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Abstract
A pesticide fate and transport model (PCPF‐B) was developed to predict the runoff of pesticides from paddy plots to a drainage canal in a paddy block based on the plot scale model (PCPF‐1). The block‐scale model now comprises three modules: (1) a module for pesticide application, (2) a module for pesticide behaviour in paddy fields, and (3) a module for pesticide concentration in the drainage canal. The PCPF‐B model was first evaluated using published data in a single plot and then was applied to predict the concentration of bensulphuron‐methyl in one paddy block in the Sakura river basin, Ibaraki, Japan, where a detailed field survey was conducted. The PCPF‐B model simulated well the concentration of bensulphuron‐methyl in individual paddy plots. It also reflected the runoff pattern of bensulphuron‐methyl at the block outlet, although overestimation of bensulphuron‐methyl concentrations occurred due to uncertainty in water balance estimation. A sensitivity analysis showed that the soil adsorption coefficient of the herbicide had the greatest influence on the concentrations and cumulative loss of bensulphuron‐methyl to the drainage canal.
Acknowledgement
This paper was produced when Thai Khanh Phong was a JSPS postdoctoral fellow at the Faculty of Agriculture, Kyushu University. Thanks are due to Dr. Son Hong Vu and Dr. Satoru Ishihara for sharing their data and to Mr. Taku Tanaka for his assistance on the water‐balance analysis. The authors would like to extend their thanks to two anonymous reviewers for their constructive comments on the manuscript.