I read the link, but didn't understand it.
I found a link to a site that calculates runoff from this site.
I put in 20 acres of drainage and a 20% slope (double what I really think it is) with a coeificiant of 1 and its telling me a 5 inch pipe will work.

Does this sound right?
Average annual rainfall - 40"
Guessing 10 acres of run off into a 1 acre pond.

If this looks right, then removing the one pipe sounds like a good idea?

Thanks for the help!