An investigation was made into the water quality and hydrology in paddy fields adjacent to Lake Yogo, Japan, where a circular irrigation system with an oxidation ditch for removal of nitroge, phosphorus, and COD was established in 1997 with the aim of saving irrigation water and reducing pollutants in drainage water. A simulation was made, and evaluated by ocmparing the simulated values for nitrogen and phosphorus in the drainage water from the agricultural land with observed data. It was concluded from the evaluation that from the agricultural land with observed data. It was concluded from the evaluation that the simulation could predict a reduction in the nutrient load entering the lake. The circular irrigation system could contribute to a reduction of the pollutant load by 44.2
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