As rice paddies are widespread sources of water pollution in the agricultural regions of the Asian monsoon area, a mechanistic understanding of nutrient loss from paddies is critical for water quality management. A 2-year experiment was conducted in a typical monsoon-affected rice field to improve our understanding of the impacts of rainfall and agricultural management practice on nitrogen (N) and phosphorus (P) loss. Samples of paddy drainage water were collected during rainfall events (n=25) and analyzed for total N (T-N) and total P (T-P) concentrations. The impacts of rainfall (amount, duration, and intensity) and agricultural management practice (transplanting and fertilization) on the event mean concentration (EMC) and loss of nutrient were assessed using regression analyses. The results showed that T-N and T-P concentrations were affected by agricultural practice; meanwhile, loss of T-N and T-P was correlated with rainfall characteristics. Specifically, the EMC of T-N but T-P was negatively (p<0.001) correlated with the number of days after agricultural practice in both years, which likely represents a decrease in nutrient availability in paddy water over time. Loss of T-N and T-P was positively (p<0.01) correlated with rainfall amount, and this suggests that the rainfall-runoff process is a key driver of nutrient loss in the study area. Our results suggest that rainfall amount and days after transplanting need to be taken into account when estimating nutrient loss from rice paddies in monsoon regions.

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Effective rainfall, which is used in paddy fields under farmers' current water management practices, was monitored during the 2008-2012 period in southern Korea. The two paddy fields (HY and JS) studied have an area of 13.69 and 8.06 ha, respectively and are composed of several paddy plots separated by irrigation canals and drains. At the HY site, the effective rainfall ranged from 446 to 794 mm and averaged 567 mm. At the JS site, effective rainfall ranged from 288 to 553 mm and averaged 426 mm. Approximately 64 and 45% of the rainfall for HY and JS, respectively, was used for crop growing in the paddy fields. These amounts were less than the recommended rainwater harvesting criteria in Korea. Since the height of the bund for each paddy plot is approximately 100-300 mm, the plots have sufficient space to hold rainfall; despite this, the field data indicate that rainfall was not used efficiently. This study confirms that farmers have little incentive to use rainfall effectively for rice paddy culture because they can use irrigation water at no charge. Some measures, such as water pricing, incentives for rainwater harvesting, improvement of drainage facilities, and further monitoring of water-use practices need to be adopted to improve water management efficiency. Copyright © 2016 John Wiley & Sons, Ltd. RÉSUMÉLa pluie efficace, qui est utilisée dans les rizières comme révélateur des pratiques de gestion de l'eau des agriculteurs, a été suivie au cours de la période 2008-2012 en Corée du Sud. Les deux champs de paddy (HY et JS) étudiés ont une superficie de 13.69 et 8.06 ha, respectivement, et sont composées de plusieurs parcelles de rizières séparées par des canaux d'irrigation et des drains. Sur le site HY, la pluie efficace variait de 446 à 794 mm pour 567 mm en moyenne. Sur le site JS, la pluie efficace variait de 288 à 553 mm pour 426 mm en moyenne. Environ 64 et 45% des précipitations pour HY et JS, respectivement, ont été utilisés pour la culture dans les rizières. Ces montants sont inférieurs aux recommandations en vigueur en Corée. Sachant que la hauteur des diguettes en limite de parcelles de paddy est d'environ 100 à 300 mm, il apparait que les parcelles ont suffisamment de volume pour contenir précipitations; ce qui indique que les précipitations n'ont pas été utilisées de manière efficace. Cette étude confirme que les agriculteurs sont peu incités à utiliser efficacement les précipi-tations pour la culture de riz paddy, car ils peuvent utiliser l'eau d'irrigation sans frais. Certaines mesures, comme la tarification de l'eau, des mesures incitatives pour la récolte de l'eau de pluie, l'amélioration des installations de drainage, et la poursuite de la surveillance des pratiques de consommation d'eau doivent être adoptées pour améliorer l'efficacité de la gestion de l'eau.

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