What Is the Hydrologic Cycle?

The hydrologic cycle involves water moving from the surface (most importantly the oceans) to the atmosphere, across the land, and everywhere in between. Environmental scientists know that the hydrologic cycle includes various processes that change water from solid to liquid to gas form and transport it to every corner of earth’s surface (and below).

In terms of water, the earth is a closed system, so water isn’t added or removed from earth; it’s simply transformed, transported, and recycled.

Since the hydrologic cycle has no beginning or end (hence the term cycle), you can jump in at any stage. A good place to start is at the oceans, where most of earth’s water is stored. This figure illustrates the major steps of the hydrologic cycle.

Credit: Illustration by Wiley, Composition Services Graphics

• Water in the oceans moves to the atmosphere through evaporation, a process that changes the liquid water to vapor, or gas.

  After the water vapor is in the atmosphere, processes of atmospheric circulation transport it around the globe.

• As the water vapor is carried over land, the atmosphere often releases it in the form of precipitation (rain or snow).

  The precipitation may stay on land in the form of snow (for a year or so) or ice (for many years), or it may move across the land as rivers and streams, and some of it will evaporate back into the atmosphere.

• The water on the surface of the earth may end up in lakes for many years, be absorbed into the soil and rocks and become groundwater, or continue to flow as runoff until it reaches the ocean again.

  Groundwater is water that flows underground toward the nearest ocean.

• Plants release water into the atmosphere through a process called transpiration. While plants lose water to the atmosphere pretty much all the time (sort of like sweating), transpiration is higher during photosynthesis, when plants release water into the atmosphere in exchange for taking in carbon dioxide.

  This exchange of water between the atmosphere and plants is a part of the hydrologic cycle that’s often overlooked.

The hydrologic cycle doesn’t occur in a straight line. Throughout the path described in this list, water is being evaporated back into the atmosphere and being added to the surface as precipitation.

Understanding the movement of water through the environment and around the planet is important in environmental science when you’re studying issues such as freshwater resources, water pollution, and climate change.