Research on Blind Inlets Shows Promise to Reduce Phosphorus Loss in the Right Locations

December 3, 2020

People installing blind inlet in farm field with farm equipment in background
Members of the Southfork Watershed Alliance and researchers installing a blind inlet on Royle Duncan farm in Hardin County, Iowa. Image courtesy of the Southfork Watershed Alliance.

AMES, Iowa — Blind inlets are a conservation tool not yet well-known in Iowa, but they are gaining interest as a “simple, cost-effective practice farmers can use to reduce phosphorus that takes the least land out of production,” according to Jo Baumann, a board member for Iowa’s Southfork Watershed Alliance in Hardin County.

Also known as a “French drain,” blind inlets can be used to replace surface inlets commonly used in the tile system that drains cropland. Surface inlets, signaled by orange perforated tubes sticking up from fields, drain low spots where water accumulates during heavy rainfall and in wet periods. Water flowing through surface inlets bypasses the soil’s filtering capacity and can rapidly transmit sediment, nutrients and pesticides to drainage networks that connect to streams.

Blind inlets re-engineer surface inlets with a combination of buried perforated pipes covered by filtering gravel and sand, overlain with a permeable landscape fabric barrier close to the field’s surface. When sited and working properly, blind inlets can reduce the amount of sediment and phosphorus entering water sources. Their benefits to farmers include that they take no cropland out of production and allow for unimpeded passage of farm equipment.

In 2017, the practice was added to the Iowa Nutrient Reduction Strategy (INRS) list of practices that can be used to reduce phosphorus. Adding blind inlets to the state’s menu of approved practices in the INRS was based in part on early field research in Iowa conducted by the USDA Agricultural Research Service and Baumann’s Southfork Watershed Alliance.

The alliance received a grant from the Iowa Department of Agriculture and Land Stewardship and the State Soil Conservation Committee to demonstrate three in-field conservation practices with potential to reduce phosphorus loss: blind inlets, filter socks around surface inlets and grass buffers. The Southfork group partnered with water quality researchers from the USDA Agricultural Research Service to study the practices’ water quality impacts, with support from the Leopold Center for Sustainable Agriculture.

A combination of the practices was installed on nine locations on five cooperating farms, along with controls that had no practices. Sites were monitored from 2013-2016 to assess water quality.

Blind inlets

The data from the Southfork project confirmed findings from research in Indiana and Minnesota showing blind inlets could significantly reduce phosphorus and sediment entering tile drains. In the Southfork, total dissolved phosphorus measured in the blind inlets was reduced by 50% or more compared to levels measured in drainage water before it entered the inlets.

However, the blind inlets’ effectiveness varied widely, based largely on crop management in the surrounding fields. They worked best where no- or low-till practices limited soil loss that otherwise soon clogged the sand and gravel filtration system.

Placement also mattered. Farmer John Gilbert, one of the Southfork board member said that the inlets worked well, except where they were installed in low spots that drained a large area. In those cases, heavy rainfall overwhelmed the inlets, causing water to back up. One of the blind inlets was installed on Royle Duncan’s farm. It is still functioning several years later, and he hopes, still improving water leaving the farm, though the water quality is no longer being monitored.

Where they work -- in addition to water quality benefits -- blind inlets are low-cost compared to most conservation practices (estimates range from $750 to $1,500, depending on the inlet’s size and the distance to transport the needed sand and gravel). They are predicted to have a lifespan of around 10 years, based on the studies in Indiana and Minnesota. That research found blind inlets maintained or slightly increased crop yields compared to conventional open surface inlets, with no significant differences in ponding or drowned-out crops.

Filter socks

In the Southfork research, the filter socks (30-foot geotextile “E-Tubes” filled with woodchips) around surface inlets were found to be even more effective than blind inlets at reducing runoff of total dissolved phosphorus. Their filter socks (made locally by Soil-Tek in Grimes, Iowa) included a dose of alum, or aluminum sulfate, known to retain dissolved phosphorus. The filter socks are also cheap (about $150-300) and easy to install. However, they need to be replaced every year or two to remain effective.   

Grass buffers

The Southfork group had mixed results with the grass buffers surrounding the inlets. Duncan had a good experience with a buffer on his farm and still maintains it. Overall, though, the researchers concluded that the buffers were less effective than expected in improving water quality since it was difficult to establish a good stand of grass in the depressions where ponding often occurs.  

Advantages and disadvantages

“None of the practices were a miracle cure, but the blind inlets and other measures could be practical ways to improve water quality on many farms,” said Baumann, speaking from her home with a view of the South Fork of the Iowa River. “We don’t want this work that the public has invested in to just sit there. We want to see these simple, cost-effective practices used where they make sense.”

In a final grant report on the project, the USDA ARS researchers concluded that there were advantages and disadvantages to each inlet protection strategy and no one approach performed well in all situations. “More research is needed to support these early assessments,” said Jeff Cook, a USDA technician involved in the study.    

The Iowa Nutrient Research Center is supporting new research that could help boost the potential for blind inlets in Iowa. The project, Reducing phosphorus export through farmed pothole surface inlets with P filters and blind inlets, is getting underway this fall, led by Amy Kaleita, a professor in the Department of Agricultural and Biosystems Engineering at Iowa State University.

“We don’t usually think of surface inlets as a major source of phosphorus, but that seems to the be case,” Kaleita said. “That’s why we are trying to consider what interventions we can do to mitigate their impacts.”


About the Southfork Watershed Alliance

The focus of the Southfork Watershed Alliance is to improve and enhance the current environmental and water quality status within the watershed of the South Fork of the Iowa River. Together with Tipton and Beaver Creek, the South Fork drains approximately 200,000 acres in Hardin, Hamilton, Franklin and Wright counties. 

This article is reprinted courtesy of Wallace's

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