The Hydrologic Cycle

The cycle of water around the Earth from clouds to rain to rivers to oceans and back to clouds is known as the hydrologic cycle. The hydrologic cycle describes the journey of a water molecule as it travels in various forms from one region of the Earth to another.

To begin our study of the hydrologic cycle, we first need to see where water is stored on our planet. These storage sites are known as water reservoirs, and they include the atmosphere; rivers and lakes; groundwater; glacial and other land ice; and the oceans and sea ice. The oceans contain nearly 97.24% of the water on the planet. Glaciers and the polar ice caps make up about 2.14% of the Earth's water. Groundwater accounts for about 0.61% of the total water on Earth while the amount of water contained in the atmosphere is seemingly trivial, a mere one thousandth of a percent (0.001%).

Water source Water volume, in
cubic miles
Percent of
total water
Oceans 317,000,000 97.24%
Icecaps, Glaciers 7,000,000 2.14%
Ground water 2,000,000 0.61%
Fresh-water lakes 30,000 0.009%
Inland seas 25,000 0.008%
Soil moisture 16,000 0.005%
Atmosphere 3,100 0.001%
Rivers 300 0.0001%
Total water volume 326,000,000 100%
Source: Nace, U.S. Geological Survey, 1967 and
The Hydrologic Cycle (Pamphlet), U.S. Geological Survey, 1984

While glaciers and icecaps may not represent a large percentage of the water on our planet, they are certainly important (think: "I'm flying, Jack!") Glaciers spawn icebergs and these floating islands of ice can be quite hazardous to ships. Since the sinking of the Titanic, the Coast Guard has maintained a special branch dedicated solely to tracking ice conditions in the North Atlantic (United States Coast Guard International Ice Patrol,

Here are some other interesting facts about glaciers and icecaps:

If all glaciers melted today the seas would rise about 260 feet.

During the last ice age (when glaciers covered more land area than today) the sea level was about 400 feet lower than it is today. At that time, glaciers covered almost one-third of the land.

During the last warm spell, 125,000 years ago, the seas were about 18 feet higher than they are today. About three million years ago the seas could have been up to 165 feet higher.

Likewise, don't be fooled by the small amount of water in the atmosphere. Clouds have a profound impact on our climate and act in a big way to regulate the amount of heat that hits our planet. Clouds make great reflectors and where they are abundant and thick, little solar radiation reaches the ground. As we talked about earlier, the relationship between clouds, climate, and phytoplankton may provide evidence for processes by which Gaia regulates the conditions of our planet. Thus, water in the atmosphere is not trivial by any means!

One important point about these reservoirs and the percentages of water that we attribute to each of them is that these reservoirs are dynamic. The amount of water in any one reservoir at any particular time is a function of the rate of input of water into the reservoir and the rate of loss of water from the reservoir. Seasonal cycles, such as snowfall, spring melt, rainy seasons, dry seasons, will change the standing stock of water in a reservoir at any given time. On geological time scales, processes such as glaciation during ice ages can change the amount of water in these reservoirs. The amount of time that a water molecule spends on average in any one of these reservoirs is known as the residence time.

Now that we have established where all the water is stored on the planet, let's take a look at how this water moves around. These are:

Not mentioned, but equally important are:

By far, the principal means by which water is removed from the oceans is through the process of evaporation. As discussed above, evaporation of water removes heat; thus the surface of the oceans are cooled as seawater turns into water vapor. In addition, salts are left behind, which makes the remaining seawater more salty. Thus, the places in the ocean that have the highest rates of evaporation also have the saltiest seawater. Another consequence of evaporation of water from the sea is the transport of this water over the land. The ocean is the primary source of all rainfall on the continents. In fact, in some regions of the world, such as the Mississippi Valley, more than 90% of the water in rainfall originated in the ocean.

As water vapor travels over the seas towards the land, it often encounters mountain ranges. I'm sure all of you have noticed that clouds tend to form on the windward side of mountains. As the warm sea breeze is forced upwards over the mountains, it cools. As it cools, it loses its capacity to hold water; hence, clouds are formed. This phenomenon by which clouds form over mountains and strip the air of its water vapor is known as the rain shadow effect.

From Mojave National Preserve Web Site,

Consider our own California coast. As warm, moisture-laden air comes into contact with the San Gabriel Mountains (or eventually the Rockies, which are even higher), the air must rise. As it rises, it cools and clouds form. If the air cools beyond a certain point, rain will begin to fall. This rainfall is typically confined to the seaward (or windward) side of mountain ranges. The cool air that passes over the top of the mountain now contains less water. As it descends, it begins to heat, holding onto its water even better. The result is that the mountains filter the water from the air and create a "rain shadow" on the landward (or leeward) side of the mountains. Many of the world's great deserts, such as our own Mohave desert, are created in this manner.

The process by which most water vapor is returned to the land or the sea is through the process of precipitation. Precipitation includes the fall of rain, snow, sleet, hail, or any other form that water might take. (People throughout the ages have reported all kinds of things falling from the sky, such as cats, dogs, frogs, ice cubes, blocks of stone, mud, pennies...we will not consider these forms of precipitation in this discussion.) There are innumerable places where precipitation might fall and find its way back to the sea. Here are a few of the possibilities:

  1. It may fall into a stream or river and be carried out to sea.
  2. It may seep into the ground and become groundwater in a process known as infiltration (water seeping through soil). This groundwater may become part of the water table that slowly flows back towards the sea.
  3. It could fall to the ground and evaporate from the soil back into the atmosphere.
  4. It could be absorbed by a plant from the soil, where it can leave the plant in a process known as transpiration (the evaporation of water through the leaves of a plant, a process that draws water and minerals to the plant).
  5. It may fall into a lake where it could be evaporated or where it might seep into the groundwater.
  6. It may become dew on a plant and evaporate to the atmosphere.
  7. It may be evaporated before it even hits the ground.
  8. It may turn into snow and rest on top of the mountain until it evaporates or melts (the process by which snow or ice evaporates directly from the solid state into the vapor state is known as sublimation).
  9. It may turn into a glacier and slowly glide downhill until it is evaporated or turned into an iceberg.
  10. It might hit a city street (or any hard surface) and be washed away as surface runoff or down a storm sewer back into a river or the sea.
  11. It might collect in a water reservoir and flow into your home through the faucet or into your toilet.
  12. It might fall directly into the sea.

Can you think of other ways that water might cycle between the land, atmosphere and sea?

As you can see, the transformations and travels of a water molecule across and within the planet can be quite extensive. The hydrologic cycle is more like a hydrologic web, with several mini-cycles played out across the globe. Tracking the movement of water in the atmosphere is the stuff of meteorologists; in the oceans, the physical oceanographers; and on land, people who study water in rocks and soil are called groundwater hydrologists.

The hydrologic cycle is probably the most distinctive and important process on this planet. It defines an awful lot of what happens not only to plants and animals, but also to rocks and mountains. Water is a powerful force, changing the shapes of continents. It is responsible for creating the Grand Canyon and Niagara Falls and is constantly changing the shape of our coastlines. As you go about your day-to-day activities, take a moment to stop and think how water affects us all. We take it for granted, but it's everywhere we look!

And lest you think water for human consumption is a vast and unlimited resource, take a look at this graph provided by the United States Geological Survey (USGS).

And here's how water use ranks in the United States.

Clearly, California consumes the lion's share of water! Use water wisely!

Here are ten water conservation tips from Clean Ocean Action,

  1. Test for a leaking toilet by adding food coloring to the tank. If any color appears in the bowl after 30 minutes, your toilet is leaking. Leaking toilets waste 200 gallons of water a day.
  2. Use water conserving plumbing fixtures and water flow constrictors on sinks and showers. If you donšt have a low-flow toilet, place two half-gallon plastic bottles filled with water in your toilet tank. This saves one gallon of water each time you flush.
  3. Run your dishwasher and wash clothes only when you have a full load.
  4. Take short showers instead of a bath. Baths can use 30 to 50 gallons of water. Showers use 5 gallons of water per minute, less if a flow constrictor is installed. [Showering with a friend or pet is also a good way to conserve water.]
  5. Check your water meter while no water is being used. If the dials are moving then you have a water leak.
  6. Donšt run water continuously when washing dishes, brushing your teeth, washing your hands and face, or shaving.
  7. Avoid using a garbage disposal. Disposal use a great deal of water. Add your garbage to the compost or trash instead of putting it down the garbage disposal.
  8. Choose plants that are native to the area you live or plants that are drought resistant for landscaping and gardens. Native plants are use to the natural amount of precipitation that occurs in the area they are found and normally do not require any additional watering This is known as Xeriscaping.
  9. Water lawn and gardens during the coolest part of the day. Use drip irrigation to apply water slowly exactly where it is needed. Collect rain from the gutter system on a house in a rain barrel to use for watering.
  10. Use a bucket of water and a spray head on the hose to wash your car. A running hose waste over 100 gallons of water in the time it takes to wash the car.

Useful links:

USGS: Water Science for Schools
A great site for understanding water and water use in the US.

USGS Glossary of water terms

Facts everyone should know about drinking water