Multisite Evaluation of APEX for Water Quality: I. Best Professional Judgment Parameterization.

Area of Science:

• Environmental modeling
• Agricultural science
• Water resource management

Background:

• The Agricultural Policy Environmental eXtender (APEX) model simulates edge-of-field water, nutrient, and sediment transport for assessing management practice impacts.
• Current APEX practice requires full site-specific calibration, demanding significant resources and data.
• Alternative parameterization methods are needed when extensive data is unavailable.

Purpose of the Study:

• To compare the performance of the APEX model for flow, sediment, and phosphorus transport using two parameterization schemes.
• To evaluate a best professional judgment (BPJ) parameterization against a fully calibrated parameterization.
• To determine the necessity of model calibration for reliable environmental impact assessments.

Main Methods:

• Analyzed 12 datasets from four locations with poorly drained soils and row-crop production under varying tillage systems.
• Compared model performance using Nash-Sutcliffe efficiency (NSE), coefficient of determination (R²), and regression slope.
• Evaluated simulations under both BPJ and fully calibrated parameterization schemes.

Main Results:

• BPJ parameterization yielded acceptable annual flow simulation (NSE = 0.7), but full calibration significantly improved it (NSE = 0.9).
• Accurate simulation of sediment and total phosphorus transport (NSE = 0.5 and 0.9, respectively) was achieved only with full calibration.
• The BPJ approach demonstrated unacceptable performance for sediment and phosphorus, indicating potential for misleading results.

Conclusions:

• Full calibration of the APEX model using site-specific water quality data is crucial for accurate simulation of sediment and total phosphorus loss.
• Relying on uncalibrated APEX (BPJ) for planning or management decisions can lead to erroneous conclusions.
• Model calibration is essential for reliable environmental impact assessments using APEX.