Multisite Evaluation of APEX for Water Quality: II. Regional Parameterization.

Area of Science:

• Environmental Science
• Agricultural Science
• Hydrology

Background:

• Phosphorus (P) Index assessment necessitates reliable, long-term P loss data across diverse conditions.
• Existing measured data are insufficient for evaluating P Index performance.
• Process-based models offer a solution for generating necessary data for P Index evaluation.

Purpose of the Study:

• To develop a regional parameterization for the Agricultural Policy Environmental eXtender (APEX) model.
• To estimate edge-of-field runoff, sediment, and phosphorus losses in restricted-layer soils of Missouri and Kansas.
• To assess the performance of the regionally calibrated model (RCM) using regional monitoring data.

Main Methods:

• Developed a regionally calibrated model (RCM) from five site-specific calibrated models (SSCMs).
• Calibrated and validated the RCM using measured monitoring data from multiple sites.
• Assessed model performance using Nash-Sutcliffe efficiency (NSE) and absolute percent bias (|PBIAS|).

Main Results:

• RCM runoff simulation performance was comparable to SSCMs (NSE > 0.72, |PBIAS| < 18%).
• RCM sediment loss simulation was poor (NSE < 0, |PBIAS| > 90%), especially for low sediment loads.
• RCM total P loss simulation was acceptable (NSE > 0.74, |PBIAS| < 30%) but underperformed SSCMs.

Conclusions:

• Runoff estimates from the RCM are acceptable for Phosphorus (P) Index evaluation.
• Caution is advised for total P-loss estimates due to poor sediment loss simulation.
• A robust regional parameterization for APEX for sediment and total P loss was not fully achieved.