I agree with Heppy.

A good analogy would be if you built an in-ground hot tub beside your swimming pool and used a connecting pipe to fill the hot tub.

The hot tub will fill up to the water level of the swimming pool. However, if you open the pool drain the water level in both will go down. If you had a check valve in the connecting pipe, then only the water level in the pool would go down and your hot tub would stay full (except for evaporation losses).

Unfortunately, it is not possible to put a "check valve" on a spring. However, it might be possible for you to cheat the spring a little bit!

Consider the pipe between the hot tub and the pool. If that pipe is 1 foot from the top of the pool, then water will go over to your hot tub every time the water in the pool rises above the pipe. If the drain is subsequently opened on the pool, then your hot tub will only drain to the level of the pipe - even if the pool is completely drained.

If one of your springs into your pond is very well defined, then you might be able to control your pond water level. Was there a nice sand/gravel spot of water inflow that you could observe when your pond was excavated?

One option would be to seal the sides and bottom of your pond and then drive a sandpoint on a 2" pvc pipe back into the location of the spring. (I would even drive it at a slight downward angle.) When the groundwater level is high, the spring will fill your pond. When the groundwater level goes down, then your pond will only drop to the level of the pipe. (Assuming your pond was perfectly sealed.) You could then make up evaporation losses with your hose if needed.

You could do a similar arrangement by building a gravel trench into the area of the spring (like a french drain) and then having a pipe connection to your pond. I would definitely recommend anti-seep collars in that situation to ensure a better seal for your pond.

Finally, you could even attempt to compact and seal your pond to the elevation of the springs, but then leave everything above that level uncompacted.

All of my hare-brained schemes are easier the more clearly the limits of your springs are defined.

Good luck on improving your pond project! It sounds like you have turned lemons into lemonade.

P.S. Another option is to just observe your water level for the first year. It may turn out that the low point of the groundwater level is still perfectly acceptable for your usage of the pond.