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We investigated nutrient concentrations, during periods of low streamflow, from medium-scale watersheds (5.1-15.3 km²) 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.

A pilot study was performed in an experimental field at Konkuk University in Seoul, Korea, to examine the effect of treated sewage irrigation on paddy rice culture and its soil characteristics. Treated sewage irrigation did not adversely affect the growth or yield of rice; instead it resulted in about 10% (with dilution) or 50% (without dilution) greater yield than in controls. The strength of treated sewage was not a limiting factor and no lodging was observed in the rice culture even with a relatively high nitrogen concentration (up to 160 mg l−1). The paddy soil also was not generally affected, although there was an indication of salt accumulation in the field irrigated with the highest strength of treated sewage. Overall, the treated sewage irrigation to paddy rice culture was thought to be an acceptable process for final sewage disposal as long as it is adequately treated and used as a supplemental source of irrigation. Copyright © 2001 John Wiley & Sons, Ltd.