My last pond fills almost exclusively through subsurface water, but I cored the upper area to force the subsurface water to enter above the full pool level so the pond will not lose water during dry periods. It has not ever dropped more than a foot in its first 3 years. We have water permeable soil lenses and pockets that make pond building "interesting" on our place, but once built correctly, the ponds hold water great. Had to pull the backhoe out of one of these pockets on the pond currently under construction. Its kinda stressful during the builds...
