The water cycle is the movement of water in all three phases throughout the earth and atmosphere. Water is constantly on the move. Groundwater water can move very slow such as only a few feet per day and ice cap water can be in about the same place for thousands of years while moisture in the atmosphere can move hundreds of miles per day. Water is a unique substance in that in that exist in all three phases at the temperatures experienced at Earth’s surface (liquid, gas, solid). The majority of the ice exists in the polar ice caps with Antarctica having the largest reservoir. The vast majority of liquid water exists in the oceans. Thus, only a few percent of the liquid water is fresh water such as in lakes, rivers and groundwater. Only a tiny fraction of the overall water on Earth is in the gas state. Much of this gaseous water vapor is in the atmosphere.

The diagram below shows some of the major features of the water cycle. Water that moves the quickest is moisture in the air, precipitation and river runoff. Liquid water under the influence of gravity will move from a higher to a lower elevation when given the chance. Thus, water in high elevation regions makes it way to rivers that carry the water to sea level. Water not only cycles between the three states of liquid, gas and solid; it also cycles between being in the atmosphere, being in the ocean, and being on land.