Quantification of uncertainty in estimated nitrate-N loads in agricultural watersheds

- The overall uncertainty in nitrate-N load was quantified through error propagation.
- Measurement uncertainty averaged 12% (expressed as CV) for the seven watersheds.
- Load calculation uncertainty ranged from 7% to 32% for a 30-day sampling frequency.
- Uncertainty depends on watershed, sampling frequency, and load calculation method.
- The results can be used to improve monitoring design balancing cost and accuracy.

SummaryEstimation of annual nutrient loads is critical for many applications in water quality management; however, measurement errors and infrequent monitoring could raise considerable uncertainty in the load estimations. The objectives of this study were to quantify the overall uncertainty in annual nitrate-N load estimates. Nitrate was selected as the study object because of the high loads common in Midwestern streams, and its important influence on hypoxia in the Gulf of Mexico. A standard error propagation method was used to quantify the uncertainty from both measurement and load calculation processes to evaluate the accumulative effect. The measurement uncertainty was assessed based on standard information; the load calculation uncertainty was evaluated through numerical simulation. The study found that uncertainty in annual nitrate-N load estimates for a 30-day sampling frequency, expressed as coefficient of variation (CV), was 14-34%. This included measurement (flow and concentration) uncertainty of 12% and load calculation uncertainty of 7-32%. These results can be used to evaluate different uncertainty sources, estimate uncertainty for similar watersheds, and design sampling programs balancing cost and accuracy.