What Are the World's Four Main Water Sources?

Surface Water

Ninety-seven percent of the world's surface water is in the oceans, which covers 71 percent of the surface area of the globe. Fresh water in lakes and rivers accounts for another 1 percent. Ocean water is 220 times saltier than fresh water. According to the U.S. Geological Survey (USGS), evaporating 1 cubic foot of ocean water yields 2.2 pounds of salt, whereas a similar sample from a freshwater lake will yield only one-hundredth of a pound. The salt in the oceans is a combination of mineral salts and decayed biological matter that have accumulated for as much as 500 million years.

Water that falls to the ground as rain, sleet and snow soaks down to become stored in sand and gravel below the surface as aquifers and underground rivers. Seeping water gradually fills the porous material below the surface, gradually raising the saturation level toward the surface. The uppermost limit of this area of saturation is known as the water table. People make wells to extract water from the ground by boring below the water table and pumping the ground water to the surface. The Earth's natural filtration keeps groundwater clean and useable.

The amount of water vapor in the air varies from trace amounts to about 4 percent, depending on the aridity and temperature of the region. There tends to be less in colder areas with little surface water and the most in moist tropical regions. When the concentration exceeds 4 percent, the vapor condenses and falls to the earth as rain or snow. While being essential for the repletion of surface and groundwater supplies, atmospheric water vapor is also the most abundant greenhouse gas. It traps energy to raise Earth's surface temperature while giving the atmosphere its characteristic blue color.

Both of Earth's poles are covered with ice, and some of that ice may have existed for as much as 8 million years. Antarctica has the majority of the world's ice, almost 90 percent, while the Greenland ice cap accounts for another 10 percent. Melting from the northern polar ice cap feeds fresh water into the oceans and into rivers that in turn feed freshwater lake systems like the Great Lakes. Glaciers cover 10 percent of all the Earth's land mass and account for the remaining small percentage of the world's ice. If all of Earth's ice were to melt at once, the USGS estimates that the sea level would rise by 230 feet.