Analysis of the water balance and the nitrogen and phosphorus runoff pollution of a paddy field in situ in the Taihu Lake basin

Nitrogen (N) and phosphorus (P) runoff loads of paddy fields in situ under conventional water and fertilizer management measures caused severe water pollution in the Taihu Lake basin. The "water balance method" was implemented to calculate the surface runoff and the N and P runoff loads of the paddy field for 2 years. The total inflow difference in the paddy field between the 2 years was 22.5% with more irrigation and less precipitation in 2013 than in 2014. The soil water depletion of the paddy field was in the range of 47.0-83.3 mm, accounting for an average of 6.4% of the total inflow. The surface runoff was mainly caused by precipitation and artificial drainage, but irrigation return flow should not be neglected in paddy fields in situ. The rainfall runoff volume accounted for an average of 42.2% of the total precipitation. The artificial drainages occurred mainly during the tillering stage of drying, during maturity stage of drying and when the water level was too high in the paddy field. More than half of the irrigation cases would occur as irrigation return flow, which accounted for an average of 11.8% of the irrigation. The variation in N and P concentrations in the surface runoff of the paddy field was predominantly governed by the times of fertilization. Compared to the irrigation volume control, the runoff pollution loads were predominantly governed by the fertilization rate. The mean reduction potential of N and P fertilization was 38.9 kg ha(-1) and 7.5 kg ha(-1), respectively. Reducing fertilizer rate is the effective way to improve the runoff pollution in paddy fields in the Taihu Lake basin.