wivell, my pond is a little less than 1/2 acre I think. There are some small ground water springs that didn't run for the last 2 years because of the drought. The springs kept my pond full for that past month while some of my neighbor's ponds dropped a few inches.
I'm embarrassed to admit it, but I still don't know for sure what the size of the watershed is. It may be 30 acres or even a little more.
Here's a video of the flow coming into the pond through the 20 inch culvert after 3 or 4 inches of rain. At the time the water was about halfway up the opening of the pipe. There is about 6 - 8 acres that run directly into the pond without passing through the pipe.
The 6 inch siphon moves a lot of water after it gets primed. My pond was at full pool when we got 4 inches of rain 2 days ago and the siphon kept up with the inflow for most of the day. When we hit the 3.7 inch mark the water started going over the emergency spillway. I had to leave the property shortly after that, but from what I can tell by the leaves and sand the emergency spillway didn't run very long.
I don't suppose that it's unreasonable to have water going over the spillway after getting a 4 inch rain, so I don't fault the pond builder for sizing the pipe at 6 inches. In a conversation with the builder I mentioned that I might want to install an 8 inch pipe myself, but that I didn't expect him to do it. I explained to him that I think most people would be fine with the system he installed, but that I was probably more fussy than most about water going over the spillway (because of outside fish migrating into my pond). When he heard my preference he insisted on installing the larger pipe himself. Anyone who gets a pond builder like the one we found is very lucky.
