AnnAGNPS Application and Evaluation in NE Indiana

The Annualized Agricultural Non-Point Source (AnnAGNPS) pollution model was developed for simulation of runoff, sediment, nutrient, and pesticide losses from ungaged agricultural watersheds. 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. Major objectives were to evaluate the ability of AnnAGNPS to simulate runoff and atrazine concentrations in uncalibrated, calibrated, and validation modes. Data sources for the model inputs included USGS Digital Elevation Model for topography, NRCS spatial SSURGO soils data, and the USDANASS cropland data layer. Observed flow data for CCW were available from a USGS gaging station, while flow and atrazine concentration data were available from a NSERL water quality sampling site at the discharge point of the MDS. 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 during model calibration, as could discharge for CCW during model validation. Initial 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 utilized in the pesticide routine, which when corrected greatly improved the results. The corrected AnnAGNPS model code could be successfully calibrated and validated for predictions of atrazine concentrations in the MDS.