Long-term seasonal trends of streamflow in Southern Taiwan

The precipitation is concentrated in the wet season, which accounts for 90% of the total precipitation in southern Taiwan. Thus, the temporal and spatial distributions of precipitation are extremely uneven, making careful management of water resources in Taiwan crucial. According to the previous study, about 72% of the precipitation becomes streamflow which is one of the main water resource in human life. Therefore, it is an important issue to conduct on trends and variation of streamflow. Four statistical tests, such as Mann-Kendall test, Theil-Sen's slope estimator, Mann-Whitney-Pettit statistics, and Cumulative Deviation method, are adapted to analyze the trend characteristics for 9 gauging stations in southern Taiwan. The results indicate that more than 70% of gauging stations in the streamflow and the baseflow are increasing in the monthly average flow during dry period in the Bazhang River Zengwen River, and Yanshui River basins. The decreasing trends are centered in the Gaoping River basin and the Donggang River basin. Simultaneously, about 65% of the changing points were in 1990 to 2000 and the differences of the average flow before and after the chaning points were large in low-flow and dry period.