Rainfall and snowmelt can runoff into streams or soak into the ground. The process of water soaking into the ground to become groundwater is known as groundwater recharge. The area on the surface where water soaks in is call the recharge area.
There are several ways that groundwater might be recharge by rain:
Rain soaks in where it falls and recharges the water table aquifer.
Rain soaks in where it fall and then moves into deep layers to recharge a confined aquifer.
Rain cannot soak in, so it moves over the ground as runoff into streams. When a stream flows through an area where water soaks in, some of the water from the stream may seep down to recharge the aquifer below.
Groundwater flows underground until it reaches a discharge zone, an area where the water is above the land surface. Springs are clearly visible discharge zones. Less obvious is the groundwater seeping into wetlands or contributing to stream flows.
If the water table is close to the land surface during the growing season, large amounts of groundwater might be withdrawn by plant roots and released into the air by plants. Evaporation can even discharge groundwater to the air in some areas.
Groundwater moves very slowly from recharge areas to discharge zones. Flow rates in aquifers are commonly measured in feet per day. It might take years, decades, or even centuries for water flow through some aquifers.
Flow rates are much faster where large openings exist, such as in fractured basalt or coarse gravel. In some cases, water might be underground only a few days or weeks before reaching the discharge zone.
Groundwater & Watersheds
Groundwater in a water table aquifer usually moves in the same direction as water flowing over the land surface. Therefore, it stays in the same watershed where the rain or snow originally fell. A watershed is the area drained by a single river system
Confined aquifers, which are much deeper than unconfined aquifers, sometimes are part of a regional groundwater flow system that does not match the surface drainage pattern. This allows groundwater to move from one watershed to another.
This section, including most illustrations, was adapted with permission from "What is Groundwater?" by Lyle Raymond, Jr. (© Cornell Cooperative Extension, Cornell University, July 1988).