The Water Cycle

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The Water Cycle
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Evaporation

• Water on Earth's surface enters the atmosphere
  through evaporation. Heat energy from the Sun
  breaks the bonds that hold water molecules
  together. The free molecules spread out and
  become a gas.

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Evaporation

• The Sun is the driving force behind the water
  cycle, which begins with evaporation. Nearly 90
  of moisture in the atmosphere evaporates from
  oceans, seas, lakes, and rivers.

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Evaporation

• On a global scale, the amount of evaporated water
  equals the amount of precipitation. This keeps
  the water cycle in balance. Although global
  evaporation equals global precipitation, it is
  not equally distributed over the earth.
  Evaporation is greater than precipitation over
  oceans. The opposite is true over land, where
  precipitation is greater than evaporation.

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Evaporation

• Evaporation requires energy and water. It takes
  about 600 calories of heat to evaporate one gram
  of water. The warmer the climate is, the more
  heat energy is available for evaporation. Most of
  the world's evaporation happens in tropical areas
  near the equator.

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Infiltration and Runoff

• Earth's surface acts like a giant sponge as
  precipitation seeps into soil and rock.
• This process is called infiltration.
• Infiltrated water replenishes groundwater, which
  can be deep underground.
• The amount of infiltration depends on several
  factors

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Infiltration and Runoff

• Precipitation intensity and durationmore water
  infiltrates when precipitation happens over an
  extended period.
• Soil propertiessandy soils absorb more water
  than clay soils.
• Soil saturationdry soil absorbs more water than
  wet soil.
• Slope of the landflat areas absorb more water
  than slopes.

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Infiltration and Runoff

• Water that doesn't infiltrate flows over the
  ground as surface runoff. If you've ever seen a
  stream of water flowing downhill during a
  rainstorm, you have seen surface runoff.
• Rivers and streams also carry surface runoff.
• About 1/3 of runoff water returns to the ocean.
• The rest reenters the atmosphere through
  evaporation and transpiration.
• Eventually, this water will condense and fall
  back to the surface as precipitation.

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