The math is simple. If you can figure out what the area is that drains into your pond and the surface area of your pond, you can calculate how much your pond should rise with a given rainfall. If the drainage area is 10x the size of your pond, your pond should rise 10x the inches of rain, plus the rain that actually falls onto your pond. But, unless the sides of your pond are vertical, the area of your pond will increase as it fills. If the ground is saturated, that water will move too, but not necessarily in the same direction as the surface runoff, and very slowly. If your pond has a cut bank on the up slope side and the bed or beds your pond is built in slope into your pond, and there is hydrologic head in those beds, and the beds are sealed on the downslope side of your pond, there might be a slight net contribution to your pond. That is, you might see it continue to rise slightly after the surface runoff stops.
