2 The water cycle
2.1 Storage of water in the hydrosphere
The hydrosphere includes the parts of the Earth that are mainly water, such as the oceans, ice caps, lakes and rivers. Various parts of the hydrosphere can be seen in Figure 2.1. The oceans are blue; snow and ice are white in the ice caps of the Antarctic, and on high mountains such as Mount Kilimanjaro. (The yellow and brown areas are deserts, and vegetation appears grey-green.)
Water moves over, on and through the Earth in a continuous cycle driven by the Sun and gravity. It is known as the water cycle or the hydrological cycle (shown by the blue arrows in Figure 2.2) and involves water as liquid, solid (ice and snow) and gas (water vapour). Water can take many different paths through the cycle, but the total volume of water in the water cycle remains virtually constant. There are two main types of water in the cycle:
meteoric water, which is fresh water derived by condensation from the atmosphere and which accumulates as surface water (rivers and freshwater lakes) and underground water;
saline water, the seawater of the oceans and many lakes.
Small amounts of magmatic water from the interior of the Earth are added to the cycle by volcanic eruptions. On the other hand, water trapped within the pores of sediment, formation water, is, at least in the short term, isolated from the water cycle. This can either be water that was originally trapped in the sediments during their formation, or water that percolated into the rocks later. Formation water is usually saline, mainly because most sediments are marine and the water trapped in the sediments would be seawater.
All parts of the hydrosphere store water temporarily, and are called reservoirs. These natural reservoirs of the hydrosphere are not to be confused with the human-built reservoirs used to store water on land: for example, the atmosphere is a reservoir, containing 13 × 1015 kg of water. The study of water movement upon and beneath the ground and the physics and chemistry of the water is called hydrology.
Water resides for different lengths of time in different reservoirs. The average length of time that water stays in a reservoir before moving to another is called the residence time for that reservoir (Table 2.1). A hydrosphere reservoir empties and replenishes at the same rate, and the residence time is calculated from the rate of replenishment in relation to the volume of the reservoir. There may be several ways in which water is transferred to and from a reservoir (Figure 2.2).
For example, the residence time for rivers, assuming the only significant transfer from rivers is by runoff:
Table 2.1 Residence times for water in the water cycle.
Residence time is a concept that can be applied to any cyclical process, not just the water cycle. Figure 2.2 shows that the transfers of the cycle are in balance: in particular, water lost to the atmosphere by evaporation and transpiration is balanced by water returned by precipitation. The residence time gives an indication of how quickly water in a hydrosphere reservoir can be renewed. The shortest residence time, 11 days, is for water vapour in the atmosphere, which is continually renewed by evaporation from the oceans and the land, and is lost by precipitation. This is a rapid subcycle of the water cycle. Subcycles involving the oceans, the ice caps and underground water are much slower (Table 2.1).
There is a large volume of fresh water locked up in the polar ice caps (Table 2.1), but these are far from the centres of population and the arid countries that need it. It is not economic to transport this water at the moment, but it may become so in the future.
Apart from the oceans and ice caps, the greatest volume of water is underground, stored in porous rocks below the Earth's surface. The shallower underground water moves quite quickly through the cycle and is fresh water, so it can be used for water resources. But it is only a small proportion of the total underground water, and its residence time is relatively short, ranging from a few weeks to a few years. Underground water below a depth of a few hundred metres moves more slowly through the cycle, and residence times are much longer, up to ten thousand years (Table 2.1). Much of this water is saline either because it has had time to dissolve salts from the rocks, or because it originates from seawater.
The world's lakes contain large volumes of water (Table 2.1) and are more accessible than the polar ice caps. However, over half of these lakes are saline (Figure 2.3), and 80% of the water in the freshwater lakes occurs in only 40 large lakes, including the Great Lakes of North America (32 × 1015 kg) and Lake Baikal in Asia (22 × 1015 kg). Rivers are very useful for water resources. Although they store very little water (Table 2.1), the water in them is rapidly renewed — it has a residence time of just a few weeks.
The water found underground and in the ice caps, lakes and rivers forms about 4% of the total in the water cycle; but because the deeper underground water, the ice caps and the saline lakes are not usable as sources of water at the moment, the amount of water that can be used for water resources is much less, only about 1% of the total. This water is distributed very unevenly, as can be appreciated when we hear about the extensive water shortages and droughts in many parts of the world. In order to understand the problems of availability and distribution of water in more detail, we shall now look at the processes in the water cycle that transfer water between the reservoirs of the hydrosphere.