Characteristics of nutrient concentrations during low streamflow in Korean watersheds with paddy fields

Abstract We investigated nutrient concentrations, during periods of low streamflow, from medium‐scale watersheds (5.1–15.3 km 2 ) with paddy fields. Data were collected both during irrigation and non‐irrigation periods, from 1996 to 2000. Mean total nitrogen (TN) concentration during irrigation periods (2.6 mg l −1 ) was significantly lower than during non‐irrigation periods (6.2 mg l −1 ), most likely due to the dilution effect of higher discharge and nitrogen removal in the forest and paddy field by denitrification. Mean total phosphorus (TP) concentration during irrigation periods (0.13 mg l −1 ) was higher than during non‐irrigation periods (0.09 mg l −1 ). Therefore, the mean TN:TP ratio during irrigation periods was 20:1, compared to 67:1 during non‐irrigation periods; hence the possibility of cyanobacterial blooms was higher during the peak of irrigation. During irrigation periods, TN concentrations in the study watersheds displayed no relationship with discharge, similar to the finding for the paddy fields. Paddy field irrigation may impact on nitrogen concentrations in low streamflow at the watershed scale. Copyright © 2007 John Wiley & Sons, Ltd.