Estimating suspended sediment discharge and its annual mass balance in the Japanese first-class watersheds

Abstract

Toward comprehensive sediment management in Japan, the present study estimated suspended sediment discharge in the downstream areas of 109 first-class river watersheds based on the relationship between suspended sediment (SS) discharge L and water discharge Q as well as the volumet-ric changes of the watersheds. Firstly, we collected liter-ature that reported L – Q relationships and we identified L – Q equations for the monitoring stations of all 109 first-class rivers. Secondly, we calculated annual average SS discharge rate L for all stations using the L – Q equations. Finally, we calculated the annual average vertical movement rate and volume change. As a result, we found that: (1) SS discharge was high in eastern Hokkaido, northern Kanto, Chubu, Shikoku regions and parts of Chugoku region, (2) average vertical movement rates were high in areas around 40° north and in the central parts of Japan, (3) the volume increase was greater in Hokkaido, areas around 40° north and central parts of Japan, (4) net increases were greater in western Hokkaido, areas around 40° north, and in Kinki region, and (5) volume increase rates were high in areas around 40° north. There were also large volume decreases in the Shinano River watershed.