risers, like the familiar orange Hickenbottom risers, are good options, Sands says. These practices allow more sediment to settle out in the field, before going down the drain. Farmers don’t much like working around intake structures, he acknowledges, but they are “a tremendous improvement over open intakes.”

Add a perennial to your rotation. “These are very effective” at controlling erosion, Helmers says.

The right thing to do

Bill Gordon raises corn and soybeans on 2,000 acres near Worthington, in southwestern Minnesota. Tiling is a necessity on his heavy, poorly drained soils. “Without it, we’d have a lot of ponding and saturated soils.” His grandfather began installing the farm’s drainage system in the 1930s, and Gordon and his father Galen continued the improvements.

Ten years ago, the Gordons upgraded a portion of their tile system that empties into a chain of three lakes. The lakes supply water for the city of Worthington. As part of the project, they installed a 26-acre de-silting basin and dam to intercept drainage water before it reaches the chain of lakes.

“Water was running off the land too fast after big rains, and took sediment with it, which ended up in Lake Okabena,” Bill Gordon says.

The de-silting basin is on land often too wet to produce good crops, Gordon says. Three water-storage ponds and surrounding grass buffers collect and filter both tile water and surface runoff before it reaches the lakes.

The Gordons donated the land for the project, and the state paid for the soil work. Meanwhile, the upland tiling improvements done in conjunction with the sediment basins greatly boosted yields, Gordon adds, more than offsetting the loss of cropland.

The Gordons and their neighbors also installed 10 miles of 60-ft.-wide grass filter strips along a creek that empties into the chain of lakes.

Drainage innovations help protect water quality

There is a growing emphasis on managing agricultural drainage water to improve both yields and water quality, says Gary Sands, a University of Minnesota Extension biosystems engineer. New techniques can help cut pollutants in drainage water without lowering drainage efficiency. Some examples:

•Two-stage ditches have gradual side slopes that mimic a natural flood plain, keeping sediment from being carried downstream.

•Shallow drainage involves placing tile lines about 3 ft. beneath the soil surface, rather than the more conventional four feet. Shallow drainage systems cut drainage water volume by up to about 40%, according to Iowa State University (ISU) research.

•Controlled-drainage water management uses an outlet structure with stacked risers or stop logs, installed in tile mains to raise and lower the field’s water table. Water can be conserved in the soil during the winter and the growing season, and then released before planting and harvesting. In the summer, the water table can be adjusted up or down according to rainfall and crop needs. The practice can reduce drainage water flow 25-35%, according to ISU research.