Reduction of lake water eutrophication caused by nitrogen and phosphorous from paddy nearby field using a circular irrigation system

Abstract 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 nitrogen, 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 comparing 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 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 % for total phosphorus and 26.9% of for total nitrogen compared with the conventional irrigation system by operating the circulation pumps for 7 hours a day during the irrigation period.