Comment by schaefer
Yes, It's from farming. To state the obvious, it's the volume of water you'd have if a foot of rain fell on an acre of field.
So, it's the unit that gets used when discussing irrigation. Or water usage that competes with irrigation. :P
Makes sense, since we usually measure rainfall in inches, it's pretty easy to look up weather records for an area to see what the minimum annual rainfall is expected to be.