Abstract
An investigation on water quality and hydrology is conducted in an agricultural watershed (74.2 ha) dominated by rice paddy agriculture, in which a circular irrigation system is implemented and some species of aquatic-plants in the drainage river are removed. The mass balance of pollutants is clarified such as nitrogen, phosphorus and chemical oxygen demand (COD) for three years. In addition, the relationship between the amounts of nutrient contents in the removed aquatic-plants and the mass balance of pollutants is discussed. From results of water quality and hydrological investigation, it is shown that the study watershed processes purification function for phosphorus and COD, partially for nitrogen. The amounts of nutrients in the aquatic-plants are 0.937 kg/ha for total nitrogen (T-N), 0.057 kg/ha for total phosphorus (T-P) and 6.0 kg/ha for T-COD. However, these contributions of the aquatic-plant removal to pollutant reduction are relatively small because of only 1 % land occupation of the aquatic-plants in the watershed.
