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The Hydrologic
Cycle
The Hydrologic Cycle involves the
continuous circulation of water in the Earth-atmosphere system. Of the many
processes involved in the hydrologic cycle, the most important are
- evaporation,
- transpiration,
- condensation,
- precipitation, and
- runoff
University
of Maryland Institute Center for Environmental Science Graphic
The
hydrologic cycle consists of inflows, outflows, and storage. Inflows add
water to the different parts of the hydrologic system, while outflows
remove water. Storage is the retention of water by parts of the system.
Because water movement is cyclical, an inflow for one part of the system
is an outflow for another. Looking at an aquifer as an example,
percolation of water into the ground is an inflow to the aquifer.
Discharge of ground water from the aquifer to a stream is an outflow (also
an inflow for the stream). Over time, if inflows to the aquifer are
greater than its outflows, the amount of water stored in the aquifer will
increase. Conversely, if the inflows to the aquifer are less than the
outflows, the amount of water stored decreases. Inflows and outflows can
occur naturally or result from human activity.
NASA
Graphic
Evaporation
Evaporation is the change of state in a substance from a liquid to a gas. In
meteorology, the substance we are concerned about the most is water. For
evaporation to take place, energy is required. The energy can come from any
source; the sun, the atmosphere, the earth, or objects on the earth such as
humans.
Everyone has experienced evaporation personally. When the body heats up due to
the air temperature or through exercise, the body sweats, secreting water onto
the skin. The purpose is to cause the body to use its heat to evaporate the
liquid, thereby removing heat and cooling the body. It is the same effect that
can be seen when you step out of a shower or swimming pool. The coolness you
feel is from the removing of bodily heat to evaporate the water on your skin.
Transpiration
Transpiration is the evaporation of water from plants through stomata. Stomata
are small openings found on the underside of leaves that are connected to
vascular plant tissues. In most plants, transpiration is a passive process
largely controlled by the humidity of the atmosphere and the moisture content of
the soil. Of the transpired water passing through a plant only 1% is used in
the growth process of the plant. The remaining 99% is passed into the
atmosphere.
Condensation
Condensation is the process whereby water vapor in the atmosphere is returned to
its original liquid state. In the atmosphere, condensation may appear as clouds,
fog, mist, dew or frost, depending upon the physical conditions of the
atmosphere. Condensation is not a matter of one particular temperature but of a
difference between two temperatures; the air temperature and the dewpoint
temperature.
The
water table is the top of the zone of saturation and intersects the land surface
at lakes and streams. Above the water table lies the zone of aeration and soil
moisture belt, which supplies much of the water needed by plants. (Image by
Hailey King, NASA GSFC.)
Precipitation
Precipitation is the result when the tiny condensation particles grow too large,
through collision and coalesce, for the rising air to support, and thus fall to
the earth.
Runoff
Runoff occurs when there is excessive precipitation and the ground is saturated
(cannot absorb anymore water). This runoff flows into streams and rivers and
eventually back into the sea.
Evaporation of this runoff into the atmosphere begins the hydrologic cycle over
again. Some of the water perculates into the soil and into the ground water only
to be drawn into plants again for transpiration to take place.
Credit: EPA, UNEP,
USGS, NASA, University of Maryland, National Weather Service |
