The Water Cycle:

The water cycle is part of the standard third grade earth science curriculum, so this should be review for you. The water cycle is one of my favorite things to teach because it is so much a part of our lives--encompassing everything from the weather to electrical power.

The water cycle or hydrologic cycle is the set processes in which water moved from one place to another. Liquid water is evaporated (converted from liquid to gas) by sunlight or wind. Water vapor moves into the atmosphere and condenses into clouds, which are accumulations of very small water droplets or ice particles. The droplets or particles can coalesce into larger particles that can fall as precipitation, either rain or snow. When the water reaches the surface of the ground, it can infiltrate (sink into the ground) or runoff (move across the surface). Runoff accumulates in channels such as streams and rivers. Water at the surface can evaporate or continue to run downhill to the sea.

The water that condenses into clouds is almost completely pure, though it does pick up impurities from the air (such as sulfuric acid). Precipitation is the source of all the freshwater. The sun purifies seawater for us. Some precipitation falls onto land at high elevation. As this water runs downhill, its energy can be tapped using dams. Hydroelectric power is essentially solar energy.

Water in the ground can be taken up by plants and transpired (released from their leaves as water vapor). Water moves very slowly underground until it eventually rejoins surface flows. The base flow of rivers is fed by groundwater. Thus, the Thames River continues to flow even during a dry spell. Some streams (so called intermittent streams) do dry up in dry weather. Water underground can be drawn from a well, which is simply a hole in the ground that goes deep enough to hit water.

Where is the water?

Fraction of total



Polar or mountaintop ice




Freshwater Lakes


The atmosphere (vapor)




Yikes! Less than one percent of all water on the planet is unfrozen freshwater, and almost all of that is underground.

All precipitation either runs off into the sea or is made into vapor again. So we can say:

precipitation = runoff + evapotranspiration

For the US this works out to:

30 inches/year = 9 inches + 21 inches
(the ocean has to make up that 9 inches or the rivers would run dry)

In other words, more than two thirds of the rainfall that lands on the US evaporates before it reaches the ocean. Some of that evaporation is promoted by the use of water for irrigation.

Water cycles through the air in about 9 to 10 days; the rivers would take about 40K years to refill the oceans.

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Posted by Anthony Benoit
Environmental Engineering Technology at Three Rivers