The age of Yodo River water in the Seto Inland Sea

Abstract

Abstract The age of Yodo River water, the largest river flowing in the Seto Inland Sea (SIS), was calculated using the constituent-oriented age and residence time theory (CART) in order to understand the long-term material transport and environmental issues in this semi-enclosed sea. The Yodo River water was found to have a mean age of 152 days for the entire SIS. Inside the SIS, the age of Yodo River water shows spatial variation ranging from 97 days in Osaka Bay, 199 days in the Harima-Nada, and 450 days in other areas. The age of Yodo River water shows an apparent seasonal variation in the SIS. Tracing water from the Yodo River reveals two pathways for its movement around Osaka Bay: one brings ~5/8 river water directly into the Kii Channel through the Kitan Strait; the other brings ~3/8 river water into the Harima-Nada through the Akashi Strait that eventually enters the Kii Channel through the Naruto Strait. The mean age of Yodo River water is 128 days at the Kitan Strait, 125 days at the Akashi Strait, 203 days at the Naruto Strait, and 271 days at the Bisan Strait. The above estimation considers the return of old Yodo River water into Osaka Bay that does not change the spatial distribution of Yodo River water age inside Osaka Bay but increases the value of water age by ~80%. Without tidal forcing, the mean ages of Yodo River water in Osaka Bay and the Harima-Nada increase by 6% and 51%, respectively, and nearly all the Yodo River water flows directly into the Kii Channel through the Kitan Strait. Without considering baroclinic forcing, the mean ages of Yodo River water in Osaka Bay and the Harima-Nada increases by 52% and 51%, respectively, while the influence of local winds and the Yodo River discharge on the age of Yodo River water are negligible.