Trend Analysis of Water Quality in the Upper Watershed of the Feitsui Resrvoir

The Feitsui Reservoir is the main source of domestic water for the Taipei region; therefore, its water quality is fo great concern for the public and government authorities. Several water sample stations have been established in the Feitsui Reservoir watershed to monitor the long-term changes of water quality. The seasonal Kendall test, an extension method of the Mann-Kendall trend test, is used in this study to assess the water quality trends during the years 1993-2002 by analyzing seasonal data from five water sample stations located in the upper watershed of the Feitsui Reservoir. Water quality indices used in this research are suspended solids (SS) and ammonia nitrogen (NH3-N). In addition, the time-series diagram and box-and-whisker plot are also conducted to assess the spatial differences of the water quality trend among the five stations and the possible causes for the changes of their water qualities. The results of the time-series diagrams indicate that, in recent years, the water contents of SS and NH3-N in most stations have been increasing, and extreme values have been appearing more frequently. The box-whisker plot indicates that the water quality is getting worse in the Huang-Ju-Pi-Liao Sation and the Ping-Lin station, which is closely related to human activities in the watershed. Also, the high peak values of the NH3-N concentration coincided with the seasons of fertilizer application. The results of the seasonal kendall test show that, except for the SS at the Huang-Ju-Pi-Liao Sation, the concentration of SS and NH3-N have increased significantly at all five stations (α=0.05). Compared with the period before the construction of Taipei-Ilan freeway, the annual average values of SS and NH3-N at the five stations have increased by 2-3 times after the construction. This supports the fact that road construction has significant influence on water quality. Although the concentration of SS and NH3-N in the upper watershed of the Feitsui Reservoir is still within safety level, it is worth noting that trend analysis points to increasing decline of watr quality in the past decade.