They spray out of the military bases, the problem is the soldiers or civilians who service the plans just fill them with water. The only reason I'm so certain these planes were spraying is because you don't service the potable water/lavatories in the 10's of thousands of gallons. The planes are usually the Evergreen 747's or the all white, no tail number See Eye Ah planes.
Isn't a mystery, it's patented.