By installing small check dams within drainage ditches beside fields, research shows that growers can stop close to 30-50% of the nitrate nitrogen (N) from flowing downstream, according to Mississippi State University (MSU) studies.
No matter how much underground tiling, no-tilling or other erosion-prevention practices are used, too much rainfall can cause crop nutrients to flow off fields and into ditches and waterways. But weirs help slow the flow, says nutrient-runoff-prevention research by Robbie Kröger, MSU assistant professor of aquatic sciences.
A small system of check dams, or weirs, about the width of a typical drainage ditch, can provide numerous locations along the drainage ditch for nutrients to be absorbed and transformed, he says. The concrete and rock, or “rip-rap” dams, occupy 10-25% of ditch bank thus still allow for drainage when heavy rain occurs. Or, if there is excessive late-winter snowfall across the northern Corn Belt, weirs could help control major runoff before the rainy season even arrives.
“Drainage ditches filter and alter nutrients before water reaches rivers,” Kröger says.
Kröger says you can easily construct these low-tech, low-maintenance weirs. First, you situate concrete or rocks to construct the dams across a drainage ditch. A slotted riser is added to retain water.
The riser should feature about a 2-ft.-wide opening to slow down water flow and nutrient runoff. “The weir height needs to be gauged off the slope of the ditch to avoid flooding agricultural landscapes upstream,” Kröger says.
Several weirs can be stair-stepped throughout the drainage ditch to provide maximum nutrient retention. Also, water collects on the downstream side of the weir to form a miniature wetland.
Kröger says a weir program can help farmers limit nutrient runoff, noting that weirs should be accompanied by “a good lush stand of vegetation to absorb and retain nutrients.”
At the MSU research location outside Yazoo City, MS, Kröger and Associate Researcher Sam Pierce have looked at nitrate concentrations coming into the ditch and the outflow of concentrations from the ditch.
Last year’s massive Mississippi and Yazoo River flooding altered their research. But data in 2009-2010 show a big reduction on concentration outflow beyond two weirs in the study. “The numbers are great,” says Kröger. “The first weir is reducing flow of nitrate N concentrations by 38%. The second weir is reducing nitrate N by 68%. If you look at the inflow and all of the outflow, you actually see an 53% reduction in nitrate N concentrations.”
Pierce adds, “Maintenance is virtually non-existent. He uses sensors situated on PVC pipe test stations to gather flow data. “These weirs can help control water flowing from a field and a wetland.”
NRCS’ Environmental Quality Incentives Program (EQIP) can likely provide financial and technical assistance to growers who install the low-grade weir systems, says Kröger, who is hoping to expand EQIP coverage to more elaborate weir systems in which ditches and other water-collection areas are reconfigured to environmentally enhance them.
He notes that weirs can be manipulated for use as farm tail-water recovery systems. “We need to understand how all these structures interact in the field, at the watershed or farm scale to assess their overall impact on downstream water quality,” he says.