Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/20/2013
Publication Date: 3/1/2013
Citation: Plotkin, S., Wang, X., Potter, T.L., Bosch, D.D., Williams, J.R., Bagdon, J.K., Hesketh, E.S. 2013. APEX Calibration and Validation of Water and Herbicide Transport under U.S. Southern Atlantic Coastal Plain Conditions. Transactions of the ASABE. 56(1):43-60. Interpretive Summary: Simulation models are valuable tools used to assess management practices, cropping systems, and other land use practices across a broad range of agricultural landscapes. Models continue to evolve providing more efficient computation and extending the scale to which they can be applied. Nevertheless their effective use requires calibration and validation using sets of measured observations. In this study, USDA-ARS Southeast Watershed Research Laboratory and USDA-NRCS scientists worked collaboratively to calibrate and validate the USDA-ARS model APEX to describe hydrologic responses and the potential for pesticide runoff from farm fields in the Southern Atlantic Coastal region of the USA. Calibration data were complied from a long-term study of tillage impacts on water quantity and quality during rotational cotton and peanut production. Once calibration was complete the model was found to effectively describe both water and herbicide losses. The study has enhanced the value of APEX by showing that the model can effectively describe these landscape scale processes. This will improve confidence in the model results. Currently the model is being used at the national in the USDA Conservation Effects Assessment Program (CEAP).
Technical Abstract: Simulation modeling is widely used to assess water contamination risk associated with pesticide use, and for evaluating effectiveness of agricultural conservation practices. Currently the Agricultural Policy/Environmental eXtender (APEX) model is being used in the USDA Conservation Effects Assessment Project (CEAP) Cropland National Assessment for this purpose. In support of the CEAP modeling efforts, APEX was calibrated and validated using a 9-year record (1999-2007) of surface runoff and tile flow, and an 8-year record (1999-2006) of soluble pendimethalin and fluometuron herbicide losses monitored at a research site maintained in a cotton-peanut rotation. Research plots were 0.14 ha with 3 each maintained in conventional and strip tillage management.Monthly surface runoff was calibrated by adjusting the Natural Resources Conservation Service Runoff Curve Number, Curve Number Index Coefficient (CNIC) and Irrigation Runoff Ratio. R2 values ranged from 0.64 to 0.79 and Nash-Sutcliffe efficiency (NSE) values ranged from 0.55 to 0.73 based for comparisons s between observed and simulated values during calibration and validation periods. The model’s performance for predicting annual runoff and fluometuron and pendimethalin runoff losses for both tillage systems and the tile drainage from the strip tilled plots was well aligned with monitored results, with R2 values ranging from 0.57 to 0.93 and annual NSE values from 0.51 to 0.89. APEX performance was relatively poor for annual tile flow in the conventional tilled plot and fluometuron leaching in the tile flow from both tillage systems. However, APEX replicated the annual means reasonably well for these three variables with the simulated annual means within 11% of the observed values. Results demonstrate APEX’s strength in simulating runoff in this landscape and potential weakness in examining tile flow.