Hydrological Modeling of the Chikugo River Basin Using SWAT: Insights into Water Balance and Seasonal Variability

The Chikugo River, the longest river on Kyushu Island, originates on Mount Aso and flows into the Ariake Sea, serving as a vital freshwater resource that supports marine ecology and regional water management. The basin experiences frequent heavy rainfall from typhoons and seasonal precipitation, making hydrological modeling essential for understanding water movement and variability. However, integrated hydrological studies in the Chikugo Catchment Area are relatively scarce. This study evaluated the performance of the Soil and Water Assessment Tool (SWAT) in simulating the streamflow, water balance, and seasonal hydrological variability in the Chikugo River basin from 2007 to 2021. Model calibration and uncertainty analysis were conducted using the Sequential Uncertainty Fitting Version 2 (SUFI-2) approach. The results indicated that the SWAT effectively captured streamflow dynamics, demonstrating strong reliability in hydrological simulations. Sensitivity analysis identified surface runoff, baseflow, and channel flow characteristics as the key factors influencing model performance. Water balance analysis revealed that evapotranspiration and baseflow play dominant roles in basin hydrology, whereas runoff remains consistently lower than precipitation because of significant water retention. These findings provided valuable insights into the hydrological processes of the Chikugo River Basin and offer a scientific basis for improved water resource management, allocation, and planning in temperate climate catchments.