Given that density altitude is a function of Pressure, Temperature, and Humidity, and most takeoff performance charts adjust takeoff distance as a function of pressure altitude and temperature, how is the humidity effect on density altitude taken into account?
Add 10% to your computed takeoff distance if it is too humid.
Humidity. Humidity is not generally considered a major factor in density altitude computations because the effect of humidity is related to engine power rather than aerodynamic efficiency. At high ambient temperatures, the atmosphere can retain a high water vapor content. For example, at 96°F, the water vapor content of the air can be eight (8) times as great as it is at 42°F. High density altitude and high humidity do not always go hand in hand. If high humidity does exist, however, it is wise to add 10 percent to your computed takeoff distance and anticipate a reduced climb rate.
Source: Density Altitude, FAA, 2008, faasafety.gov
It's worth noting that Airbus makes no mention of humidity in their performance guide, Getting to Grips with Aircraft Performance. So simply follow the FAA's advice, and any correction factors mentioned in your aircraft's flight manual.