Estimation of Nitrogen and Phosphorus Budgets in Nishinoko and Water Purification Function of the Constructed Wetland

[Synopsis] The purpose of this study is to estimate nitrogen and phosphorus budgets of Nishinoko and to quantify the water quality purification function in the constructed wetland that receives agricultural drainage before flowing to Nishinoko. The study had been conducted from February 2009 to January 2010. Water samplings and flow measurements were made twice a month during irrigation period and once a month during nonirrigation period. Measurement points were established at the lower reach of six rivers, and the inflow and outflow points of the wetland. As a result of analysis, total loadings to Nishinoko were approximated to 27 tons for T-P and 220 tons for T-N. Total outflow loadings out of Nishinoko to Biwako were approximately 180 tons for T-N and 26 tons for T-P. Those results revealed that Nishinoko played a role of water purification, accounting for 60％ and 70％ of annual T-N and T-P loadings to Nishinoko, respectively, during the irrigation period. The result of this study also showed that, on average, 5.4 kg/day or 24％ of T-N and 1.7kg/day or 30％ of T-P were removed in the constructed wetland