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?

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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?

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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.

Distribution of fresh water in Ethiopia

In Ethiopia, there are 12 major river basins/valleys, 11 lakes, 9 saline lakes, 4 crater lakes and more than 12 major swamps (Mekiso, n.d.). The average annual flow of water from all the 12 river basins is estimated to be 123.25 billion m3 (Figure 4.3). Several of the major rivers cross to neighbouring countries. For example, the Abbay River flows to Sudan and Egypt, and the Omo to Kenya. It has been estimated that 95% of Ethiopia’s annual run-off flows out of the country in these cross-boundary rivers (Waterwiki, n.d.).

Figure 4.3 Map of Ethiopia showing main river basins.

In Ethiopia, it is estimated that 54.4 billion m3 of surface run-off and 2.6 billion m3 of groundwater could be developed for use by people. However, the amount of rainfall, river flow and groundwater is highly variable across the country and depends on location and altitude. Some areas have sufficient water, while others don’t have enough. Figure 4.4 shows maps of rainfall and groundwater availability during drought in Ethiopia. Most permanent springs and streams exist only in the highlands in the west of the country. In areas below 1500 m above sea level, which is more than 55% of the country, there is hardly any surface run-off and very few permanent springs and streams.

Figure 4.4 Rainfall (above) and groundwater (below) availability during drought in Ethiopia. (MacDonald et al., 2001)

Based on the two maps in Figure 4.4, which regions of Ethiopia have the least available water?

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Somali and Afar regions have the lowest mean annual rainfall in the country. Somali also has very little available groundwater. Afar has relatively high groundwater availability.

The availability of water resources is also uneven over the year. At some times of the year you might have too much water, leading to flooding. At other times there may not be enough water, leading to drought.

Last modified: Friday, 29 July 2016, 12:18 PM