K. Inosako, I. Yoshida, A. Sugano1
Paddy fields are classified into two types in regard to water quality. One is a purification type and another is a pollution type. High concentration water is discharged from paddy fields where low concentration water is irrigated. That is, such paddy fields belong to a pollution type. Generally speaking, the water environment in a hilled rural area is good. These suggest that paddy fields in a hilled rural area have potential for being pollutant sources. Since a hilled rural area is closely related with a lower area through a river, the pollution of the area is an important problem for a regional water environment.
The purpose of this study is to understand the relation between the discharges (COD, Nitrogen, Phosphorus) from paddy fields in a hilled rural area and the regional water environment. Thus in this paper, paddy fields and a river in a hilled rural area are selected as the research areas.
As a result, the average CODMn, T-N, and T-P concentrations of the outflow water from a paddy field block were 2.6, 2.4, and 1.2 times those of the river, respectively. It was estimated that the outflow water makes the river water concentration increase by 7~8% in CODMn and 6% in T-N. Therefore, it is suggested that the CODMn concentration increased by the outflow water required the distance of 2 km for decreasing to the value before mixing by the calculation under the assumption that only self purification has an effect on purification of the river. It is clear that the outflow from paddy fields has a great influence on the river water. Therefore, it was suggested that it is important to prevent ponding water from ischarging to a receiving river by the appropriate water management of paddy fields.
