Figure 1The water cycle is thecontinuous movement of waterbetween Earth and its atmosphere.Section 1Water Resources269ObjectivesDescribe the distribution of Earth’swater resources. Explain why fresh water is one ofEarth’s limited resources.Describe the distribution of Earth’ssurface water.Describe the relationship betweengroundwater and surface water ina watershed.Key Termssurface waterriver systemwatershedgroundwateraquiferporositypermeabilityrecharge zoneSECTION 1Water ResourcesThe next time you drink a glass of water, think about where thewater came from. Did you know that some of the water in yourglass may have been part of a rainstorm that pounded the Earthlong before life existed? Or that water may have been part of adinosaur that lived millions of years ago. Some of the water wedrink today has been around since water formed on Earth bil-lions of years ago. Water is essential to life on Earth. Humans cansurvive for more than a month without food, but we can live foronly a few days without water.Two kinds of water are found on Earth. Fresh water—thewater that people can drink—contains little salt. Salt water—thewater in oceans—contains a higher concentration of dissolvedsalts. Most human uses for water, such as drinking and agricul-ture, require fresh water.The Water CycleThe Earth is often called “the Water Planet” because it has anabundance of water in all forms: solid, liquid, and gas. Water is a renewable resource because it is circulated in the water cycle, as shown in Figure 1.In the water cycle, water molecules travelbetween the Earth’s surface and the atmosphere. Water evapo-rates at the Earth’s surface and leaves behind salts and otherimpurities. Water vapor, which is a gas, rises into the air. Aswater vapor rises through the atmosphere, the gas cools and con-denses into drops of liquid water that form clouds. Eventually thewater in clouds falls back to Earth and replenishes the Earth’sfresh water. The oceans are an important part of the water cyclebecause almost all of Earth’s water is in the oceans.Copyright© by Holt, Rinehart and Winston. All rights reserved.