GLOBAL WATER DISTRIBUTION The world’s total water supply amounts to approximately 1386 cubic metres. Figure 4.1 (first bar) shows that over 97 per cent of this water is saline and mostly located in the oceans. Only a tiny proportion of this total (2.5 per cent) is the fresh water that humans require to sustain life (middle bar). Of this fresh water, over two-thirds is locked up in glaciers and ice caps and almost all the rest is stored as groundwater (water in the soil or held in pores of rock). Surface fresh water (third bar), which is the source of the water people consume around the world, represents only slightly more than 1 per cent of the total fresh water available (0.0067 per cent of the Earth’s total water). Almost 90 per cent of the planet’s fresh water is stored in ice and lakes with the remaining 10 per cent being held in five other main stores. Considering that rivers are the main source of water for the world’s population, it is clear that human life depends on just 0.0002 per cent of the planet’s total water. Fresh water 2.5%
Other saline water 0.9%
Oceans 96.6%
Total global water
Surface/other fresh water 1.2%
Groundwater 30.1%
Glaciers and ice caps 68.7%
Ground ice and permafrost 69.0%
Fresh water
Surface water/other fresh water
Atmosphere 3.0% Living things 0.26% Rivers 0.44% Swamps, marshes 2.6% Soil moisture 3.8% Lakes 20.9%
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WATER AND ITS MANAGEMENT
∆ Fig. 4.1 The locations of water on Earth.
THE WATER CYCLE The volume of water on Earth has remained much the same for billions of years because it moves continuously and very efficiently around the world. The water cycle, driven by the sun’s energy, also known as the hydrological cycle, describes this movement of water between the oceans, the land and the atmosphere and is summarised in Figure 4.3. r The sun heats the water held on the Earth’s surface in oceans, seas and lakes. Some of this water is turned into water vapour (water in the form of an invisible
∆ Fig. 4.2 Victoria reservoir, Sri Lanka – rivers are the world’s most important source of fresh water.