Stratospheric water vapor represents a major uncertainty for current climate simulations. We assess the robustness of simulated stratospheric water vapor and its trends by using the Lagrangian chemical transport model CLaMS driven by both ERA-Interim and JRA-55 reanalysis data. Based on these simulations, we particularly analyze the impact of the quasi-biennial oscillation (QBO) and of Major Stratospheric Warmings (SSW) on the amount of water vapor entering the stratosphere during boreal winter. According to the ERA-driven simulation, the amplitude of water vapor variations related to the QBO amounts to 0.5 ppmv. The additional effect of SSWs strongly depends on the QBO phase, with a pronounced drying of about 0.3 ppmv about 3 weeks after the SSW during easterly QBO phase, and a weaker drying effect during westerly QBO phase. We discuss, the robustness of these results and of long-term water vapor trends with respect to the meteorological reanalysis data used. Furthermore, sensitivities of stratospheric water vapor are quantified with respect to the dehydration scheme (e.g., nucleation barrier, ice sedimentation), and to small-scale mixing by varying the parameterized mixing strength in the CLaMS model.