Tuesday, April 15, 2008

What is the Water Cycle?

Cole from State College, PA, wrote in to ask if the Spring-time saying “April showers bring May flowers” is true. The saying points to the theory that the large amount of rain observed at this time of year is thought to assist in the growth of plants and flowers that generally bloom in May. However, flowers actually bloom at different times during the year depending on their location, so timing of rainfall that aids in the health and growth of flowers is different depending on that location. Too much rain can also negatively affect the plants by making them more susceptible to diseases or killing the roots. This question brings up another good question, though - where do “April showers” come from? To answer that, we’ll need to learn about the water cycle.

The water cycle is a term used to describe the continuous movement of water in and around the Earth. About 70% of our Earth is covered by water, which amounts to approximately 333 million cubic miles! So that’s a lot of water in constant operation – but how is it in a constant cycle?

Two major components of the water cycle are evaporation and condensation. Evaporation occurs when a substance goes from the liquid to the gaseous state, and condensation occurs when a substance goes from a gaseous to a liquid state. These processes happen on Earth with the help of the sun. The sun heats the surface of water causing it to evaporate into water vapor (gaseous water), which rises into the atmosphere. This water vapor then cools and becomes clouds, which eventually condense into water droplets. Depending on the temperature of the atmosphere, the water then precipitates (falls back to the Earth’s surface) as rain, sleet, hail or snow. Some of this precipitation falls on trees or other plants and can evaporate again into the atmosphere. The precipitation can also continue to the ground, and now the water is considered runoff water. This runoff water can then get into the ground and accumulate where it is eventually stored in aquifers, which are large, natural storage tanks of groundwater that can be used later if needed. The runoff water can also form or add to lakes and streams, which can also then freeze into snow caps or glaciers. Water that falls to the ground and stays in the soil ends up evaporating and returning back to the atmosphere – you can see how this is a continuous cycle! The water in aquifers, though, can accumulate there for thousands of years. Aquifers are actually our major sources of drinking water.

So consider the long journey water has taken the next time it rains, snows, hails or sleets. Maybe it end up as your drinking water or maybe it will end up in your local water reservoirs. Perhaps, it will just evaporate back into the atmosphere to come back to the Earth’s surface as rain another day.

Little Lion Experiment
This experiment will allow you to create a small-scale model of the water cycle using common items found around your house. You will need: plastic wrap, a large bowl (preferably one that is clear), a weight (a paperweight will work), small container (a clean, empty yogurt cup works well), a rubberband or piece of string, tap water, paper and pencil. You will also need access to sunlight.

Steps: 1) Place the small container in the middle of the large, clear bowl so the opening of the small container is up. 2) Fill the bowl with some water (at most half full) but be careful not to fill the small container inside. 3) Cover the bowl with plastic wrap. 4) Fasten the plastic wrap around the bowl’s rim with the rubberband or string. 5) Put a weight on top of the plastic wrap in the center. 6) Put the demonstration on a window sill or somewhere that it will be in contact with the sun. 7) Record your observations of the experiment every 10 minutes on your paper (you should conduct this experiment for at least an hour).

What did you observe? Hopefully you saw that the heat of the sun evaporates the water, which rises, condenses on the cool plastic, and falls into the small container similar to how rain falls. Now that you know how to make your own model of the water cycle, change some of your materials in the experiment. For example, use salt water instead of tap water. Or, you could use ice water (a mixture of water and ice chips) instead of tap water. Were you still able to observe the water cycle?