Water is composed of two atoms of hydrogen and one atom of oxygen bonded together. Despite its simplicity, water has remarkable properties. Water expands when it freezes and has high surface tension because of the molecules’ polar nature that they tend to stick together. Without water, life might not exist on Earth, and it certainly would not have the tremendous complexity and diversity that we see.

Earth’s oceans contain 97 percent of the planet’s water, and just 3 percent is freshwater with relatively low concentrations of salts. Most freshwater is trapped as ice in the vast glaciers and ice sheets of Greenland and Antarctica. A storage location for water such as an ocean, glacier, pond, or even the atmosphere is known as a reservoir. A water molecule may pass through a reservoir very quickly or may remain for much longer. The amount of time a molecule stays in a reservoir is known as its residence time.

Hydrologic Cycle

Because of the unique properties of water, water molecules can cycle through almost any-where on Earth. The water molecule found in a glass of water today could have erupted from a volcano early in Earth history. In the intervening billions of years, the molecule probably spent time in a glacier or far below the ground. The molecule surely was high up in the atmosphere and maybe deep in the belly of a dinosaur.

Water is the only substance on Earth that is present in all three states of matter – as a solid, liquid, or gas. Along with that, Earth is the only planet where water is present in all three states. Because of the ranges in temperature in specific locations around the planet, all three phases may be present in a specific location or region. The three phases are solid (ice or snow), liquid (water), and gas (water vapor).

Water is continuously on the move. It is evaporated from the oceans, lakes, streams, the surface of the land, and plants (transpiration) by solar energy. It is moved through the atmosphere by winds and condenses to form clouds of water droplets or ice crystals. It comes back down as rain or snow and flows through streams, into lakes, and eventually back to the ocean. Water on the surface and in streams and lakes infiltrates the ground to become groundwater. Groundwater slowly moves through the rock and surficial materials. Some groundwater returns to other streams and lakes, and some go directly back to the oceans. (Earle, 2019)

Because Earth’s water is present in all three states, it can get into various environments around the planet. The movement of water around the Earth’s surface is the hydrologic (water) cycle. Water changes from a liquid to a gas by evaporation to become water vapor. The Sun’s energy can evaporate water from the ocean surface or lakes, streams, or puddles on land. Only the water molecules evaporate; the salts remain in the ocean or a freshwater reservoir. The water vapor stays in the atmosphere until it undergoes condensation to become tiny droplets of liquid. The droplets gather in clouds, which are blown about the globe by the wind. As the water droplets in the clouds collide and grow, they fall from the sky as precipitation. Precipitation can be rain, sleet, hail, or snow. Sometimes precipitation falls back into the ocean, and sometimes it falls onto the land surface.

When water falls from the sky as rain, it may enter streams and rivers that flow down to oceans and lakes. Water that falls as snow may sit on a mountain for several months. Snow may become part of the ice in a glacier, where it may remain for hundreds or thousands of years. Snow and ice may go directly back into the air by sublimation, the process in which a solid changes directly into a gas without first becoming a liquid. Although it is hard to see water vapor sublimate from a glacier, it is possible to see dry ice sublimate in the air.

Snow and ice slowly melt over time to become liquid water, which provides a steady flow of freshwater to streams, rivers, and lakes below. A water droplet falling as rain could also become part of a stream or a lake. At the surface, the water may eventually evaporate and reenter the atmosphere.

A significant amount of water infiltrates into the ground, and soil moisture is an important reservoir for that water. Water trapped in the soil is essential for plants to grow. Water may seep through dirt and rock below the soil through pores infiltrating the ground to go into Earth’s groundwater system. Groundwater may enter aquifers that may store freshwater for centuries. Alternatively, the water may come to the surface through springs or find its way back to the oceans. Plants and animals depend on water to live, and they also play a role in the water cycle. Plants take water from the soil and release substantial amounts of water vapor into the air through their leaves, a process known as transpiration. NASA has an excellent online animation of the hydrologic cycle.

People also depend on water as a natural resource. Not content to get water directly from streams or ponds, humans create canals, aqueducts, dams, and wells to collect water and direct it to where they want it.

The table above displays water use in the United States and globally (Estimated Use of Water in the United States in 2005, USGS). It is important to note that water molecules cycle around. If climate cools and glaciers and ice caps grow, there is less water for the oceans, and sea level will fall. The reverse can also happen.