Groundwater

Rain soaks into the ground and seeps downward through tiny gaps between grains of sand, soil, and rock. That open space is called porosity, measured as the percent of the rock's volume that is empty. Below a certain depth, every pore is full, and the top surface of that fully soaked zone is the water table. Above it, pores hold a mix of air and water; below it, water alone. A well is just a hole drilled down past the water table so water can seep in and be pumped out.

Porosity alone does not make a good aquifer. The pores also have to connect to each other so water can actually flow through them toward a well; that property is permeability. A rock can be porous but nearly useless as an aquifer if its pores are sealed off from one another, like tiny isolated bubbles. The best aquifers, think loose sandstone or gravel, combine high porosity with high permeability, so they store plenty of water and let it move. This is also why groundwater drains slowly: it creeps through narrow, winding paths grain by grain, not rushing through open channels like a river in a cave.

That slow creep explains the drought puzzle: water filling a well today may have fallen as rain months or years ago, far upstream in the aquifer, and is only now arriving. It also connects to the water table's rise and fall. Recharge from rainfall refills the pores from above; pumping drains them from below. When a town pumps faster than rain recharges the aquifer, more water leaves than enters, and the water table sinks year after year.