Abstract
The examined dam (Nishitakao Dam in Touhaku National Irrigation Project) is possible to be eutrophicated because of an excess of nutrient load effluent from livestock area and fodder fields on the dam basin. Although now, the water quality is categorized as an eutrophic lake, the water-bloom has not occurred. It is considered that the dam operation which they exhaust the dam of water at the end of irrigation period affects the formation of water environment, The aim of this study is to examine the dynamics of water quality, including the effect of the term-end discharge, an ecosystem water quality model was constructed in this study. Simulation results showed that, under the condition without term-end discharge, the maximum value of Chemical Oxygen Demand (COD) was decreased because the inflow which imports pollutant to the dam decreased by 26%. However, it was also shown that the concentration of Total Nitrogen (T-N) increased gradually and the concentration of Total Phosphate (T-P) kept almost no changes
