What do Earth’s surface and human beings have in common? Answer: They both consist mostly of water. Water covers about three-quarters of Earth’s surface. Water makes up about two-thirds of the human body. In fact, water is a large part of every living thing.
Water is essential for living things to grow, reproduce, and carry out other important life processes. For example, plants use water, plus carbon dioxide and sunlight, to make their food in the process of photosynthesis. Animals and other organisms eat plants or eat other organisms that eat plants. Water is also essential as an environment for living things. Both fresh water and salt water provide habitats for many kinds of living things.
Water on Earth
Although Earth has lots of water, the amount of water that humans can use is very small. About 97 percent of Earth’s water is the salt water found in the ocean. People have named different parts of the ocean, but in fact these parts are all connected, so they really form a single world ocean.
Only about 3 percent of Earth’s water is fresh water. Most of that fresh water is found in the huge masses of ice near the North and South poles. Less than 1 percent of the water on Earth is available for humans to use. Some of this available fresh water is found in lakes, rivers, and streams. Other fresh water is located under the ground. This underground water is called groundwater. It fills the small cracks and spaces between underground soil and rocks.
The Water Cycle
Water is always moving from one place to another. The continuous process by which water moves through the living and nonliving parts of the environment is called the water cycle. In the water cycle, water moves from bodies of water (such as oceans, rivers, lakes, and streams), land, and living things on Earth’s surface to the atmosphere and back to Earth’s surface.
The sun is the source of energy that creates the water cycle. The sun’s energy warms water in oceans, rivers, and lakes. Some of this water evaporates—changes into a gas called water vapor. Smaller amounts of water evaporate from the soil, from plants, and from animals (through their skin). Water vapor rises in the air and forms clouds. As water vapor cools in the clouds, it condenses, or changes into liquid water drops. When water drops in the clouds become heavy, they fall back to Earth as precipitation—rain, snow, sleet, or hail.
Precipitation is the source of all fresh water on or under Earth’s surface. The water cycle renews the supply of usable fresh water on Earth.