Water Cycle

Water is constantly in motion. Sometimes quickly, as in a fast-flowing river, but sometimes it moves quite slowly, like in underground aquifers, glaciers and deep ocean currents.

The water cycle, is often shown as a simple circular cycle (as in the accompanying diagram) in which water evaporates from the ocean, is carried over land, falls as rain, and then travels back to the ocean through rivers.

Although a drawing of the water cycle oversimplifies the actual movement of water, the diagram is a useful tool. The actual path any given water molecule follows in a complete water cycle can be varied and complex and may not follow the exact path shown by a diagram.

It is also important for students to understand that water cycle diagrams do not show the amount of time that a water molecule may take as it travels through the water cycle. For instance, water starting in the Antarctic may take over 250 years to travel along the bottom of the Pacific Ocean before it re-surfaces near Alaska.  Water can also remain frozen in a glacier or ice sheet for thousands of years before re-melting.

Water may also change state, back and forth, from a liquid, gas and solid (condensing, evaporating, etc.) as it travels through the cycle. Water even travels underground, where it seeps through the spaces between grains of soil, sometimes coming to the surface as artesian springs.

Living organisms also move water around. Water, is either directly consumed as liquid or extracted from food and then carried within bodies. It leaves the organism as a gas during respiration, is excreted or may evaporate from the skin as perspiration. Plants are the major biotic movers of water. Their roots collect water for distribution throughout the plant. Some of the water will be used in photosynthesis, but most travels to the leaves where it is easily evaporated.

Although water vapor is invisible, fog and clouds do give some indication of water vapor in the atmosphere. Water condensation, seen as early morning dew or even on a cold glass, is one visible example of the water vapor present in our air. In clouds, water molecules condense and collect on microscopic dust particles into droplets until they become heavy enough that gravity pulls the water down as precipitation- in the form of rain, snow, sleet or hail.

Education Connection
The water cycle is more than a diagram it has significant impacts on our daily lives, local and global ecosystems and even economic systems. The resources in this collection can help teachers take students beyond just a minimal knowledge of a simple diagram of the cycle. This collection provides real-time and historic data sources that track and measure the water in different portions of the water cycle; satellite images shows water vapor in the atmosphere, interactive maps can be searched to show precipitation, snow depths, river flows and evaporation rates. Included are also lessons, games and hands on activities that model the complexity of the cycle. The Background resources allow students and teachers to investigate the water cycle holistically and in its individual parts. These resources allow teachers to implement student-directed and place-based projects about the water cycle.

Adapted from NOAA Office of Response and Restoration Water Cycle Game