A Model Study On The Age Distribution Of River Water And Its Control Factors In The Semienclosed Sea

Water age is defined as the time elapsed since the departure of a water parcelfrom an area, where its age is prescribed to be0, to its arrival at a water body ofinterest. Water age is such one auxiliary variable that quantitatively reflects theenvironmental functions of dynamic processes and helps to understand the transporttimescales of land-based materials in the sea. However, little study has considered theage of river water in the semienclosed sea. To understand the age distribution of riverwater and its control factors in the semienclosed sea, the age of Yellow River water inthe Bohai Sea and Yodo River water in the Seto Inland Sea was calculated by athree-dimensional model based on the constituent-oriented age and residence timetheory （CART）. The Yellow River and Yodo River is the largest river that feeds intothe Bohai Sea and Seto Inland Sea, respectively. Significant results and usefulconclusions have been achieved:Yellow River water has a mean of3.0years for the entire Bohai Sea. The spatialvariation of the water age is significant:1.7years in the Laizhou Bay but3.9years inthe Liaodong Bay. The vertical variation in age of Yellow River water in the BohaiSea is small except for the area close the Yellow River mouth and the southern part ofCentral Basin in spring and summer. Except for the Laizhou Bay, the age of YellowRiver water in the entire Bohai Sea shows weak seasonal variation. The age of YellowRiver water at the Bohai Strait is3.4years. The age of Yellow River water at theBohai Strait is lower at the southern side than at the northern side throughout a year.But the vertical variation in the age of Yellow River water at the Bohai Strait is small.The exclusion of the return of old Yellow River water to the Bohai Sea does notchange the spatial distribution of water age for the four seasons but reduces theabsolute value of water age greatly. Without the reentry process, the mean age ofYellow River water in the entire Bohai Sea decreases to1.2years. Among all of thedynamic processes associated with circulations, the tide is the most dominant factor to affect the age of Yellow River water. Without tidal forcing, the mean age of YellowRiver water was reduced by more than50%. The winds are a secondary factor incontrolling the age of Yellow River water. Without winds, the mean of Yellow Riverwater increases by20%³0%. Without baroclinic force or changes in Yellow Riverdischarges have only slight influences on the age of Yellow River water in the BohaiSea.Yodo River water has a mean of152days for the entire Seto Inland Sea. Thespatial variation of the water age is significant:97days in the Osaka Bay,199days inthe Harima-Nada but450days in other regions of the Sea Inland Sea. The verticalvariation in age of Yodo River water in the Seto Inland Sea is small except for OsakaBay throughout a year. The age of Yodo River water in the Osaka Bay and entire SetoInland Sea both shows strong seasonal variation. After discharging into the Osaka Bay,Yodo River water divides into two branches. One of them (⁰.3×10¹⁰m³) flows intothe Harima-Nada through the Akashi Strait, and then flows into the Kii Channelthrough the Naruto Strait. The other branch (⁰.5×10¹⁰m³) flows into the Kii Channeldirectly through the Kitan Strait. Only a little Yodo River water flows into otherregions of the Seto Inland Sea. The age of Yodo River water at the Naruto Strait andKitan Strait is203days and128days, respectively. In winter, the age at the KitanStrait is lower at the eastern side than at the western side and the age distribution inthe vertical direction is homogenous. While in summer, the age at the Kitan Strait islower at the western side than at the eastern side and the age in the vertical directionshows strong stratification. The exclusion of the return of old Yodo River water to theSeto Inland Sea does not change the spatial distribution of water age for the fourseasons but reduces the absolute value of water age greatly. Without the reentryprocess, the mean age of Yodo River water in the entire Seto Inland Sea decreases to69days. Among all of the dynamic processes associated with circulations, thebaroclinic force is the most dominant factor to affect the age of Yodo River water.Without baroclinic force, the mean age of Yodo River water in the Osaka Bay andHarima-Nada both increases by more than50%. The tide is a secondary factor incontrolling the age of Yodo River water. Without tidal forcing, the mean of Yodo River water in the Osaka Bay and Harima-Nada increases by6%and51%,respectively. Without winds or changes in Yodo River discharges have only slightinfluences on the age of Yodo River water in the Seto Inland Sea.To understand the age distribution of river water and its control factors in thesemienclosed sea, the age of Yellow River water in the Bohai Sea is compared withthe age of Yodo River water in the Seto Inland Sea. In the semienclosed sea, the waterage varies significantly in the horizontal direction. The age of river water is smallestaround the river mouth and increases in proportion to the distance away from the rivermouth. The vertical variation in age of river water in the area close the river mouth islarge. And the age of river water in the area close to the river mouth shows strongseasonal variation. Yellow River water flows out of the Bohai Sea through the BohaiStrait, while Yodo River water mainly flows out of the Seto Inland Sea through theNaruto Strait and Kitan Strait. The exclusion of the return of old river water to thesemienclosed sea does not change the spatial distribution of water age for the fourseasons but reduces the absolute value of water age greatly. Changes in riverdischarges have only slight influence on the age of river water in the semienclosed sea.Among all of the dynamic processes associated with circulations, tide is the mostdominant factor to affect the age of Yellow River water in the Bohai Sea. And windsare a secondary factor. While baroclinic force is the most dominant factor to affect theage of Yodo River water in the Seto Inland Sea. And tide is a secondary factor.