Actually, there is no "Drain". But I have drained the unit by removing the bleed adapter. Unfortunately, you have to take the unit apart to do that. (More work than it's worth) However, after the summer hot weather goes away, I will take it apart, drain the water, and clean out any dirt, since it can get nasty cold in my area, and ice could damage some parts in the unit.
BTW: The unit uses water in two ways. First the water evaporation cools the air, so water is used. Second, the bleed system removes minerals from the water that are built up due to evaporation.
Example: If you put some water in a glass and wait until the water is gone, you will see a residue in the bottom of the glass. The residue is the minerals in the water. What you don't see is as the water is evaporating the concentration of minerals in the left over water increases dramatically.
The bleed brings in "Fresh" water to dilute the concentration preventing the minerals from building up too fast on the cooling pads of the unit. All evaporative units have a bleed system.