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The Hydrologic Cycle
Water is the source of all life on
earth. The distribution of water varies from place to place. Some locations have plenty of
water, others have very little. Water exists on earth as a solid (ice) and in liquid or
gas form (water vapor).
Image adapted from:
http://danpatch.ecn.purdue.edu/~epados/ground/src/title.htm,
Ground Water Primer EPA Region 5 and Agricultural & Biological Engineering, Purdue
University website.
Water transport takes place on or below
the earth's surface by flowing glaciers, rivers, and ground water flow. Oceans, rivers,
clouds, and rain, all of which contain water, are continuously changing; surface water
evaporates, cloud water precipitates and rainfall infiltrates the ground, etc. The cyclic
transfer of water vapor from the earth's surface via evapotranspiration into the
atmosphere, from the atmosphere via precipitation back to earth, and through runoff into
streams, rivers, and lakes, and ultimately into the oceans, is called the Hydrologic Cycle -
five main processes* make up this cycle:
 Condensation
- the process of water vapor in the air turning into liquid water
Precipitation
- any or all forms of water particles that fall from the atmosphere, such as rain, snow,
hail, and sleet.
Infiltration - movement of
water, typically downward, into soil or porous rock.
Runoff - excess rainwater or snowmelt
that is transported to streams by overland flow, tile drains, or ground water.
Evapotranspiration - a collective term
that includes water lost through evaporation from the soil and surface-water bodies and by
plant transpiration.
The continuous circulation of water,
powered by the sun which evaporates water from oceans, rivers, lakes, and even from trees
and land, is fundamental to the availability of water. Water that sinks into the soil
flows through underground reservoirs, or aquifers, as groundwater. This same water
sustains life.
Therefore how we take care of our air,
soil, and land is directly related to the quality and quantity of water we need for
survival.
*Definitions from the National
Water-Quality Assessment Glossary; http://water.usgs.gov/nawqa/glos.html
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