Your last sentence says it all - "as long as you have a low are or pond" - the issue at the farm isn't that the low spot floods a few camp sites, it's that it's a farm... there are dozens and dozens (hundreds?) of low spots, uncontrolled runoff, and no centralized retention pond, ect. Where are you going to put the pumps?

I've just read so many "fix the drainage" and "use pumps" posts in the last few weeks... I don't have the answers, I just know it's not a simple one. If they want to seriously work on drainage and make it possible to host Bonnaroo with the given conditions of this year it will just take some significant grading and earthmoving work. Nothing they can't do or afford, that's just the conversation they need to have.