I would have thought that as evaporation is a function of temp/humidity and time that the impotant thing is to get the required amount of water in a given time. To acheive this one could use one large bucket or a chain of smaller buckets. the advantage of the smaller buckets being that any one does not significantly reduce the effort if it is unavailable, and there is a contiuous stream of water arriving
.When say attacking a flame front, an aproach can be set up with a chain of aircraft arriving sequentially allowing a continuous
pattern of overlapping drops to be made on the front.
One aircraft, of any capacity cannot make such an effort.