The processes involved in water entering and leaving the groundwater system are known as recharge and discharge.
Processes of aquifer recharge and discharge can occur both naturally or be influenced by human activity. For example, in some parts of Australia surface water is injected into the ground through man-made boreholes, so that water is stored and used more efficiently than it is at the surface (known as Managed Aquifer Recharge). Groundwater pumping and use for town water supplies or agricultural purposes are examples of artificial discharge.
Recharge occurs when surface water, either from direct precipitation or from rivers and lakes, percolates downwards through the microscopic spaces in the soil and rock profile. Eventually, the infiltrated water may make its way into an underground water-bearing rock formation, known as an aquifer. Recharge mainly occurs in areas where parts of the aquifer are exposed at or close to the surface. Recharge is commonly expressed as the amount of water which fills an aquifer over a given period of time, and is usually measured in millimetres-per-year.
Discharge represents the outflow of groundwater from underground aquifers. This can occur diffusely across the landscape, such as when vegetation is reliant on groundwater. Discharge also occurs at point-source areas, such as natural springs. In some cases, groundwater also discharges into surface water bodies such as rivers and lakes.