The Water Cycle: How it works, a simple and detailed explanation with definitions, diagrams, and a visualization activity!

Ever wondered what keeps our streams flowing? Or why rivers don’t run out of water? Or where does the water bubbling out of the ground at natural springs come from? The answer to all these questions actually begins with our sun! The sun is like an engine, providing the necessary energy to drive the water cycle. The water cycle is an ongoing process of moving water from the oceans to the land and back.

The water cycle begins over our oceans, where the energy from our sun causes huge amounts of water to evaporate every day. When water evaporates, it doesn’t disappear – it becomes water vapor: water in gas form rather than liquid form. Ocean water can evaporate, but salts in the ocean cannot. Because salt cannot evaporate, evaporation separates freshwater out of the ocean. Freshwater is non-ocean water that is usable by land plants and animals, including humans! Evaporation is an important earth process that makes human life possible because it separates the freshwater from the saltwater.

As the evaporated water vapor rises up in the atmosphere, it cools and condenses, or starts forming tiny liquid droplets. As larger droplets form, they fall from the clouds as precipitation, such as rain or snow. Most precipitation will fall directly back into the ocean. However, sometimes clouds that form over the ocean will move over a continent and precipitation falls on the land. When rain falls on the land, some of it flows over the surface of the land, which we call runoff.  Little streams may flow into larger streams and rivers, or temporarily be stored in non-moving bodies of water like ponds and lakes. Any water on the earth’s surface, including streams, rivers, ponds, lakes, and oceans is called surface water. Ultimately, all surface water will do one of three things: evaporate again, flow all the way back to the ocean, or infiltrate…

To infiltrate, in other words, means to seep into the ground. When water infiltrates it fills the tiny gaps in the soils and rocks below our feet! This water is called groundwater! In the water cycle, water may infiltrate into the ground at any point between where it falls on the land as precipitation and when it flows back into the ocean. Like surface water, groundwater also moves or flows, but much more slowly than surface water. Some groundwater bubbles back out of the ground onto the surface in a spring or a gaining stream: a stream that is fed by groundwater rather than runoff.  Plants will also take up groundwater through their roots and release it back into the air as water vapor through the tiny pores in their leaves, in a special type of evaporation called transpiration. 

So far, we’ve looked at water in liquid and vapor forms. But what about water in solid form such as ice and snow? It turns out ice and snow play an important part in the water cycle too. When snow falls on land, it can build up in one place, or accumulate. When temperatures increase, such as in springtime, the snow that built up through the winter will melt and flow into our surface and groundwaters. In certain cold and snowy areas, such as in tall mountains or near the poles, snow accumulates faster than it melts. This forms glaciers and ice sheets: long-lasting bodies of ice, made from compacted snow. Most of the fresh water on our planet is currently stored as ice in glaciers, ice sheets, and snow! Scientist in the field of snow hydrology study these important stored waters!

Saltwater like ocean water is toxic to humans and land plants and animals. All the water we drink, bathe in, clean with, and use to grow plants must be freshwater. Freshwater is only a tiny fraction of the water on earth: about 97.5% of earth’s water is saltwater. That means only 2.5% of the earth’s water is freshwater. Of that 2.5% freshwater, most is not usable by humans. Over two-thirds of the earth’s freshwater is frozen in glaciers, ice caps, and snow, and a little less than a third is groundwater. Surface water like rivers, lakes, and ponds are just a fraction of a percentage of earth’s total fresh water – and just a tiny part of a fraction of a percentage of earth’s total water overall! If you’re having trouble visualizing this, check out the free at-home activity guide to help you and your child or students visualize the percentages of freshwater from home in the kitchen or bathtub!

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• Continue to Part 2 of this post about the water cycle – covering the ways humans interact with the water cycle!
• Skip to Part 3 about the water cycle – covering the role of mountain watersheds!
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Free Project: Water on Earth’s Surface

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References and Further Reading

1. Plummer, C. C., Carlson, D. H., & Hammersley L. (2019) Physical Geology. New York, NY: McGraw-Hill Education. (16th ed., pp. 232 – 320).
2. U.S. Geological Survey. (n.d.). How Much Water is there on Earth? Available: https://www.usgs.gov/special-topic/water-science-school/science/how-much-water-there-earth?qt-science_center_objects=0#qt-science_center_objects

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