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Parameterizing V-notch Weir Equations for Flow Monitoring in a Drainage Control Structure
Published on: April 25, 2025
Saturated buffer performance under alternative weir settings: Implications for design and management.
Gabriel M Johnson1, Thomas M Isenhart1, Christopher Hay2,3
1Department of Natural Resource Ecology and Management, Iowa State University, Ames, Iowa, USA.
Seasonal management of saturated buffers effectively removes nitrate-nitrogen (NO3-N) from tile drainage. However, extended full drainage periods can lead to significant NO3-N losses, necessitating careful site consideration.
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Area of Science:
- Agricultural Water Management
- Environmental Science
- Soil Science
Background:
- Saturated buffers are crucial edge-of-field practices for mitigating nitrate-nitrogen (NO3-N) runoff from subsurface drainage.
- The influence of seasonal weir elevation management on NO3-N removal efficiency in saturated buffers remains understudied.
Purpose of the Study:
- To assess the impact of control box weir elevation management on NO3-N removal in a saturated buffer.
- To compare in situ treatment performance against design predictions from USDA NRCS conservation practice standard 604.
Main Methods:
- Monitoring a 253 m saturated buffer (approx. 6 ha) for flow and NO3-N load from 2022-2024.
- Adjusting weir elevations to "full drainage," "growing season," and "fallow season" settings.
- Analyzing treatment efficiency and flow bypass during different management periods.
Main Results:
- During a 29-day "full drainage" period, 25% of flow and 28% of NO3-N load bypassed the buffer.
- NO3-N removal efficiency was higher in 2022 compared to 2023 and 2024.
- Actual treated flow exceeded predictions, while peak drainage flow was lower, indicating higher-than-designed treatment capacity utilization.
Conclusions:
- Seasonal weir management can achieve substantial NO3-N removal in saturated buffers.
- Extended "full drainage" periods require careful management to prevent significant NO3-N losses.
- Discrepancies between observed and predicted performance suggest a need for revised saturated buffer design methodologies.

