Groundwater is water that has collected at some point. Lake, aquifer, whatever. Over X many years rain has pooled in this spot.
If there is X amount of rain coming in each year and you use less than that, by sending it on down the river/whatever no worries. (as long as you’re not dumping things in the river that are gonna suck for people downriver.
If you use more than that, well there’s going to be less water in the groundwater next year. Also the people downriver probably don’t get as much water, so they’re groundwater will also probably be lessened if they don’t cut back.
Groundwater tends to be millions upon millions of gallons. It takes a while to use up, especially since it’s being replenished occasionally.
But if you’re using more than is coming in it doesn’t matter that it will “eventually” come back around. At some point there’s going to be a dry spot in the loop where previously there’s been a water deposit.