APEX Model Assessment of Variable Landscapes on Runoff and Dissolved Herbicides

Abstract

Variability in soil landscapes and their associated properties can have significant effects on erosion and deposition processes that affect runoff and transport of pesticides and nutrients. Simulation models are one way in which the effects of landscapes on these processes can be assessed. This study evaluated the effects of variations in landscape position on runoff and dissolved atrazine using a calibrated APEX model. Fourteen agricultural plots (18 x 189 m2) in the Goodwater Creek watershed, a 7250 ha agricultural area in north-central Missouri, were simulated with the farm- and field-scale Agricultural Policy/Environmental eXtender (APEX) model. Plots were under three different tillage and herbicide management systems for three grain crop production systems. Each plot contained three landscape positions: summit, backslope, and footslope along with two transition zones. Runoff was measured and samples were collected from 1997 to 2002 during the corn year of the crop rotations. Runoff samples were analyzed for dissolved atrazine. The APEX model was calibrated and validated with event data from each plot during the corn growing years from 1997 to 1999 and 2000 to 2002, respectively. APEX reasonably simulated runoff and dissolved atrazine concentrations with annual coefficients of determination (r2) values ranging from 0.60 to 0.98 and 0.52 to 0.97, and Nash-Sutcliffe efficiency (NSE) values ranging from 0.46 to 0.94 and 0.45 to 0.86 for calibration and validation, respectively. The calibrated model was then used to simulate variable sequencing of landscape positions and associated soil properties as well as variable lengths of landscape positions. Simulated results indicate that as the length of the backslope increased while the steepness remained constant, so did the volume of runoff discharged and the atrazine concentrations at the plot outlet. In addition, the highest level of simulated runoff occurred when the backslope position was located adjacent to the outlet. Results from this study will be helpful to managers in placement of conservation practices on sensitive landscapes for improvement in water quality.