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:
 Groundwater
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