Evaluation of the amount of nitrogen and phosphorus runoff load from rivers based on the high-frequency survey

Abstract

The investigations of the water quality of the Umeda River in the Aichi prefecture of Toyohashi city, were examined once a day for one year and more intensively at the time of 18 rainfall events to evaluate the annual runoff loads of the total nitrogen (TN) and total phosphorous (TP). The river water level was recorded automatically at 10-minute intervals. Compared with clear weather days, loads of TN and TP during heavy rain increased 28 and 102 times at most. The appropriate annual runoff loads of TN and TP were calculated using the following methods: the constant-concentration/flow method, the constant concentration method, and the LQ method using either the logarithmic method or the direct method. Annual runoff loads of TN and TP obtained from a detailed survey were 474t/year and 60t/year. A better estimated annual load of TN was obtained by LQ methods (the direct method) based on low flow and high flow, respectively. On the other hand, the annual load of TP was obtained by the LQ method (the direct method). Continuous observation data will be needed to obtain the appropriate values for the load from diffused pollution sources.