Seasonal Rainfall Variability in Reservoir Watersheds of Northern Taiwan

ABSTRACT The East Asian monsoon rainfall is crucial and serves as a sufficient water resource for reservoir watersheds in East Asia. Among them, the Feitsui Reservoir (FR) is the primary source of water supply in northern Taiwan, especially for Taipei City. As a result of global warming, Taiwan has experienced major droughts, resulting in insufficient water being stored in reservoirs, except in the FR. This study discovered that precipitation from tropical cyclones (TCs; 29.97% of annual rainfall) was not the dominant source of water in the Feitsui Reservoir Watershed (FRW); instead, the results indicated that the water resources of FRW were contributed by non‐TC rainfall (~70%), where the northeasterly monsoon resulted in heavy rainfall in autumn–spring (48.24%) and the mei‐yu and summer monsoon contributed to the others (21.79%). Due to the interaction between the monsoon and topography, asymmetric rainfall patterns were observed in this study. Specifically, rainfall from autumn to spring was concentrated in the eastern part of the FRW and northern Taiwan, while it was distributed on the opposite side during the mei‐yu and summer seasons. Under global warming conditions, spring rainfall in the FRW has shown a decreasing trend from 1990 to 2020, whereas a significant increase was observed in mei‐yu rainfall. Our findings explain the seasonal rainfall characteristics and regional climate variability in the FRW and northern Taiwan. This study can be used as a reference for evaluating strategies for adjusting water resources to achieve the ultimate goal of a stable water supply in Taiwan.