Combined PEST and Trial-Error approach to improve APEX calibration

The Agricultural Policy Environmental eXtender (APEX), a comprehensive hydrologic model well-suited for small watersheds, requires understanding of the input parameters for improved calibration. The “trial and error” method for calibrating the APEX model has been used very commonly in previous studies. In this study, the automatic calibration software Parameter Estimation (PEST) was combined with the conventional trial-and-error method to improve APEX calibration. The proposed Combined PEST and Trial-Error (CPTE) approach can overcome: (i) weaknesses of “Trial-Error” method in terms of tediousness and subjectivity involved in the decision to end a calibration, and (ii) drawback of PEST in that it may lead to biased simulation due to ignoring local specific condition. A case study was developed to verify the CPTE approach. The results based on APEX runoff simulation indicate that the CPTE approach greatly improved the calibration of APEX model with respect to model performance criteria. Coupling inverse modeling and trial-error manual method can be an efficient and effective alternative in calibrating the APEX model.