The pollutant loads from a paddy field watershed due to agricultural activity

Observations were performed in a small agricultural watershed for four consecutive irrigation periods in order to characterize fluctuations in the characteristics of pollutant runoff from paddy fields. During the puddling and rice-planting period and at the beginning of the mid-summer drainage period, both the pollutant concentrations and pollutant loadings of suspended solids, total nitrogen, and total phosphorus increased. In contrast, the pollutant net loading of total nitrogen was negative during the intermittent irrigation period. These results indicate that changes in the specific agricultural activities in the paddy fields cause temporal fluctuations of the pollutant runoff. Previous studies which focused on paddy fields have dealt with unit loading for entire irrigation period only. However, the unit loading for the entire irrigation period cannot take into account pollutant loading fluctuations due to differences in agricultural activity. Thus the unit loading of pollutant should be evaluated for each agricultural activity during the irrigation period.