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Groundwater – A major link in the hydrologic cycle
Groundwater circulates as part of the hydrologic cycle. As precipitation and other surface water sources recharge the groundwater it drains steadily, and sometimes very slowly, towards its discharge point.
Groundwater does not stay underground forever, and it does not lie still waiting for us to draw it from a well. The hydrologic cycle is the series of transformations that occur in the circulation of water from the atmosphere onto the surface and into the subsurface regions of the earth, and then back from the surface to the atmosphere. Precipitation becomes surface water, soil moisture, and groundwater. Groundwater circulates back to the surface, and from the surface all water returns to the atmosphere through evaporation and transpiration.
When precipitation falls on the land surface, part of the water runs off into the lakes and rivers. Some of the water from melting snow and from rainfall seeps into the soil and percolates into the saturated zone. This process is called recharge. Places where recharge occurs are referred to as recharge areas. 
Eventually, this water reappears above the ground. This is called discharge. Groundwater may flow into streams, rivers, marshes, lakes and oceans, or it may discharge in the form of springs and flowing wells.
Groundwater discharge can contribute significantly to surface water flow. In dry periods, the flow of some streams may be supplied entirely by groundwater. At all times of the year, in fact, the nature of underground formations has a profound effect on the volume of surface runoff. While the rate of discharge determines the volume of water moving from the saturated zone into streams, the rate of recharge determines the volume of water running over the surface. When it rains, for instance, the volume of water running into streams and rivers depends on how much rainfall the underground materials can absorb. When there is more water on the surface than can be absorbed into the groundwater zone, it runs off into streams and lakes.
The residence time of groundwater, i.e., the length of time water spends in the groundwater portion of the hydrologic cycle, varies enormously. Water may spend as little as days or weeks underground, or as much as 10 000 or more years. Residence times of tens, hundreds, or even thousands of years are not unusual. By comparison, the average turnover time of river water, or the time it takes the water in rivers to completely replace itself, is about two weeks.
This table illustrates the estimated depth and residence time of the world's water supply:
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