Distribution of water resources
About two-thirds of the Earth’s surface is covered with water. The total amount of water on the Earth is about 1400 million km3 (UNEP, 2002). Of this, around 97.5% by volume is held in the oceans and is salt water. Only 2.5% (or about 35 million km3) is fresh water. Figure 4.2 shows the proportions of Earth’s water found in different parts of the environment.
Figure 4.2 Distribution of water resources on Earth. (Note that the size of the smaller segments has been exaggerated for clarity.)
From Figure 4.2, which percentages of fresh water are estimated to be stored in the forms of: ice and permanent snow (glaciers), groundwater, and as surface and atmospheric water?
Around 69% is stored in the form of ice and permanent snow, about 30% as groundwater,and only about 0.4% of fresh water is stored as surface and atmospheric water.
Fresh water is water with a dissolved salt concentration of less than 1%. Globally, fresh water is distributed unevenly. About three-quarters of global annual rainfall occurs in countries containing less than one-third of the world’s population. About 80% of the world’s water run-off is concentrated in countries in northern and equatorial regions, which have relatively small populations. For example, the Amazon River in South America accounts for 20% of global run-off each year. The area drained by the Amazon is huge, but it is sparsely populated. In Africa, the Congo River and its tributaries account for 30% of the entire continent’s annual run-off, but the Congo’s area contains only 10% of Africa’s population.
Why is it a problem that the huge areas of the Amazon and Congo rivers have large amounts of fresh water but only a relatively small number of people?
The problem is that the fresh water is unequally distributed. A large proportion is found in places that are remote from the majority of the population so the water is not available to them.
An additional problem is that rainfall throughout much of the developing world is highly seasonal. The seasonal rains may last for only between one and three months, which can leave people short of fresh water during the dry season