Tracking Freshwater Plume Using Ocean Radar in Ariake Bay, Japan

A freshwater discharge from a river is a crucial factor to understand water circulation and ecological system near the river mouth due to the changes of seawater density and the nutrient supply for the ocean. Therefore, water pollution such as red tide is strongly dependent upon the amount of water discharged from river and the pathways of them. It is clear that there is a need to understand the behavior of the freshwater discharge from rivers in order to predict and prevent the red tide. Remote sensing techniques will offer a useful tool for the measurement compared to the in-situ measurements conducted in most previous field since they provide information over a large span of spatial and temporal scales. Ocean radar is a useful remote sensing technique for measuring the surface current in ocean with high resolution. In recent years, some field measurements indicate the effect of salinity on the receiving signal intensity (e.g. [1]). However, the detail of the effect is not thoroughly investigated. In this paper, we investigated the effect of salinity on the receiving signal intensity of the ocean radar through the theory and field measurement. Furthermore, we made an attempt to track the freshwater plume from Chikugo River in Ariake Bay using the receiving signal intensities of the ocean radar with VHF band.