Abstract: With the increasing demand for continuous water quality monitoring, the layout of online water quality monitoring stations becomes a more significant issue. This study uses the Hydrologic Simulation Program Fortran (HSPF) model to simulate the quantity and quality of water in the Tamsui River basin, Taiwan. Of the nine hydrological stations compared, most have a Nash-Sutcliffe efficiency coefficient greater than 0.7, and all have a correlation coefficient greater than 0.7. At most of the 27 water quality stations compared, the symmetric mean absolute percentage errors are below 25% for the simulations of biochemical oxygen demand, ammonia nitrogen, suspended solid, and dissolved oxygen. Based on the change rate and substandard rate of water quality, we suggest a concept of water quality-sensitive river section to determine a prior arrangement of the online water quality monitoring locations. We point out the shortage of the existing monitoring locations and recommend to add a new station separately at Sansia Bridge on the Sansia Tributary, Baoqiao Bridge on the Jingmei Tributary, and Jiangpei Bridge on the Keelung Tributary.

Key words: water quality simulation, online monitoring, site layout, water quality-sensitive river section, Tamsui River