How does the hydrologic cycle move water through the ecosystem?

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sciftw | High School Teacher | (Level 1) Educator Emeritus

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The hydrologic cycle, or water cycle, is the continuous movement of water throughout the environment.  In very simplified language, it's water on the ground, water in the air, and water falling between the two locations.  

The water cycle, as with any other process, is driven by energy, specifically thermal energy.  I'll start with water on the ground.  Most likely the water is in liquid form, but it could be in solid form.  Let's stick with liquid form.  To move water from being a liquid on the ground to being vapor in the air, the water needs to vaporize.  There are two ways to vaporize liquid water.  One is to boil it, which is not common in the water cycle.  The other is to evaporate the water.  Evaporation is vaporization below a boiling point.  In the water cycle, the liquid water absorbs enough thermal energy to cause water molecules at the surface to vaporize and break away.  That water vapor rises into the atmosphere.  

If heat energy was added to vaporize the water, then heat energy must be lost in order to return it to its liquid state.  That process is called condensation.  As the water vapor rises, the air temperature drops.  As the air temperature drops, the relative humidity increases.  Once the air temp has cooled to where the air is fully saturated (100% humidity), condensation occurs.  That temperature is known as the dew point.  If you were watching all of this from the ground, you would have seen a cloud develop.  If it happened at ground level, you would have called it fog.  

Condensed water vapor is not heavy enough, at this point, to fall back to Earth.  After enough condensation occurs the droplets begin to clump together and get heavier and heavier.  Once heavy enough, they begin to fall toward Earth.  This is called precipitation.  It could be rain, snow, sleet, hail, or even a mixture of those.  Once that water is on the ground, the cycle starts over.  

That's the water cycle in a nutshell.  Water evaporates, water condenses, and water precipitates back to the ground.  

Slightly more advanced in the water cycle is a process known as transpiration.  It works exactly like evaporation except it is happening out of plant life.  Leaves contain tiny little pores called stomata, and water can evaporate into the air from those pores.  That's called transpiration.  

Water that's on the ground doesn't typically sit in a single location, so another step of the hydrologic cycle is called runoff.  It's the movement of water along the surface of the ground through natural or artificial channels.   

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