Study region: Upper Blue Nile Basin (UBNB), Ethiopia.

Study focus: We explored the potential of using the globally available actual evapotranspiration (ETa) dataset in the model calibration processes to enhance hydrological model plausibility for the large UBNB. We compared three calibration strategies: conventional single-point calibration based on streamflow data, spatially explicit ETa-based calibration, and a multi-variable approach incorporating both streamflow and ETa data.

New hydrological insights for the region: Our results underscore the limitations of single-variable calibration in capturing the heterogeneity of the UBNB, particularly in the estimation of ETa. By integrating ETa into the calibration process, multi-variable calibration offers improved performance across both streamflow and ETa simulations, providing valuable insights into basin dynamics and internal processes. This approach, leveraging ETa as a signature of basin heterogeneity in the calibration, demonstrates significant promise for enhancing the plausibility of hydrological models in the complex, and large UBNB while maintaining computational simplicity. We used SWAT+, which is the most recent version of the most used hydrological model in the UBNB, SWAT. Thus, this study provides a benchmark for the employment and calibration of the SWAT+ model.