抄録
TAMEIKE (irrigation ponds) have been constructed for the last 1,300years. The size of these ponds ranges from small to large. The majority are small. Some of the small TAMEIKE have been abandoned because new water resources for irrigation were developed. High concentrations of nitrogen were observed not only in some of these abandoned ponds but also in ponds that are currently in use. Then authors conducted a survey about these ponds. The results show that the use of watershed for these TAMEIKE plays an important rolls in regard to the nitrogen concentrations of the TAMEIKE. Therefore, watershed management for upland fields is important in order to reduce the nitrogen concentrations of the TAMEIKE. To preserve forest between the TAMEIKE and the watershed is an another importance.
