Assessment of the Effect of Cyclic Irrigation on Reducing Nitrogen Effluent Loading from a Paddy-Field District

Abstract

Nitrogen (N) effluent loading discharged from paddy fields has a large impact on water quality in downstream areas. Cyclic irrigation, which reuses drainage water for irrigation purposes, is considered an effective water pollution prevention measure in paddy fields. In this study, we evaluated the effectiveness of cyclic irrigation in the reduction of N loading from a paddy-field district. We measured temporal variations in N concentration in drainage water and estimated the N mass balance for the study district for three consecutive years since 2007. We found that cyclic irrigation reduced N loadings. Mean N effluent loadings during the cyclic irrigation were 4.4 kg/ha and were smaller compared to lake water irrigation (non-cyclic irrigation). The hydrological structure and effect of a cyclic irrigation system can be characterized by using three parameters: ratio of the N concentration in drainage water to that in irrigation water; intensity at which drainage water is reused; and ratio of surplus irrigation water to the total amount of irrigation water. We conclude that the efficient use of irrigation water is important for reducing N effluent loadings from the paddy-field district because N in drainage water may not be qualitatively purified.