Framework to parameterize and validate APEX to support deployment of the nutrient tracking tool

The Agricultural Policy Environmental eXtender (APEX) model is the scientific basis for the Nutrient Tracking Tool (NTT). NTT is an enhanced version of the Nitrogen Trading Tool, a user-friendly web-based computer program originally developed by the USDA. NTT was developed to estimate reductions in nutrient losses to the environment associated with alternative practices. The relatively easy access and ease with which the interface can be used has provided opportunities to demonstrate NTT in locations throughout the country; however, the absence of a clearly defined, consistent approach to parameterization and validation has raised questions over the reliability and consistency of simulated results. In this study: guidelines for parameterization and validation of APEX were developed based on literature review findings and the authors' experience; and a case study was provided to illustrate how the developed guidelines are applied. The developed guidelines are in the form of recommendations covering essential phases of model simulation studies as well as a clear interpretation of model performance evaluation criteria thresholds and model simulation performance results. These guidelines were successfully applied in the central Ohio case study. The most sensitive water yield parameters and their respective reasonable range of values were determined. Simulated monthly and annual water yield values were within 5% and 15% of observed values during the calibration and validation periods, respectively. Overall, the developed guidelines together with the illustrative case study example are intended to serve as the framework to parameterize and validate APEX to support nation-wide deployment of NTT. This framework can be easily modified and used in additional APEX and other modeling studies.