Framework for Parameterizing and Validating Apex to Support Ntt.

The Agricultural Policy Environmental eXtender (APEX) is the process model underlying the Nutrient Tracking Tool (NTT). The NTT provides users, primarily farmers and crop consultants, with the opportunity to compare the effects of two scenarios, practice combinations, or other alternative conditions on indicators of interest using multiple years of historical or simulated weather. The local-project driven approach has provided opportunities to demonstrate NTT in locations throughout the country; however, there is no clearly defined, consistent approach to parameterization and validation to increase the users’ confidence regarding reliability and consistency of the results. Recently, a comprehensive literature review of APEX and NTT applications was conducted by USDA Agricultural Research Service scientists whose findings, in addition to the scientists modeling experience, were used to develop parametrization and validation guidelines. A succinct description of these guidelines will be presented. The use of these guidelines is demonstrated for flow in a case study headwater sub-watershed in central Ohio. The developed guidelines were in the form of recommendations covering essential phases of phases related to model parameterization and validation. The most sensitive flow parameters and their corresponding range of values will be presented. Preliminary results showed that the 95% confidence interval for simulated monthly and annual flow were 41.21 ± 0.19 mm and 506.61 ± 7.64 mm, respectively. The uncertainty intervals for simulated mean monthly and annual flow varied from 40.28 mm to 43.41 mm and 489.49 mm to 521.81 mm, respectively. The simulated mean monthly and annual flows were within 5% of the measured values.  Overall this case study together with the model parameterization and validation guidelines developed are intended to serve as the framework for a larger effort by USDA to parameterize and calibrate APEX to increase confidence in and to support deployment of NTT.