Moisture transport within atmospheric rivers is driven by a complex combination of processes, including convergence of moisture from different origins which change over the atmospheric river’s life cycle. The water vapor budget within an atmospheric river enables us to understand moisture sources and sinks (horizontal flux, evaporation and precipitation). Here, we focused on the water vapor budget of the exceptional atmospheric river associated with the storm Dennis that led to record-breaking precipitation on February 15th 2020. We used the WRF model to simulate the event and applied our new water vapor budget approach to the tracked atmospheric river. We also performed two sets of sensitivity experiments: one reducing the tropical moisture, and the other modifying the ocean evaporation to assess how these two main moisture sources affect the water vapor balance within the atmospheric river. We also study changes in the atmospheric river, cyclone and associated precipitation at landfall in the sensitivity experiments. For Dennis, tropical moisture played a prominent role in the early stages of the atmospheric river, while ocean evaporation became critical later. Additionally, the reduction of evaporation and also of tropical moisture is related to a decrease in precipitation over Europe. This study offers a new approach to understanding the evolution of atmospheric rivers and highlights the importance of different moisture processes. It provides a case study that helps to unravel feedback mechanisms and the impact of different perturbations on the water vapor balance of atmospheric rivers.