Your county may have a gis system that shows elevation contours. Usually these systems have tools to measure area. If you stare at the contours for a few minutes, you should be able to figure out how to draw a line that separates water that would run into your pond vs water that runs away from it. The line should form a loop. The area inside the loop is your watershed.

I'm not super experienced with this stuff, but it seems you may want to keep the pond quite small, maybe even smaller, since it doesn't sound like you're dealing with a lot of watershed. Maybe deepen it and surround it with good sized rocks so the water level can change 6-12" without looking anemic and exposing muddy banks. Or maybe you have plenty of inflow and the soil is just leaky...

Last edited by crazyj; 12/20/16 09:20 PM.