Non-point pollutant reduction in a paddy-field watershed using a circular irrigation system

The pollutant fluxes of nitrogen and phosphorus were elucidated for 3 years in a paddy-field watershed where a circular irrigation system was implemented. The annual net outflow loads of pollutants, i.e. the outflow load minus the inflow load, were as follows: the total nitrogen loads for the first, second and third year were − 5.46, + 58.7 and − 4.20 kg ha−1 yr−1, respectively, and the total phosphorus loads were − 2.87, + 1.13 and − 1.70 kg ha−1 yr−1, respectively. The positive net outflow loads for the second year were strongly affected by a considerably large amount of precipitation. It is concluded that the study watershed purified nitrogen and phosphorus compounds during normal hydrological years. The reason for the good purification performance in spite of no significant concentration decrease was that a large amount of water, usually more than 1000 mm excepting precipitation, flowed within the watershed by paddy-field irrigation. The study watershed seemed to have the desirable function of pollutant removal, with the deposition being more effective than the flushing in the river, and the retention time being sufficient for the purification mechanisms in paddy fields. Although the circular irrigation system was originally designed not for the purpose of pollutant removal but to supply water for paddy-field irrigation, it can contribute to reducing non-point pollutants.