Soil moisture (VWC) and atmospheric dryness (vapor pressure deficit, VPD) have an increasing impact on tree transpiration and productivity with climate change. However, the interaction of VWC and VPD on canopy transpiration (Ec) of plantations is still unclear, especially in water-scarce areas. Here, we selected Pinus tabulaeformis Carr. and Platycladus orientalis (L.) Franco plantations in the semiarid loess hills of China to examine the impacts of soil and atmospheric water conditions on Ec during the growing seasons (May–September) of 2015–2020. The results showed that the magnitudes of Ec were similar in the two plantations throughout the study period. The lowest Ec occurred in September because of little precipitation and low atmospheric evaporative demand, while Ec was highest in July due to the increase in soil water availability and atmospheric evaporative demand. There were obvious inflection points in the response relationship between VWC and VPD with Ec for both plantations. Under non-drought conditions, Ec reached a maximum in both plantations and was mainly impacted by solar radiation (Rs) and air temperature. As VPD increased, Rs still was the dominant factor, but canopy conductance decreased significantly, resulting in negative correlations between Ec and VPD. Under soil drought and compound drought conditions, Ec was mainly controlled by Rs and VWC, and the inhibitory effect of VPD on Ec was enhanced. In addition, the ratio of Ec to precipitation was close to 50% for both plantations at the interannual scale. Considering that soil water depletion in the Loess Plateau will accelerate with climate change and increasing drought events, the increasing water use stress will further hinder tree gas exchange, inhibiting tree growth. It is thus necessary to pay special attention to the impact of soil and atmospheric drought on water use for forest management in the Loess Plateau and similar regions of the world.