The water table does not freeze, and your pumping it down will just be a waste of time. I would guess that the purpose of it is in the offchance that you would have to drain your pool, you would want to drop a pump in there and get the water down so the pool does not float.

And in case you ever do have to pump the water down, you need to pump for as long as it takes to get the water to slow down coming into the pit, which would be better known as a "well-point".