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• Rainfall amount, duration, and intensity significantly affect Ec from diurnal to seasonal timescales. • Rainfall timing has a considerable impact on the magnitude, timing, and intra-daily distribution of Ec. • The responses of Ec to rainfall vary among canopy phenology periods. • The driving mechanisms of biophysical variables to Ec are more complex in days without rainfall events. Rainfall patterns affect vegetation water use through canopy transpiration (Ec) and are becoming increasingly variable due to global warming. However, Ec and its controlling mechanisms in boreal forests have rarely been investigated under various rainfall patterns. In this study, we measured the sap flow using thermal dissipation method in a boreal larch forest (Larix gmelinii) and observed biophysical variables simultaneously (e.g., air temperature, solar radiation, relative humidity, vapor pressure deficit, wind speed, soil water content, rainfall, and leaf area index) over two consecutive growing seasons (May–September) in 2021 and 2022. The responses of Ec to rainfall patterns were evaluated among three rainfall categories (classified based on rainfall amount, duration, and intensity), five types of rainfall timing, and two phenology periods (i.e., leaf expansion and defoliation periods). The results showed that daytime Ec significantly decreased, while nighttime Ec was less variable with increasing rainfall amount, duration, and intensity. The responses of Ec to rainfall categories with different rainfall amount, duration, and intensity were inconsistent at the monthly scale. The magnitude and diurnal dynamics of Ec were greatly influenced by the rainfall timing, and these effects got more considerable with increasing rainfall amount, duration, and intensity. Surprisingly, Ec showed opposite responses to rainfall in leaf expansion and defoliation periods, suggesting an enhanced impact in the leaf expansion period while a diminished impact in the leaf defoliation period. The significantly affected biophysical factors of Ec varied dramatically among rainfall categories. The interactions between biophysical factors and Ec were more complex and diverse in days without rainfall events than days with rainfall days. These findings highlight the importance of rainfall patterns in affecting Ec, provide new insight into the interactions between rainfall patterns, Ec, and biophysical factors, and are important supports for improving evapotranspiration simulations in the context of climate change. [ABSTRACT FROM AUTHOR]