2 The global water cycle
The flow of water through the land, the atmosphere and the sea is shown in Figure 3.
The route by which most water enters the atmosphere is evaporation from the sea. Much smaller amounts of water enter the atmosphere from the land and from rivers and lakes.
Water vapour in the atmosphere condenses into clouds and falls as precipitation (rain, hail or snow). The distribution of rainfall across the planet is very uneven, with some regions receiving rain all year round and some receiving none at all (Figure 4). Some of the water that falls on the land runs into streams and thence into lakes, rivers and the sea; some of it evaporates back into the atmosphere and a lot of it percolates deep into the ground, where it becomes groundwater. Groundwater, extracted by means of wells, is an important source of water for people, especially in those parts of the world where rainwater is insufficient to meet their needs. Percolation through the ground purifies water by filtration so that water that emerges from the ground at natural springs is typically very free of microbes.
Transpiration is the release of water vapour by plants (Figure 3). Plants take in carbon dioxide from the atmosphere and release oxygen. To maintain a flow of nutrients through their stems and leaves they take up water in their roots and release it as water vapour through tiny holes in their leaves, called stomata, through which they also take in carbon dioxide. A recent study suggests that, because carbon dioxide levels in the atmosphere are rising through the burning of fossil fuels, plants don't need to keep their stomata as wide open as they used to in order to obtain the carbon dioxide they need. As a consequence plants are now releasing less water into the atmosphere than they did in the past (Gedney et al., 2006; Matthews, 2006). This represents a very subtle consequence of climate change that affects the global water cycle and makes the important point that the global ecosystem is very complex, and that a change in one component can have wide-ranging and unexpected consequences.
An important component of the water cycle, not shown in Figure 3, is human intervention in the form of sanitation systems. Water-borne human waste is collected in sewers, treated in sewage plants and returned, as cleaner water, to the water cycle, into rivers or the sea. The primary function of sewage treatment is to break down faeces and remove harmful microbes from the water. Sewage plants also have a role to play in removing harmful chemicals from water. In heavily populated parts of the world, water passing through sanitation systems represents a substantial proportion of the water flow in rivers. The River Lea, which runs from Hertfordshire in England into the River Thames, would probably cease to flow for much of the year were it not for the output from sewage plants (Brown, 2002).
The population in many of the world's megacities is growing so fast that the development of effective sewage systems is not keeping pace. In cities such as Karachi, in Pakistan, the water supply, mostly from groundwater, is heavily polluted by untreated sewage and contains high levels of bacteria (Rahman et al., 1997).