To answer the ditch pipe question, the water level will balance between the pond and the ditch level....If the water in the ditch is higher than your pond, it will flow into the pond, otherwise the water will flow out of your pond into the ditch. The pipe being undewater has nothing to do with it.
I probably didn't describe the plan accurately. Here is a sketch:
The two piping options are in brown and red. The ditch is shown full, and the pond is almost empty. From what little I remember from physics, until the level of the pond reaches the outlet of the pipe, the flow would be proportional to the head, which would be 1 foot for the brown pipe or 3 feet for the red one. Once the pond reaches the outlet of the pipe, then the head would be just the difference in levels for the ditch and the pond.
One thing to consider when having the pipe under water in your pond is that fish can swim out of it as the levels balance and the flow slows.
Good points. I would have the pipe entrance slightly above the pond full-water line, so there should not be any back flow when the ditch is empty. Maybe I should put a screen over the outlet to keep critters from trying to swim into the pipe.
I suspect with normal rain amounts in your area, a 10 acre watershed is adequate, but your local NRCS agent should be happy to let you know for sure. Having an extra,controlable water supply could never be a bad thing!
The pond ought to look great! What species of birds are the nesting houses for?
We had a pretty severe drought here until recently, and a lot of the nearby ponds dropped severely. But even during the drought, we would get an occasional rain that would fill the ditch for a few hours. I thought that backup water source would help during dry periods.
The bird house is for purple martins. During nesting season I have WEB cameras in one of the gourds and overlooking the house. Right now the cameras are just pointing towards the pond, in case anyone wants to monitor the progress.
http://www.purplemartin.tv/charles_branch_combined/default.htm
