Assessment of agricultural watershed water budget considering upstream water budget against reservoir discharge and agricultural water supply

This study proposes a watershed baseflow and direct runoff assessment methodology considering agricultural reservoirs, consisting of an assessment method that reflects the upstream water budget in the reservoir water balance and a watershed analysis using a linked modeling technique. The linked model integrates the watershed model with the reservoir operation model, allowing for the estimation of watershed runoff, agricultural water supply, reservoir discharge, and irrigation return water. It was calibrated and verified to ensure result applicability beyond "Satisfactory." The assessment method considering the reservoir separates the inflow into direct outflow and baseflow, analyzed the reservoir water balance for both, and reflects them in the outflow (discharge, supply). This approach enables the assessment of temporal variations in inflow, storage, discharge, and agricultural water use. The monthly BFI in the reservoir storage, it gradually decreased during the flood season and subsequently increased. This was reflected in the discharge assessment, allowing us to reflect the reservoir condition at each time. The resulting average watershed baseflow index was 69.0 %, 1.5 % higher than the value without reservoir consideration. This study is significant in integrating upstream watershed hydrology and reservoir operations into overall watershed assessment. By evaluating the impacts of reservoirs and agricultural water use on watershed conditions, it supports the development of management frameworks such as evaluating watershed unit improvements and formulating reservoir operation strategies to enhance watershed water cycles.