Water – here, there and everywhere

How much water is there in the world? Scientists estimate over one billion cubic kilometres (one cubic kilometre of water would fill 300 Olympic-sized swimming pools). Water covers nearly three quarters of the earth's surface in oceans as well as rivers, lakes, snow and glaciers. There is water in the atmosphere and water underground. Water evaporates and returns to the land surface in what is known as the hydrologic cycle.

In the hydrologic cycle, water evaporates from the ocean into the atmosphere, from there it can precipitate back into the ocean, or onto the land surface. From the land, it can evaporate or transpire back into the atmosphere, or flow overland or percolate underground before flowing back into the ocean. The distribution of the water around the globe varies from season to season and year to year, but the total quantity of water on the earth's surface remains essentially constant. The hydrologic cycle is discussed in detail in Freshwater Series No. A-1, "Water – Nature's Magician".

Although water exists in other forms in the hydrologic cycle, this issue in the Freshwater Series focuses on surface water as it is this water which we see in our everyday lives. Most of the earth's water is salty or permanently frozen. Figure 1 illustrates the proportion of fresh water that is available to us from the world's water supply. 
Figure 2 lists quantities that scientists have estimated for the various types of water that make up the world's supply. These amounts should be regarded as indicators of the relative quantities of water on earth. Owing to the difficulties in estimating volumes of water on a global scale, especially water underground, estimates can vary considerably. What is important is the overall picture that these estimates give. 

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