Here's a very useful, nearly useless piece of information. Maybe helpful for the OCD among us (me).
The average surface area of the top and bottom of a 5 gal bucket is about one square foot - if you forget about OCD precision and go with some rounding.
There are 43,560 square feet in an acre.
An acre-inch of water is about 27,154 gallons. Or, to put it another way, when it rains an inch, that's about how many gallons fall an each acre of your field. To wiggle an acre down to square feet, one square foot gets something like 0.6 gallons of water. So, your 5-gallon bucket adds nearly 8-inches equivalent rainfall - assuming all the world is a straight line.
The hydrodynamics (is there such a thing?) are a little different when you water by the bucket. The only point is, one bucket of water per tree seem more than adequate!
Wasn't that fun?