Water cycle

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The Water Cycle is the natural flow of water through the grand ecosystem (Earth). The amount of water on Earth remains relatively constant over time but the state of individual water molecules changes throughout the cycle.  Water undergoes the physical processes of evaporation, condensation, transpiration, sublimation, deposition, infiltration and precipitation as it travels into and out of the atmosphere and through a variety of reservoirs such as rivers, oceans, and glaciers.  Throughout the cycle, water takes the form of liquid, solid and gas.

Water is constantly in motion. Sometimes quickly, as in a fast-flowing river, but oftentimes it moves quite slowly, as in underground aquifers.  The water cycle is often shown as a simple circular cycle in which water evaporates from the ocean, is carried over land, falls as rain, and then is transported back to the ocean through rivers. This depiction tends to oversimplify the actual movement of water as there are many places, or compartments, where water may be found during its cycle.  The actual path any given water molecule follows within the complete water cycle can be quite varied and complex. Furthermore, simple water cycle figures are unable to convey the period of time that any given water molecule may spend within certain compartments.  For instance, the Antarctic Bottom Water, deep ocean water formed in the Antarctic, takes over 250 years to travel along the bottom of the Pacific Ocean before it re-surfaces in the Aleutian Islands.  (to address students' alternative conceptions of the simplistic water cycle model, see Water cycle game)

Water may change state from a liquid to a gas or solid as it travels along its path. Water in its liquid form is the most visible and obvious. Water can be seen flowing in rivers and surging in ocean waves.  Water even travels underground, though slowly, where it seeps through the spaces between grains of soil, sometimes coming to the surface as artesian springs. Although gravity works on ground water, geologic formations play a critical role in determining which direction water actually travels while underground.  Living organisms also move water about. Water, either directly consumed as liquid or extracted from food, is carried within bodies. It then leaves as a gas during respiration, is excreted, or may evaporate from the skin as perspiration.  Plants are the major biotic movers of water. Their roots collect water for distribution throughout the plant. Some is used in photosynthesis, but most travels to the leaves where it is easily evaporated or transpired. Plants and their roots are also a major determining factor in the ability of a landscape to retain surface runoff.  Although most water vapor cannot be seen, fog and clouds do give some indication of water vapor in the atmosphere. Water condensation, seen as early morning dew or even on a cold glass, is one visible example of the water vapor present in our air. In clouds, water molecules condense and collect on microscopic dust particles until they reach such a weight that gravity pulls the water down as precipitation.


Illustration of the Water Cycle

Physical Processes of the Water Cycle

  • Evaporation is the process by which liquid water turns into gaseous water vapor. Although lower air pressure helps promote evaporation, temperature is the primary factor. During the water cycle, some of the water in the oceans and freshwater bodies is warmed by the sun and evaporates, leaving impurities behind. As a result, the water that goes into the atmosphere is cleaner than it was on Earth.
  • Condensation is the opposite of evaporation: when a gas changes into a liquid. When the water droplets formed from condensation are very small, they remain suspended in the atmosphere. These millions of droplets form clouds in the sky or fog at ground level. Water condenses only when there are small dust particles present around which the droplet can form.
  • When the temperature and atmospheric pressure are right, small droplets of water in clouds form larger droplets and precipitation falls to Earth in a variety of forms (rain, snow, sleet, and hail).  Much of the water that returns to Earth as precipitation runs off the surface of the land, flowing downhill into streams, rivers, ponds, and lakes. Surface runoff is an important part of the water cycle because it results in a large portion of precipitation returning again to the oceans.
  • Infiltration is the process by which rainwater soaks into the ground, through the soil and underlying rock layers. Some of this water ultimately returns to the surface at springs or other low spots downhill; most of it remains underground (groundwater). As the water travels through the soil and rock layers, many of its impurities are filtered out, helping to clean the water.
  • As plants absorb water from the soil, that water moves from the roots through the stems to the leaves. Once the water reaches the leaves, some of it evaporates from the leaves, adding to the amount of water vapor in the air. This process of evaporation through plant leaves is called transpiration.
  • Sublimation occurs when solid water (snow or ice) changes directly to water vapor without going through the intermediate liquid phase.  This can be observed in the freezer when ice cubes seem to shrink in the trays without first melting.
  • Deposition is the opposite of sublimation:  when a gas changes directly to a solid (water vapor turns to ice).  This is commonly known as frost.

Handy Link for further information: http://www.planetguide.net/book/chapter_2/water_cycle.html