Play It Again, Sam!

The water cycle is one example of a cycle in nature. The water cycle is of particular interest because it impacts our lives in many ways. Parts of the water cycle include the weather that we experience, the amount of water that we must add to our lawns in the summer time, the location of bodies of water for water storage and recreation, and the importance of water conservation. Here's another way to think about the importance of the water cycle. Approximately three-fourths of the Earth is covered with water--salt water. Of this water, approximately one percent is the fresh water on which we depend. The fresh water that we use and its continuous replacement is a result of the water cycle.

Water is a unique substance. It is one of the few materials on the Earth that exists naturally as a solid, liquid, or gas. Changes in state, such as solid to liquid or gas to liquid, are caused by changes in energy. Changes in this energy, which ultimately comes from the sun, are measured by changes in temperature.

When we think of the water cycle, we often first think of bodies of water on the surface of the Earth, such as lakes, reservoirs, oceans, rivers, and streams. Water from these surfaces enters the water cycle upon evaporation. Evaporation occurs when increases in energy (from the sun) is great enough to turn a liquid into a gas or water into water vapor. Water vapor is also added to the atmosphere by transpiration. Transpiration is the release of water by plants. Plants collect water through their roots and lose it as it evaporates into the atmosphere through small openings on the undersides of their leaves. Many of our activities also add water vapor to the air: hanging our clothes out to dry, sprinkling our lawns on sunny days, splashing water at the swimming pool. Interestingly, only pure water evaporates. This means that the mud in puddles, the salt in the ocean, and pollutants stay on the surface of the Earth. Only pure water turns into water vapor. In this way, we can get fresh water from salty ocean water.

Once water vapor is in the air, it often stays there. This water vapor is called humidity. Humidity can vary from 0% in the deserts to 100% right before a summer rain storm. Warm air holds much more water than cold air. When water vapor cools, it condenses. We can see the condensation of water in the form of clouds. We can also see condensation whenever water vapor comes in contact with cold air or cooler objects. Look on your mirror after a hot shower--this is condensation in action. In nature, dew and frost are a result of water vapor condensing onto the cooler surfaces of plants and windows. Water vapor condenses when it reaches higher elevations because the air is cooler.

As water vapor cools into clouds, many things can happen to it, depending on the temperature. If the clouds stay relatively warm, the water vapor will collect into larger and larger drops until they are too heavy to stay aloft. Rain is one form of precipitation. Precipitation returns water from the atmosphere back to the surface of the Earth. If the clouds are cold enough, other forms of precipitation may occur. Water vapor may turn directly into snow in a process called sublimation (moving directly from a gas to a solid or a solid to a gas). Hail may be formed when rain drops are tossed high into the clouds and colder temperatures. As these small drops freeze, they are dropped lower into the clouds, coated with water, and then blown back up into the atmosphere to freeze again. When too heavy to be blown around in the clouds, hail stones drop to the Earth.

When water returns to the Earth, it can be absorbed into the soil. This process is called percolation. Water will trickle through the tiny spaces between the soil particles and eventually collect above an impermeable rock layer. This water and saturated soil above the rock layer is called ground water. Ground water can move to lower elevations and depression through underground "rivers." The top level of the ground water is called the water table. Any depression in the Earth below the water table appear as lakes and ponds.

Different types of soil will vary in their ability to absorb water. When water is not absorbed by the ground, it may create surface runoff. Runoff water travels over the surface of the ground and causes soil erosion. Evidence of this type of erosion can often be found in road cuts and other unprotected soil surfaces. Plant roots are important aids in the prevention of surface erosion.