If you have a ground water pond then the pond will seek the level that matches the water table. If your pond has more water than surrounding water levels it will go down and if you get rain, run off or the water table comes up so will your pond. You should mount an aluminum ruler on a stake or mark a PVC pipe and drive it in the shallows of the pond. Track your water height up and down for a while and you'll get a feel for what the factors are that make your water go up and down.
As temps go up you will lose some from sun/evaporation and those with wind exposure will lose more from evaporation than those with sheltered ponds.
IF you don't like the level your pond is at now (and it probably is about where it is going to be since you haven't mentioned super heavy rains or unusually hot weather).. Then you either add to it with a well to keep it above the level it wants to seek or you plug access to the ground water table. Adding water works best if you have a big well and efficient well motor and a good power source near the pond. Some use gasoline powered pumps connected to a nearby stream. It can get costly especially since you CAN raise your pond level, but you usually CANNOT raise the water table level all around you.
Most of use either learned to like the level that our pond sought for itself, or we used soilfloc powdered floculant/polymer to seal the bottom of the pond and then could more easily keep it at a higher level.