Long-term changes in pollutant load outflows and purification function in a paddy field watershed using a circular irrigation system

A long-term investigation on the water quality and hydrology was carried out for 8 years and 7 months (from October 1991 to April 2000) in a paddy field watershed using a circular irrigation system. The annual amount of rainfall ranged from 1270 to 2226 mm and it was found that the amount of irrigation water tended to decrease as rainfall increased. Phosphorus and chemical oxygen demand (COD) concentrations tended to decrease with the river flowing down, whereas nitrogen concentrations showed no significant difference. The annual outflow loads (sum of the net-outflow load during irrigation periods and the outflow load during non-irrigation periods) of total nitrogen (T-N), total phosphorus (T-P), and COD ranged from 13.6 to 75.3 kg ha−1 yr−1, −3.55 to 2.21 kg ha−1 yr−1, and −24.7 to 48.5 kg ha−1 yr−1, respectively. The negative values for T-P and COD loads indicated that the study watershed performed a purification function. The change in annual pollutant loads was primarily attributed to the amount of hydrological water volumes (the annual amount of rainfall or that of rainfall plus irrigation water) for T-N and COD loads and partially for T-P load. In addition, the purification function was related to the hydraulic retention time, and the study watershed allows sufficient retention for pollutant purification for phosphorus and COD contents and partially for nitrogen content.