Characteristics of Nitrogen and Phosphorus Runoff from Paddy Fields during the Irrigation Period

Our aim was to quantify and characterize total nitrogen (TN) and total phosphorus (TP) in runoff from paddy fields in agricultural areas of Tottori, Japan. Freshwater pollution and eutrophication usually involve excess input of nutrients such as N and P. Nonpoint source water pollution is more difficult to control than point source pollution, and the contribution from agricultural nonpoint sources has been increasing. We investigated the characteristics of pollutant runoff from four paddy fields of different sizes, in 2003 and 2004. The N and P export was investigated during rainfall events by examining the variation in TN and TP in runoff, the correlation between discharge and pollutant concentration and loading, and the event mean concentration (EMC). The average TN and TP concentrations in surface water in May and June were 7.3 and 2.2 mg L–1, respectively, in 2003, and 12.4 and 2.0 mg L, respectively, –1 in 2004, and were higher than in other months [2003: 1.132 (TN) and 0.206 (TP) mg L–1; 2004: 1.243 (TN) and 0.284 (TP) mg L). –1 TN and TP loadings tended to increase with increasing discharge in most paddy fields. The EMCs were similar among the four paddy fields, but rainfall events differed. The calculated annual pollutant loadings (TN and TP) per unit area were 0.084 (2003) and 0.010 (2004) ton N y ha, and 0.029 (2003) and 0.004 (2004) ton P y ha in one field; those of the other fields –1 –1 –1 –1 were similar. Thus, pollutant contributions from paddy fields as nonpoint agricultural sources are substantial, and continuous examination of discharge water is important for the control and management of agricultural nonpoint source pollution