Standard equation is: rainfall, minus evaporation and seepage will get you net gain/loss of pond water levels. If you have a spring, then this could make seepage a negative number. If this equation yields a pond filling at a rate that you find satisfactory, then you'll be happy. If not, then you'll need an alternate source for pond water. I fill a couple of my ponds with well water, so they don't need a watershed. Otherwise you would need runoff from a watershed to fill your pond.
