Application of sediment conservation health diagnosis indicators in watersheds—example of the Taipei water source domain

Health diagnosis indicators including runoff ratio, denudation depth, vegetation cover ratio, turbidity, and sediment concentration were used to assess sediment conservation problems in the Taipei Water Source Domain. Apart from comparing the health of various sub-watersheds based on health indicators, reference values of health indicators were determined and used to assess changes in the health of individual sub-watersheds. According to the results, the average runoff ratios during dry season at the Feitsui Reservoir and Quchi gauging station were 1.15 and 0.76, which indicated the area has excellent water retention capacity. The denudation depth in the Zhakong River sub-watershed, the chief source of sediment in the watershed, was 1.22 cm/year. A forest cover ratio of 92.59% throughout the whole watershed indicated vegetation cover was generally very good. Turbidity and sediment concentration at the Fushan station were higher than those at other stations, and commonly soared after typhoons or torrential rains, which affected downstream tap water quality. In summary, the whole watershed has excellent water retention capacity and good vegetation cover; the chief problem is the presence of landslides in the Zhakong River sub-watershed, which causes downstream turbidity to soar in the wake of typhoons and torrential rains.