Entropy of a Vaidya black hole

In the case of the brick wall model for a static black hole, it is shown that the Bekenstein-Hawking entropy with the geometric character comes from the contributions of fields in the vicinity of the horizon. According to this idea, the entropy of a Vaidya black hole as the simplest nonstatic case is calculated by using the assumption of local equilibrium near the horizon. The difference from the static hole is that the result proportional to the area of the horizon relies on a time-dependent cutoff. We analyze the notion of the local equilibrium in black hole thermodynamics and find that it is maintained for the holes with a mass far greater than the Planck scale.