Atmospheric Rivers (ARs) are defined as corridors of enhanced integrated water vapor transport (IVT) and produce large fractions of annual precipitation in regions with complex terrain along the western coastlines of mid-latitude continents (e.g., 30-50% along the U.S. West Coast in California). This study investigates this relationship among landfalling ARs, IVT, and watershed mean areal precipitation (MAP) for a 38-year period over California. On average, the daily average IVT magnitude at different coastal locations explains ~34% of the variance in annual watershed MAP across all 140 watersheds with large spatial variability across California. Further investigation of the IVT magnitude and direction at coastal locations illustrated that adding water vapor transport direction increases the explained variance in annual MAP to an average of 45%, with highest values (~65%) occurring in watersheds over North-Coastal California. Similar investigation of the lowertropospheric water vapor flux vector at 850 hPa and 925 hPa revealed further increases in the explained variance in annual MAP to an average >50%. The results of this study (1) emphasize the importance of IVT direction and water vapor flux altitude to watershed MAP, (2) align well with previous studies for select locations that highlight the importance of upslope (i.e., lower tropospheric) water vapor flux during landfalling ARs and precipitation, and (3) motivate the development of AR-related and watershed-centric forecast tools that incorporate IVT direction and water vapor flux altitude parameters in addition to IVT magnitude.