Saturated Buffer
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1. Overview of saturated buffers
A saturated buffer is a conservation drainage practice that helps reduce nitrate loss from subsurface-drained farmland. In this system, some of the water from the tile drain is rerouted into perforated pipes running along the ditch bank, buried under a vegetated buffer strip. Water exits the perforated pipes and slowly moves through the soil of the buffer strip, where nitrate is removed by a natural process called denitrification before reaching the ditch (Figure 1). Denitrification is the microbial conversion of nitrate into a harmless nitrogen gas.
A saturated buffer system works by combining three processes (Figure 1):
(1) Controlled drainage, which manages how much water leaves the field through the tile drains.
(2) Subsurface buffer flow, which treats the water as it slowly moves through the soil.
(3) Buffer strip, which helps reduce and treat surface runoff and uptake nutrients from the subsurface buffer flow.
This bulletin shares results from a study (Ghane et al. 2025) comparing a field with a saturated buffer to a similar field with free drainage. The side-by-side paired-field setup gave a clearer picture of saturated buffer performance and helped isolate the effects of subsurface buffer flow and controlled drainage.
2. Performance of saturated buffers and their slope suitability
Based on 2023–2024 results, the saturated buffer system reduced annual drainage discharge and nitrate load by 49% and 66%, respectively, in a field with a 1.1% slope (Figure 2).
While some existing guidelines for suitability of the saturated buffer have recommended field slopes of 2-8% for saturated buffer suitability, our research showed that even flatter fields can be highly effective. These results, measured directly against a control field, demonstrate that saturated buffers can provide measurable water-quality benefits on fields with slopes as low as 1%, supporting their broader use.
3. Controlled drainage vs. subsurface buffer flow
3.1. Controlled drainage dominated nitrate load removal
Based on 2023–2024 results, the controlled drainage component reduced 53% of the total nitrate load. This was made possible by managing the weirs in the upstream chamber of the control structure, which regulates how much water leaves the field.
The subsurface buffer flow component reduced 13% of the total nitrate load (Figure 3), achieved by directing a portion of the drainage water into the buffer.
These findings confirm that controlled drainage was the dominant nitrate removal mechanism and highlight the often-underappreciated role of water level management in the upstream chamber of the structure in saturated buffer systems.
3.2. What happens with managing weirs in the upstream chamber?
Weir management in the upstream chamber raises the water level in the perforated collector drain and laterals near the edge of the field. This forces water out of the pipe perforations and increases lateral seepage through the soil. In this case, the perforated pipes function as additional distribution pipes and expand the treatment area.
3.3. What if we have a two-chamber structure?
Three-chamber structures are commonly used for research monitoring, while two-chamber structures are typically used in practical applications. A two-chamber control structure can function similarly to a three-chamber system. In such cases, the upper chamber continues to operate as a controlled drainage, contributing to flow regulation and nitrate reduction.
4. Cost-effectiveness of saturated buffers
Based on 2023–2024 results, controlled drainage alone was more cost-effective in removing nitrate at $24/lb-N, compared to $36/lb-N for the full saturated buffer system.
Because the subsurface buffer flow component provided only modest nitrate reduction, its added cost reduced the overall cost-effectiveness of the system. In this analysis, we assumed the field is already equipped with a buffer strip that requires no additional cost. Further research is recommended at other sites.
5. Effect of weir management on nitrate load removal
Weir depth significantly impacts nitrate load removal. High-level weir settings in the middle chamber were far more effective than low-level settings (Figure 4). When the outlet was set at 1 ft depth below ground surface, nitrate-N removal averaged 0.068 lb/ac per day, 115% more than low-level weir settings. The high-level setting maintained a greater hydraulic gradient, raised the water level, and promoted greater seepage into the buffer. This enhanced nitrate load reduction.
6. Backflow of water from buffer into the control structure
A negative grade in the distribution pipe and deep weir setting caused the pipe to pull water back into the structure instead of distributing it into the buffer. Raising the weir stopped the backflow and improved buffer performance.
Design Tip 1
If distribution pipes are installed with a negative grade for self-cleaning (that is, flushing of sediments), use a high weir setting to prevent backflow into the structure (Figure 5). You can also install the pipes at zero grade to avoid backflow altogether. A zero-grade installation (on contour) also maximizes distribution pipe length.
Design Tip 2
Avoid lowering the distribution pipe to the bottom of the control structure (Figure 6). It is better to connect it near the pipe’s original depth. Dropping the pipe to reach the bottom causes backflow, pulling water into the structure when the outlet is lowered for trafficability. If a drop is unavoidable, keep the weir high to reduce backflow.
7. Apparent vs. actual performance
To compare apparent and actual performance, we first applied the method from earlier studies, which calculated performance based on load reduction relative to the water flowing over the upstream chamber (Figure 6). We then calculated actual performance using a free drainage control field.
Our study found:
• Actual nitrate-N reduction: 10.9 lb/ac per year
• Apparent nitrate-N reduction: 2.8 lb/ac per year
The earlier method underestimated saturated buffer performance because the flow over the upstream chamber had already undergone nitrate reduction due to reduced discharge from controlled drainage (see Ghane, 2025). In other words, some nitrate removal occurs before water enters the subsurface flow component.
When the water table in the upstream chamber is raised and perforated pipes are present near the field edge, these pipes act as additional distribution pipes, boosting lateral seepage and enhancing nitrate removal. Under these conditions, saturated buffers are more effective than previously reported.
We recommend that future studies use a paired-field design to more accurately evaluate performance.
8. Summary
• Considering the combined impact of controlled drainage and the subsurface buffer flow, saturated buffers are more effective than previously reported.
• This was the first study to measure actual saturated buffer performance using a paired-field design.
• Fields with slopes as low as 1% are viable for saturated buffer installation.
• Controlled drainage is a key source of nitrate load reduction in saturated buffer systems, enabled by upstream weir management.
• Use high-level weir management to reduce backflow and boost performance.
• If installing distribution pipes on a negative grade, use high-level management to prevent backflow.
Acknowledgement
This work was partly funded by the Michigan Department of Agricultural and Rural Development (791N7700580) and the Classic Conservation Innovation Grant (USDA-NRCS-NHQ-CIG-20- GEN0010808) from the US Department of Agriculture’s Natural Resource Conservation Service.
Expert Reviewed
This Extension bulletin has been peer reviewed by external experts.
References
Ghane, E., AbdalAal, Y., Tehrani, A. (2025). Paired-field evaluation of a saturated buffer reveals significant water-quality benefit through upstream weir management. Agricultural Water Management. 318, 109664. https://doi.org/10.1016/j.agwat.2025.109664
Ghane, E. (2025). Controlled drainage beyond flat fields (E3536). Michigan State University Extension Bulletin. www.canr.msu.edu/drainage
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