Mann-Kendall test and Theil-Sen estimator for long-term spatial and temporal trends of streamflow in Taiwan

Climate changes have significantly altered the global precipitation pattern in recent years, resulting in uneven temporal and spatial distributions of precipitation, and even precipitation polarization. Taiwan, located in a subtropical climatic zone, experiences discernible dry and wet seasons, making the polarization of precipitation in the country much more obvious. Since stream flow is strongly influenced by precipitation, such polarization results in great stream flow differences between the wet and dry seasons. Thus, it is essential to investigate the trend and variability of stream flow. Little research has been done on trend characteristic analysis of annual average flow in the Taiwan area. This study focused on stream flow data gathered over 30 years at 34 stations located in 4 water resource districts of Taiwan. The Mann-Kendall test was employed in the trend analysis of annual average flow, while the trend slope was assessed using the Theil-Sen estimator, thereby calculating long-term variation in flow at every station. Our observations showed no remarkable flow trend variations in South Taiwan, while only two out of eight flow stations in north Taiwan exhibited significant trend oscillations, namely Ximen Bridge and Fu-Shan stations in Lanyang and Danshui rivers, respectively In central Taiwan, two out of six stations, i.e., Ta-Lu-keng and Sung-Mao stations in Houlong and Dajia rivers, respectively, showed markedly increasing trends. Among 11 flow stations in east Taiwan, Chih-Pen and Ma-An-Chi stations in Zhiben and Hualian rivers, respectively, demonstrated significant flow strengthening, whereas Yen-Ping station in Beinan river reflected the opposite behavior. The results of this study provide references for future water resource planning in hydrology studies at the regional scale.