courtesy of: USGS
As you know, the Earth is a watery place. But just how much water exists on, in, and above our planet? About 70 percent of the Earth’s surface is water-covered, and the oceans hold about 96.5 percent of all Earth’s water. But water also exists in the air as water vapor, in rivers and lakes, in icecaps and glaciers, in the ground as soil moisture and in aquifers, and even in you and your dog.
Water is never sitting still, though, and thanks to the water cycle, our planet’s water supply is constantly moving from one place to another and from one form to another. Things would get pretty stale without the water cycle!
All Earth’s water in a bubble
This drawing shows various blue spheres representing relative amounts of Earth’s water in comparison to the size of the Earth. Are you surprised that these water spheres look so small? They are only small in relation to the size of the Earth. This image attempts to show three dimensions, so each sphere represents “volume.” The volume of the largest sphere, representing all water on, in, and above theEarth, would be about 332,500,000 cubic miles (mi3) (1,386,000,000 cubic kilometers (km3)), and be about 860 miles (about 1,385 kilometers) in diameter.
The smaller sphere over Kentucky represents Earth’s liquid fresh water in groundwater, swamp water, rivers, and lakes. The volume of this sphere would be about 2,551,000 mi3 (10,633,450 km3) and form a sphere about 169.5 miles (272.8 kilometers) in diameter. Yes, all of this water is fresh water, which we all need every day, but much of it is deep in the ground, unavailable to humans.
Do you notice that “tiny” bubble over Atlanta, Georgia? That one represents fresh water in all the lakes and rivers on the planet, and most of the water people and life of earth need every day comes from these surface-water sources. The volume of this sphere is about 22,339 mi3 (93,113 km3). The diameter of this sphere is about 34.9 miles (56.2 kilometers). Yes, Lake Michigan looks way bigger than this sphere, but you have to try to imagine a bubble almost 35 miles high—whereas the average depth of Lake Michigan is less than 300 feet (91 meters).
Water is on and in the Earth
The vast majority of water on the Earth’s surface, over 96 percent, is saline water in the oceans. But it is the freshwater resources, such as the water in streams, rivers, lakes, and groundwater that provide people (and all life) with most of the water they need every day to live. Water sitting on the surface of the Earth is easy to visualize, and your view of the water cycle might be that rainfall fills up the rivers and lakes. But, the unseen water below our feet is critically important to life, also. How would you account for the flow in rivers after weeks without rain? In fact, how would you account for the water flowing down this driveway on a day when it didn’t rain? The answer is that there is more to our water supply than just surface water, there is also plenty of water beneath our feet.
Even though you may only notice water on the Earth’s surface, there is much more freshwater stored in the ground than there is in liquid form on the surface. In fact, some of the water you see flowing in rivers comes from seepage of groundwater into river beds. Water from precipitation continually seeps into the ground to recharge the aquifers, while at the same time water in the ground continually recharges rivers through seepage.
Humans are happy this happens because people make use of both kinds of water. In the United States in 2005, we used about 328 billion gallons per day of surface water and about 82.6 billion gallons per day of groundwater. Although surface water is used more to supply drinking water and to irrigate crops, groundwater is vital in that it not only helps to keep rivers and lakes full, it also provides water for people in places where visible water is scarce, such as in the desert towns of the western United States. Without groundwater, people would be sand-surfing in Palm Springs, California instead of playing golf.
Just how much water is there on (and in) the Earth? Here are some numbers you can think about:
- If all of Earth’s water (oceans, icecaps and glaciers, lakes, rivers, groundwater, and water in the atmosphere was put into a sphere, then the diameter of that water ball would be about 860 miles (about 1,385 kilometers), a bit more than the distance between Salt Lake City, Utah to Topeka, Kansas. The volume of all water would be about 332.5 million cubic miles (mi3), or 1,386 million cubic kilometers (km3). A cubic mile of water equals more than 1.1 trillion gallons. A cubic kilometer of water equals about 264 billion gallons.
- About 3,100 mi3 (12,900 km3) of water, mostly in the form of water vapor, is in the atmosphere at any one time. If it all fell as precipitation at once, the Earth would be covered with only about 1 inch of water.
- The 48 contiguous United States receives a total volume of about 4 mi3 (17.7 km3) of precipitation each day.
- Each day, 280 mi3 (1,170 km3)of water evaporate or transpire into the atmosphere.
- If all of the world’s water was poured on the contiguous (lower 48 states) United States, it would cover the land to a depth of about 107 miles (145 kilometers).
- Of the freshwater on Earth, much more is stored in the ground than is available in lakes and rivers. More than 2,000,000 mi3 (8,400,000 km3) of freshwater is stored in the Earth, most within one-half mile of the surface. But, if you really want to find freshwater, the most is stored in the 7,000,000 mi3 (29,200,000 km3) of water found in glaciers and icecaps, mainly in the polar regions and in Greenland.
Where is Earth’s water located?
For a detailed explanation of where Earth’s water is, look at the data table below. Notice how of the world’s total water supply of about 332.5 million mi3 of water, over 96 percent is saline. And, of the total freshwater, over 68 percent is locked up in ice and glaciers. Another 30 percent of freshwater is in the ground. Rivers are the source of most of the fresh surface water people use, but they only constitute about 300 mi3 (1,250 km3), about 1/10,000th of one percent of total water.
Note: percentages may not sum to 100 percent due to rounding.
Sphere representing all of Earth’s water
If you just want an image of all water on, in, and above the Earth, here it is.
Performing the calculation
Here are some numbers we used in creating this diagram, and looking at them will help explain the relative sizes of the Earth and our water ball.
Using Mr. Shiklomanov computations (shown in the table below) of the volumes of water on, in, and above the Earth we start off with an estimated total volume of all water to be about 332,500,000 mi3, or 1,338,000,000 km3. You should first realize that numbers like these can only be estimated, and the numbers in the table are also rounded. To draw the sphere on the map we needed the diameter of the circle (putting the 3-dimensional sphere into a 2-dimensional circle, to draw it).
The volume of a sphere is: 4/3 x Π (“pi” or 3.1416) x radius3
To put the unknown radius on one side, the formula we need to use is:
Radius = The cube root of (3/4* 3.1416 x 332,500,000)
Then, the radius = cube root of 79,378,300
That gives the radius of our sphere to be about 429.8 miles (692 km), with a diameter of about 860 miles (1,384 km), which is the diameter of the largest blue ball you see floating on the western United States.