How does rain refill a groundwater supply?

A town's well draws water from an aquifer: a layer of sediment or rock whose connected spaces let water move. New water can enter this underground supply, but rain falling above it does not all arrive at the well. Refilling depends on the route water takes.

First, rain must enter the ground. Some stays near the surface, where roots take it up or evaporation returns it to the air. Water that continues downward may eventually reach the saturated zone, where the connected spaces are filled with water. This addition is called recharge. The top of a shallow, unconfined saturated zone is the water table.

Recharge can happen over a broad area or through particular places such as a streambed. A layer that lets water through easily provides a different route from a thick clay layer that slows it. A deeper aquifer may receive much of its water where its rock layer reaches the surface far away.

The refill and the withdrawal do not necessarily happen together. A well may remove water today that entered the ground long ago. Recharge also has to help support springs and streams, not just wells. Understanding a groundwater supply therefore means following water into, through and out of an entire landscape.