Implementing multi-site calibration and reservoir operations in SWAT+ : insight for water balance management in the Awash River Basin, Ethiopia

ABSTRACT Accurate water balance estimation is essential for sustainable water resource management, especially in large-scale studies. This research investigates the dynamics of water availability and distribution to enhance agricultural productivity using the Soil and Water Assessment Tool (SWAT+) in Awash Basin, Ethiopia. We propose a multi-site calibration approach for large-scale streamflow simulation and mass balance estimation, integrating both observation and soft data. This approach aims to improve the representation of reservoir operations and irrigation practices within the model and to assess how calibration and management practices affect model performance. Streamflow simulated and mass balance components at the Awash-Awash station outlet were estimated after the model was calibrated and validated using monthly streamflow data from 1997 to 2019. Irrigation practice and the Koka Reservoir release rule were implemented using decision tables. Our findings indicate that the operations of the Koka Reservoir, combined with effective irrigation practices, significantly improved streamflow simulation and water balance estimation at the downstream gauging station. The SWAT+ model demonstrated satisfactory to very good performance in replicating both streamflow and water balance components in the study area. The methodological framework presents a valuable tool for sustainable water resource management that can be adopted in other river basins.