If all the world's water could be placed in a 26 gallon container, then...
about 3 quarts, or 3% would be fresh water
1/2 quart, or 0.5% would be available fresh water, and
1/2 teaspoon, or 0.003% would be fresh water available for human use
Learning Topic: The Water Cycle

What is the Water Cycle?

The water cycle (also called the hydrologic cycle) is the water movement in the atmosphere, on the surface and below the surface of the earth. As the name implies, it is a cycle which means that water moves (cycles) through these processes. The amount of water on the earth is fairly constant, which is why it is so important to protect the quality of the water that we have. Over 70% of the earth’s surface is oceans. Approximately 97% of earth’s water is found in the oceans; with only 3% freshwater. Of the freshwater, over 68% is frozen in icecaps and glaciers and 30% is in groundwater. Less than 1% of earth’s water is usable for our daily needs.

Water is a renewable, natural resource that continually moves through the environment in the water cycle.

The key components in the water cycle include precipitation, infiltration, runoff, evaporation and condensation.


When water falls to the earth from clouds, it is called precipitation.  Precipitation can be in the form of rain, sleet, snow, hail or freezing rain.  Rain is the most common form of precipitation.  Precipitation is the point in the water cycle where the moisture returns to the earth.


Infiltration is the amount of precipitation that enters into the subsurface soil.  Water that seeps into the ground can reside in one of two locations.  It may be found in the “zone of aeration”, which is where the spaces in the soil contain both water and air.  The water may also infiltrate into the “zone of saturation” which is where all of the spaces within the soil or the cracks in the bedrock are filled with water.  Precipitation that infiltrates recharges the groundwater and provides water to our wells.


Surface runoff is precipitation that moves over the land.  It is the water that neither infiltrates nor evaporates upon its initial fall to earth.  Approximately one third of precipitation that runs over land runs off into streams, rivers and back to the oceans.  The remainder infiltrates into the groundwater or evaporates from soil or plants (transpiration).

Evaporation and Transpiration

Evaporation occurs when water changes from a liquid to a gas or vapor.  This is the main route through which water returns to atmospheric water vapor.  Though we may not always see it, there is always water vapor in the atmosphere. Approximately 90% of evaporated water in the water cycle comes from the oceans. 

Both plants and soils release water vapor to the atmosphere.  Plants release moisture from their leaves.  The return of water to the atmosphere from plants is called transpiration.  Water taken up by the roots is released through the pores in the leaves, returning water to the atmosphere. Transpiration from plants provides approximately 10% of moisture in our atmosphere.


For precipitation to occur, water vapor has to change into liquid water.  The process of condensation changes water vapor from a gas to a liquid.  When the weight of the water (millions of water particles) becomes heavy enough, it falls to the ground in some form of precipitation. 

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