EVALUATION OF SEASONAL IMPACTS OF RIVER INFLOW ON PHYSICAL FIELDS IN HARIMA NADA BY USING OCEAN MODEL

Abstract

Over the past three decades, oligotrophication in Harima Nada, Japan, has caused ecological and fisheries impacts, leading to the implementation of nutrient management in coastal waters. As a semi-enclosed sea with limited exchange with the open ocean, its hydrodynamics are influenced by river inflow and atmospheric conditions. In this study, we investigated the seasonal impacts of river discharge on the physical fields of Harima Nada using a model system that coupled the Regional Ocean Modeling System (ROMS) with the Weather Research and Forecasting (WRF) Model by comparing simulations with and without river inflow. Including river inflow improves the reproducibility of seawater temperature and salinity across the basin, reducing the annual RMSE by 0.13 °C and 1.59 PSU, respectively. As a combined effect of temperature and salinity, water column stability reflects the seasonal influence of freshwater input, with stratification strengthening under high discharge and weakening under low discharge. Freshwater source analysis indicates that the Yodo River and the Kako River are the dominant contributors, with seasonal mean fractions of 22% and 16%, respectively. Overall, river discharge influences density stratification throughout the entire water column, suppressing vertical mixing and improving model performance. When stratification is weak, the influence of river input throughout the water column is correspondingly reduced. The results offer practical guidance on managing treated effluent and implementing coastal monitoring strategies.