Various materials, including plastics and nutrients, are transported from land to ocean. This transport is closely linked to water movement, and material sources are typically distributed over broad areas rather than concentrated at a single point. To clarify the dominant pathways and processes that determine the age of materials entering the ocean, we developed a hybrid model for Kinki region, Japan and its surrounding ocean. This model combines a land river-ocean seamless model that simulates water flow field with a particle tracking model that tracks the movement of materials. 
The land-river-ocean seamless model is based on JORRO and uses a two-layer configuration: a freshwater layer representing land surface and river waters, and an oceanic layer representing sea water. Surface elevation and river pathways are derived from the HydroSHEDS database. Friction coefficients are prescribed to be larger over land surfaces than along river changes, reflecting differences in flow resistance. We find that the mean age of particles at river mouths to be about 2days when the friction coefficient on land is set to ten times that of the river. To identify the factors controlling the age distribution, sensitivity experiments were conducted to examine the dependence of particle age on initial particle position and land-surface friction.
The results indicate that water age is more sensitive to friction to land-surface friction than to initial positions, as the friction coefficient controls water velocity and thus, the residence time on land. To examine the impact of weather events, additional experiments were performed to examine changes in age distribution at river mouths changes after rainfall. The age distribution was largely determined by the newly precipitated water, and its influence persists for several days after the rainfall event.

age distribution,model;material transport