Simulating internal watershed processes using multiple SWAT models.

Area of Science:

• Environmental science
• Hydrology
• Water quality modeling

Background:

• Effective watershed management relies on accurate water quality models.
• Models are typically calibrated and validated at the basin outlet for large-scale assessments.
• Understanding model performance at finer scales is crucial for targeted management.

Purpose of the Study:

• To evaluate the field-scale performance of watershed models calibrated at the basin outlet.
• To assess the ability of these models to capture farm field-scale hydrological and nutrient dynamics.
• To identify discrepancies in process representation within watershed models at different scales.

Main Methods:

• An ensemble of watershed models was calibrated using in-stream observations at the basin outlet.
• Model performance was evaluated against discharge and nutrient data at the farm field scale.
• Discrepancies in surface runoff, subsurface drainage, and nutrient loading were analyzed.

Main Results:

• All models demonstrated strong performance at the watershed outlet.
• Upstream performance varied significantly, indicating scale-dependent accuracy.
• Models tended to over-predict discharge via surface runoff and subsurface drainage.
• Models under-predicted phosphorus loading through subsurface drainage and nitrogen loading via surface runoff.

Conclusions:

• Watershed models calibrated at the basin outlet may not accurately represent field-scale processes.
• Caution is advised when using these models to evaluate field-scale management strategies without localized data.
• Model improvements for field-scale dynamics require incorporating data at that resolution.