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Forest transpiration plays a vital role in the regional water budget and water supply security of the semi-arid Loess Plateau of northwest China. A thorough understanding and accurate predictions of the variation in the transpiration of forests with important tree species, e.g., <i>Armeniaca sibirica</i> (L.) Lam., are critical for land and water management. Owing to the extreme climatic seasonality and interannual variability, detailed information on the seasonal variation in the transpiration of <i>Armeniaca sibirica</i> plantations and its response to climatic and soil moisture/temperature fluctuations is limited. Therefore, in this study, the sap flux density and meteorological and soil moisture/temperature conditions were continuously monitored during the growing season (May to October) in 2019–2020. The results show the four following features: (1) The mean daily transpiration of the <i>Armeniaca sibirica</i> plantation was 0.31 mm·day⁻¹; (2) the daily transpiration varied nonlinearly with increasing potential evapotranspiration (PET). Transpiration first increased rapidly until reaching the PET threshold of 4 mm·day⁻¹ and then slowly increased within the PET range of 4–8.5 mm·day⁻¹, but thereafter, it decreased slightly when PET exceeded 8.5 mm·day⁻¹; (3) the daily forest transpiration varied with increasing relative extractable soil water content (REW) and soil temperature (ST) following a saturated exponential function; i.e., it first increased until reaching a threshold of 0.5 of REW or 14 °C of ST, but thereafter tended to stabilize; (4) models for estimating the daily forest transpiration were established. According to these models, PET had the greatest limiting effect (32.17%) on forest transpiration during the observation period, while REW and ST showed lower limits at 7.03% and 3.87%, respectively. The findings of this study are useful for understanding and managing the hydrological effects of forests in the semi-arid Loess Plateau as a typical dryland with seasonal droughts.