Jinman, before the pond was built, I had an underground spring that brought water to the top of the ground. While we were digging the pond, we were in a severe drought, and never found the first drop of water. I thought sure we would find the spring, but never did.
I have about 15 acres surrounding the pond - mostly all woods. We get runoff from that acreage. This past summer we had more rainfall than in past summers, so the water level stayed even with the overflow pipe. It will be interesting to see how much the water level changes when we have a typical dry North Carolina summer.
Bob