Estimation of Watershed Nitrate Loads on Drained Agricultural Lands

Jiang, Yan, M.S.A.B.E., Purdue University, December 2013. Estimation of Watershed Nitrate Loads on Drained Agricultural Lands. Major Professor: Jane R. Frankenberger. Estimation of nutrient load is critical for many applications in water quality management; however, infrequent data monitoring and measurement error 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, and develop a statistical model to predict nitrate loads based on subsurface drainage characteristics in Indiana watersheds. 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. Results showed that the estimated measurement uncertainty, expressed as coefficient of variation (CV), ranged from 11.7 to 12.4%, and the load calculation uncertainty CV for a 30-day frequency ranged from 7% to 32%. The estimated overall uncertainty ranged from 14 to 34% in annual nitrate-N load estimates for a 68% confidence interval.