Calibration and validation mismatch

[Anna Zam kottayil]

I am currently working on SWAT model calibration and validation using SWAT-CUP, and I would like to seek your guidance regarding an issue I am facing with the model performance.

For my study, I have used data from 2015 to 2025, considering 2015 as the warm-up period. During calibration, I obtained a Nash–Sutcliffe Efficiency (NSE) value of 0.82, which indicates a very good model performance. However, during validation, the NSE value dropped to 0.51.

I understand that a decrease in NSE during validation is expected, but I am concerned about whether this level of reduction is acceptable or if it indicates potential issues in my model setup. I would like to know if this discrepancy could be due to factors such as parameter overfitting during calibration, variability in input data, or issues related to the selected calibration/validation periods.

Could you please guide me on:

- Whether this validation performance is acceptable for hydrological modeling studies

- Possible reasons for the drop in NSE during validation

- Any steps I can take to improve the validation results

I would really appreciate your insights and suggestions.

Thank you for your time and support.