Assessing uncertainty in conservation practice performance: effect of sample frequency on bioreactor nitrate removal

Area of Science:

• Agricultural Science
• Environmental Science
• Water Resource Management

Background:

• Edge-of-field monitoring is vital for evaluating conservation practices like denitrifying bioreactors in tile-drained systems.
• Current sampling frequencies for nutrient loss may not be adequate for practices with both inlets and outlets.

Purpose of the Study:

• To determine the uncertainty associated with common sampling strategies for monitoring bioreactor performance.
• To identify optimal sampling frequencies for accurately quantifying nutrient removal by bioreactors.

Main Methods:

• Utilized Monte Carlo simulations to analyze high-frequency nitrate-nitrogen (NO3-N) sensing data from seven bioreactors.
• Assessed four annual performance metrics (inlet/outlet losses, removal efficiency, load removed) under three sampling scenarios.
• Varied sampling frequencies to quantify uncertainty in estimated bioreactor performance.

Main Results:

• Optimal sampling frequency is influenced by drainage area size, discharge variability, and NO3-N concentration variability.
• Research-oriented monitoring (scenario 1) achieved ±10% uncertainty with sampling every ~2 days.
• Practitioner-oriented methods (scenario 2) and event-based sampling (scenario 3) showed higher uncertainty or overestimation.

Conclusions:

• Monitoring conservation practice performance, particularly bioreactors, necessitates more frequent sampling than monitoring subsurface nutrient loss alone.
• Findings offer benchmark uncertainty levels for denitrifying bioreactor monitoring across various edge-of-field objectives.
• Event-based sampling is not recommended due to potential overestimation during high flow events.