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

Area of Science:

• Agricultural Water Management
• Environmental Monitoring
• Nutrient Management

Background:

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

Purpose of the Study:

• To quantify the uncertainty associated with common sampling approaches for monitoring bioreactor performance.
• To determine optimal sampling frequencies for accurately assessing bioreactor efficiency and nutrient removal.

Main Methods:

• Utilized Monte Carlo simulations to subsample 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.
• Evaluated the influence of drainage area size, discharge variability, and NO3-N concentration variability on sampling frequency.

Main Results:

• Optimal sampling frequencies were influenced by drainage area size, discharge, and NO3-N concentration variability.
• A sampling frequency of approximately every 2.1 days achieved ±10% uncertainty for research-oriented monitoring (scenario 1).
• Practitioner-oriented methods (scenario 2) and event-based sampling (scenario 3) required more frequent sampling or led to overestimation, respectively.

Conclusions:

• Monitoring conservation practice performance, such as denitrifying bioreactors, necessitates more frequent sampling than monitoring subsurface nutrient loss alone.
• Findings provide benchmark uncertainty levels for various edge-of-field monitoring objectives related to bioreactor performance.
• Results highlight the need to adjust sampling strategies based on specific monitoring goals and site characteristics.