NONPOINT 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 wa s implemented. The annual net outflow loads of pollutants, i.e. the ou tflow 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 conside rably large amount of precipitation. It is concluded that the study wa tershed purified nitrogen and phosphorus compounds during normal hydro logical years. The reason for the good purification performance in spi te of no significant concentration decrease was that a large amount of water, usually more than 1000 mm excepting precipitation, flowed with in the watershed by paddy-field irrigation. The study watershed seemed to have the desirable function of pollutant removal, with the deposit ion being more effective than the flushing in the river, and the reten tion time being sufficient for the purification mechanisms in paddy fi elds. 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. (C) 1997 Elsevier Science Ltd.