Atrazine Prediction in Northeast Indiana

The Annualized Agricultural Nonpoint Source (AnnAGNPS) pollution model was developed for simulation of runoff, sediment, nutrient, and pesticide losses from ungauged agricultural watersheds. This article describes the first documented calibration and validation of AnnAGNPS for prediction of atrazine loading. Here, the model was applied to the 707 km2 Cedar Creek watershed (CCW) and the 45 km2 Matson Ditch sub-catchment (MDS), which are predominantly (>85%) agricultural, with major crops of corn and soybeans. Atrazine herbicide is of significant concern, as the St. Joseph River is the source of drinking water for the city of Fort Wayne, Indiana, with Cedar Creek being the main tributary. Major objectives were to evaluate the ability of AnnAGNPS to simulate runoff and atrazine concentrations in uncalibrated, calibrated, and validation modes. In an uncalibrated mode, flow discharge predictions by AnnAGNPS were satisfactory at the CCW scale but could be improved through calibration. Flow discharge for both CCW and MDS could be well matched with observed values during model calibration and validation. AnnAGNPS predictions of atrazine concentrations in runoff water were very poor, and it was impossible to improve the results through any type of calibration. Inspection of the model source code revealed a unit conversion error in the runoff value being input to the pesticide routine, which when corrected greatly improved the results. The corrected AnnAGNPS model code could be satisfactorily calibrated and validated for predictions of atrazine concentrations in the MDS, but not in the CCW where only coarse measured data were available.