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The Chikugo Barrage is located at 23 km upstream from the mouth of the Chikugo River, which is the largest river in the Northern Kyushu Region of Japan. Main purposes of the Chikugo Barrage are flood control, water supply, maintaining flow of the Chikugo River and preventing seawater intrusion from the Ariake Sea. There are two rivers flowing into the reservoir, the Chikugo River (main stream) and the Homan River (a tributary). According to the intake of water supply and irrigation water, and the operation of the Chikugo Barrage, previous researches reported that the hydraulic retention time (HRT) in the reservoir of the Chikugo Barrage became longer and resulted in high growth of phytoplankton in the reservoir during 1985-2008. In this study, relationship between loading from upstream, HRT and algal productivity on water quality in the Chikugo Barrage Reservoir was analyzed by using water quality model. Calculated results confirm that chlorophyll-a (Chl-a) in the reservoir increased when inflow loading was high or when the HRT in the reservoir was long. Algal productivity significantly affected on the nutrient level and also led to the increase in COD and SS in the reservoir. Without loading from the Homan River, simulated result of COD in the reservoir becomes lower which indicates that loading from the Homan River has subsidiary impact on algal growth in the reservoir. Loading control in the Homan River Basin is suggested as an effective measure for water quality management in the Chikugo Barrage Reservoir.
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