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Letting Nature ‘Do the Work’ of Agricultural Drainage | Ohio Sea Grant

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Letting Nature ‘Do the Work’ of Agricultural Drainage

12:00 pm, Sun June 23, 2024 – Agricultural drainage ditches that are “self-forming” could benefit water quality, new research shows

In northwest Ohio, drainage systems allow farmers to remove excess water from surface soil and ensure that crops can grow. Yet that increased flow of water can contribute to nutrient runoff downstream, fueling harmful algal blooms on Lake Erie.

Recently, researchers found that agricultural drainage ditches designed to mimic wetlands and let nature “do the work” can counteract nutrient runoff and actually provide water quality benefits.

a person holding a tape measure along a ditch

Researchers excavate a two-stage ditch floodplain to assess sediment and nutrient deposition.

Such “self-forming” ditch designs were developed by Dan Mecklenburg and other researchers at The Ohio State University. In their latest project, funded by the Ohio Department of Higher Education’s Harmful Algal Bloom Research Initiative (HABRI), the team studied how nutrients and sediments are retained within different drainage channels.

“How can we take our understanding of how natural stream systems function and incorporate that into the engineering design of ditches?” said Jon Witter, an associate professor in the Agricultural and Engineering Technologies Division at Ohio State Agricultural Technical Institute (ATI) within the College of Food, Agricultural, and Environmental Sciences. “We’re trying to understand how much it’s trapping, and how much might be released back into the environment.”

Agricultural drainage ditches are an important piece of infrastructure for farming, Witter said, and if they’re functioning properly, they simply transport water downstream to rivers and lakes. However, environmental problems can arise in these channels. Ditches can fail, with banks collapsing and introducing sediments and nutrients into the system, or those materials can build up in the system, which costs money to remove or “dip out.”

Since the early 2000s, researchers at Ohio State have worked to improve the engineering designs of ditches. While a conventional “trapezoidal” ditch is simply effective at moving water through a small width of space, a “two-stage” design can do the same while also stabilizing banks, reducing the need for maintenance, and protecting existing habitat. The two-stage ditch design, developed by Professor Andy Ward of Ohio State ATI, includes a deep channel with vegetated floodplain “benches” on either side.

diagram with three different ditches

A diagram showing the design differences between conventional, two-stage, and self-forming agricultural drainage ditches.

More recently, researchers looked at fluvial geomorphology — the processes that shape natural rivers — to envision how ditches could become more self-sustaining. By widening the area around the channel to create a floodplain, water will interact with vegetation and sediment to create its own stream system, Witter said.

“The idea is that over time, sediment will be supplied from the watershed, flowing water will rework that sediment, vegetation will come in and stabilize it, and then you have the benefit of trapping all those sediments and nutrients in place,” he said.

Essentially, nature will decide what becomes of the site, whether it mimics a wetland or fills in with herbaceous vegetation. Researchers found that the self-forming ditches have a much greater capacity to trap and store sediments than prior designs, Witter said.

To quantify this, the team worked to conduct a formal analysis of self-forming ditches at nine sites around Ohio. They measured repeated cross sections of the channels to see how fast sediments were building up and what nutrients were being stored.

containers sitting on dirt surrounded by vegetation

Researchers collected and installed resin cores, pictured here, on site to determine phosphorus and nitrogen mineralization rates.

From the analysis, researchers found that across all the sites, the ditches accumulated a little less than an inch per year of sediment. Based on their predictions, the systems should trap material for multiple decades in most cases, Witter said.

The self-forming sites trapped significant amounts of nutrients as well, the study found. Each site accumulated an average of 52 pounds of phosphorus per year — totaling almost 800 pounds of phosphorus over the past 10-15 years. Over decades as the sites mature, some of the projects are expected to trap more than 3,000 pounds of phosphorus.

“The vegetation that comes in is almost like a layer of armor,” Witter said. “It’s going to keep those sediments and a lot of those nutrients there long-term.”

Notably, the team found that retention rates of nutrients in the ditches were comparable to that of natural and constructed wetlands.

Another aspect of the project involved measuring rates of nitrogen and phosphorus mineralization, a long-term process where nutrients are converted to their inorganic forms and can eventually release into the environment. The team extracted soil cores from the sites and found that only very small quantities — .3 to 1 gram per square meter — are being released through mineralization each year.

“We found that we’re trapping a whole bunch more than we’re releasing, and that’s very consistent with findings in other published literature that’s out there,” Witter said. “The numbers are probably the worst case scenario, so we found that what appeared to be our worst case wasn’t that bad. The rate that you’re losing nutrients is so much slower that it’s like a bottleneck.”

Drone footage of a self-forming ditch.

From this work, Witter said he hopes to help farmers understand the benefits of using conservation ditch designs. While widening ditches costs money and removes farmland, trapping sediments and nutrients will benefit ecosystems and society as a whole. Recently, Ohio Governor Mike Dewine’s H2Ohio Initiative has incentivized building conservation ditches.

Those trapped materials could be also reused by farmers as a fertilizer alternative or soil amendment, Witter said. His team has tested applying sediment buildup from ditches to farm soil to improve crop yields. Early results are promising, he said, and he hopes to study the concept more in the future.

“The vegetation that comes in is almost like a layer of armor. It’s going to keep those sediments and a lot of those nutrients there long-term.”
Jon Witter

“So maybe this becomes more of a circular, self-sustaining sort of system where farmers get the drainage they need but society also benefits,” Witter said. “I think this will help farmers want to start to adopt this, utilizing it as a system that enhances their drainage.”

Moving forward, Witter hopes to expand the research to include more sites with agricultural drainage ditches. His colleagues, Ohio State research engineer Dan Mecklenburg and Jessica D’Ambrosio, Ohio Agriculture Director for The Nature Conservancy in Ohio, are also continuing to share resources from the research with engineers and agencies.

For more about Dr. Witter’s HABRI research, watch his Freshwater Science webinar or contact him at witter.7@osu.edu.

Ohio Sea Grant is supported by The Ohio State University College of Food, Agricultural, and Environmental Sciences (CFAES) School of Environment and Natural Resources, Ohio State University Extension, and NOAA Sea Grant, a network of 34 Sea Grant programs nation-wide dedicated to the protection and sustainable use of marine and Great Lakes resources. Stone Laboratory is Ohio State’s island campus on Lake Erie and is the research, education, and outreach facility of Ohio Sea Grant and part of CFAES School of Environment and Natural Resources.

ARTICLE TITLE: Letting Nature ‘Do the Work’ of Agricultural Drainage PUBLISHED: 12:00 pm, Sun June 23, 2024
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