Water is the most common substance on the planet and covers 70% of the Earth’s surface. Most of the Earth’s water (97.25%) is salt water found in the oceans, while 2.75% is freshwater found in the icecaps, glaciers, groundwater, lakes, rivers, and atmosphere. Water is also present in plants, animals, and soil. Less than 1% of the Earth’s water is considered to be available freshwater. While saltwater in the ocean is useful for shipping, recreation, and food, and supports a wide variety of plants and animals, freshwater is far more usable by humans.

Some of the Earth’s water is located underground as groundwater (i.e., water that fills all of the spaces between earth particles), as water vapor in the atmosphere; or is frozen in the icecaps and glaciers. However, most of the Earth’s water is surface water because it is found on the surface of the Earth in oceans, lakes, and rivers. 

The total amount of water on Earth is estimated to be 370,000 quadrillion (370,000 + 18 zeros) gallons or 1.4 billion cubic kilometers. Of this total, approximately 0.01% is found in lakes. The Great Lakes are the largest body of fresh surface water on Earth! They contain about 84 percent of North America’s surface fresh water and about 21 percent of the world’s supply. Only the polar ice caps contain more fresh water. (For additional information, see Great Lakes Physical Features and Population transparency master.) Michigan has the longest freshwater coastline of any political subdivision in the world, being bounded by four of the five Great Lakes, plus Lake Saint Clair Michigan has 64,980 inland lakes and ponds. A person in Michigan is never more than six miles (10 km) from a natural water source or more than 85 miles (137 km) from a Great Lakes shoreline.

The hydrologic cycle, or water cycle, is the continuous movement of water above, below, and on the surface of the Earth.

Note: Students should know that the water cycle and the hydrologic cycle are interchangeable terms for the same process.

The hydrologic cycle moves water from one location on Earth to another. Along the way, water may change state from liquid to gas (water vapor) or solid (snow and ice). For example, water evaporates from the ocean into the atmosphere and forms clouds, which may be blown to the Great Lakes, where they will condense into rain or snow and precipitate onto Michigan’s land. Next, the rain and melted snow will run off into wetlands or inland lakes, which then empty into rivers that flow to the Great Lakes. The water in the Great Lakes may again evaporate into clouds, continuing the water cycle. As water moves through the water cycle, it passes through all terrestrial and aquatic ecosystems; passing through all plants, animals, and humans; and is used in some way in all economic activities. The transfer processes for water as it moves from one part of the water cycle to another include: condensation, precipitation, infiltration, runoff, evaporation, sublimation, and transpiration.

Water comprises about 70% of a person’s body mass. There is no more important nutrient than water—clean water is essential for human health! Water makes up a part of every living plant and animal. The health of the American people and the economic growth of Michigan and the nation depend on the availability of clean freshwater. Water is fundamental to life and is a basic requirement for all of our agricultural, industrial, and recreational activities, as well as for the health of the natural environment.