Abstract
Runoff characteristics of nitrate nitrogen (NO3-N) concentrations of drainage water were investigated in Yatsu agricultural catchment during rainfall events. With regard to the changes in the NO3-N concentrations for each rainfall event, the NO3-N concentrations showed a decreased tendency according to the increases in runoff by rainfall. When the runoff and the NO3-N concentrations for rainfall events during the irrigation or non-irrigation periods were plotted in a scattered diagram, they were significantly related to each other using a power function. The NO3-N concentrations during the irrigation period were below those at the similar runoff during the non-irrigation period, suggesting that irrigation water and a nitrogen removal function of paddy fields affected the No3-N concentrations during rainfall events. In special, the direct runoff was strongly related to the No3-N concentrations. In addition, the NO3-N concentrations decreased when the antecedent rainfall during the 5 days preceding the rainfall event (pre-5-day rainfall) was great, indicating that the pre-5-day rainfall affected the changes in the NO3-N concentrations. The results suggested that irrigation water, direct runoff, and antecedent rainfall affected the NO3-N concentrations of drainage water during rainfall events.
