Water use and the water cycle
Water use and the water cycle

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Water use and the water cycle

1 Water use

1.1 Water as a resource

Water is arguably the most important physical resource as it is the only one that is essential for human survival; we would die very quickly without it. Other physical resources can make life more comfortable, but water makes life possible. If you were shipwrecked on an island, a priority would be to find a source of fresh water fairly quickly; the thought of looking for other physical resources on the island, such as minerals for construction, fuel for warmth or metals for tools would come much later, if at all.

Water is the most common substance on the Earth's surface. It exists on land, in the oceans and in the atmosphere. Water may be pure or contain dissolved substances, particularly salt, in ocean water, some lakes and deep underground (see Box 1.1). 'Water resources' are usually fresh water, with a low content of dissolved, suspended and biological substances. Although water is abundant on Earth, fresh water is much less so, being only a few per cent of the total water, and in some places on Earth it is very scarce.

Water differs from most other resources, as it is a renewable resource, continuously driven around in a cycle by the energy of the Sun and the Earth's gravity. For example, rain falling on land flows away to the sea in rivers, and so is lost as a fresh water resource. But rain will fall again, replenishing the rivers. The rainfall may be irregular or seasonal, so the renewability of water is not instantaneous but has a delay of a few days or even many years. In comparison with the time needed to form other resources, such as sand and gravel, coal, or petroleum, this time is very short, so water is regarded as a renewable resource.

Problems are caused by the presence of too little water (droughts) or too much water (floods). The 1970s and early 1980s were a time of widespread drought in the Sahel zone of Africa, and in the southern republics of the former USSR, and, on a smaller scale, parts of Britain experienced drought conditions in 1975-76 and 1984, 1989-90, and 1995. Floods were particularly prevalent in 2000, with widespread flooding in Mozambique in Africa, and in the UK.

Box 1.1 Properties of water

Water is one of the few substances that commonly exist in each of the three physical states on Earth, as a solid (ice or snow), liquid (water) and a gas (water vapour). What is unusual about water is that its solid form has a lower density than its liquid form, which is rare in other substances. This means that ice floats on water, so water freezes from the top down rather than the other way around. If water froze from the bottom up, then aquatic life would be forced upwards as the water froze, and eventually end up stranded on the surface of the pond, lake, river or sea, instead of being able to survive underneath the ice in liquid water.

Water is an extraordinary solvent; no other liquid dissolves such a large variety of substances, which is why it is ideal for scientists, industrialists and cooks, but unfortunately it means that water can be easily polluted.

Box 1.2 Historical water use

Early humans were hunter-gatherers, roaming from place to place, but their movements were controlled by the need for water. When humans started to grow food, establishing permanent settlements, water was needed for crops and animals as well as for people, so settlements have always grown up near reliable sources of water. Most major towns lie on the banks of rivers, or, where there are no streams or rivers, settlements exist where underground water can be reached by digging wells.

The management of water resources has a long history. The oldest known dam in the world was constructed in Egypt about 5000 years ago, and was used for storing drinking and irrigation water. Farmers in Arabia at this time used the craters of extinct volcanoes as storage tanks for irrigation water and dug deep wells for their drinking water. Excavated ruins in India of similar antiquity retain the remains of water supply and drainage systems, which included baths and swimming pools.

The Romans had sophisticated systems for water supply and sewage disposal in their cities, including great aqueducts to bring clean water from surrounding hills (Figure 1.1).

Figure 1.1
Figure 1.1 Roman aqueduct, Pont-du-Gard, France.

The amount of water needed for individual survival is very small. The island castaway could live on 1 or 2 litres of drinking water a day if he or she also had a source of food. However, our everyday water requirements are greater than those of the castaway. Even at subsistence level, water is needed for cooking and washing as well as for drinking. The minimum requirement is about 5 litres per person per day, provided there is enough rainfall for growing food. The typical quantity of water used at subsistence level in developing countries is 20-40 litres per person per day. Here people often have no access to safe drinking water; water may be taken directly from a river, pond or well, and used without any form of treatment. However, if water is taken from a well, it is likely to be of good quality because of the purifying properties of the rock through which it has passed.

In high-income, industrialised countries, where a piped water supply is usually available, much more water is used — a typical per capita figure is 500 litres per person per day. Much of this extra demand comes from industry, agriculture and power generation, but part is due to the greater use of water for domestic purposes. The convenience of a piped water supply makes it easy to use water for many purposes other than drinking, cooking and basic washing — for example, washing machines, dishwashers, car washing and flushing WCs (Figure 1.2). Indeed, in some communities, notably the drier regions of the United States, more than half the domestic water supply is used for watering lawns and flower gardens, a need that would not be thought of in a subsistence-level community. However, on a global scale, only 8% of water use is for domestic purposes. This is illustrated in (Table 1.1). Table 1.2 gives an indication of the qualities of water needed for various domestic purposes, for gowing food and for manufacturing.

Figure 1.2
Figure 1.2 The average household water use by category for England and Wales, by purpose, for 1997-8 (Environment Agency, 2001).

Table 1.1 Global water use, 2001.

Domestic use (%)Industrial use (%)Agricultural use (%)
global82270
high-income countries115930
low- to middle-income countries81082

Table 1.2 Typical water requirements for various domestic purposes, food growing and manufacturing.

DomesticManufacturing or industrialFood growing or agriculture
Use or productQuantity of water needed/litresUse or productQuantity of water needed/litresUse or productQuantity of water needed/litres
WC, per flush6-101 kg (litre) beer81 kg wheat1000
bath80-1701 kg paper1001 kg rice4500
shower, per minute5-101 kg bricks1-21 kg sugar beet1000
washing machine, per loadup to 801 kg steel5-2001 kg potatoes550
dishwasher, per load25-351 kg aluminium8000
1 kg fertiliser600
1 kg refined crude oil15
1 kg synthetic rubber3000
car380 000
Note: 1 litre of water weighs 1 kg, and 1 m3 is 1 × 103 litres and weighs 1 tonne.

Rainfall provides most of the water needed by crops in large areas of the world, but in some areas the rainfall is insufficient or seasonal, and vast quantities of irrigation are needed to grow crops (Box 1.3). On a global scale, more water is used for irrigation than for anything else. Agriculture accounts for 70% of the global water use (Table 1.1), with most used for irrigation.

Industry is the second largest user of water on a global scale, making use of water in many ways. From Table 1.2 you can see that it generally needs much more than a litre (a kilogram) of water to produce a kilogram of product. In addition to the purposes listed in Table 1.2, water is used for building, in the preparation of processed food (such as food-canning), for cleaning, in ore-processing, for waste disposal and for cooling. Electricity generation requires vast quantities of water for cooling. In addition, the energy of falling water is used to turn turbines to generate hydroelectric power.

Box 1.3 Irrigation

Irrigation is used to grow crops in areas where they could otherwise not be grown, or to improve the yield. Around 15% of the world's arable land is irrigated. In arid regions, farming would be impossible without it, as in the deserts of Egypt and Saudi Arabia.

Elsewhere, irrigation is used to supplement rainfall and to overcome the high variability of rainfall in semi-arid regions, where rainfall occurs for only a short period during the year and in unpredictable quantities.

Irrigation provides security against crop failure and is not confined to arid and semi-arid areas; it is used often, but irregularly, on high-value crops in areas of greater rainfall such as England.

Irrigation is most common in Asia, where it is used particularly for rice-growing in flooded fields. In some parts, rivers are allowed to flood the fields in the rainy season, and rice and other crops are planted as the floods recede. In other places, seasonal rainfall is stored in reservoirs for use in the dry season. Water may also be pumped from underground; this occurs particularly in the south-western USA and the Sahara.

In addition to the flooding of fields, water is fed to crops through channels, by spraying, or by drip feed from holes in pipes (Figure 1.3). The method used varies in cost and efficiency of water use: developing countries usually use channels in the soil, which are cheap but inefficient (much of the water is not used by the crop). Irrigation is especially important for these countries, which may have insufficient rainfall for agriculture and cannot afford to import food. Countries may also use large amounts of water for irrigation to grow high-value crops, such as fruit in the south-western USA (as will be discussed in Box 1.4).

Figure 1.3
Figure 1.3 Drip irrigation in Jordan. Hoses are laid out across the fields, with holes at regular intervals in the hoses. Individual plants, in this instance grape vines, are supplied by each hole.

If used inappropriately, irrigation can lead to a major problem — salts present in the irrigation water can accumulate in the soil as the water evaporates, a process called salinisation. This causes a decline in crop yields until eventually the soil becomes useless for agriculture, but it can be prevented by using enough irrigation water to wash the salts through the soil, and draining this water from the fields.

Water is used in the production of energy, but energy is also needed to supply water: from the human energy needed to carry water from a river or raise it from a well, to the electrical energy needed to pump water around the distribution system to houses and industry. Also part of the energy budget for water resources is the energy used in building dams, digging tunnels, building water treatment plants and so on. Given enough energy, water resources can even be 'created': seawater, for example, can be desalinated to fresh water for an energy cost of about 300 megajoules per cubic metre (300 MJ m−3). (A joule, J, is the SI unit of energy; 1 MJ is 106 J.)

Water is also used for transportation, on the sea and on rivers, lakes and canals. Most canals were built for transporting goods, sometimes linking navigable rivers.

They are still important in parts of Europe, but the 3200 km of canals that were built in Britain during the late 18th and early 19th centuries are now little used for their original purpose, as goods can be moved more quickly by rail or road.

Recreational uses of water include angling, sailing and swimming, which can take place in reservoirs as well as in rivers, lakes and the sea. Water also has indirect uses, such as providing salt and fish for food.

The uses of water can be divided into consumptive uses, where water is used in such a way as to be temporarily lost as a resource, and non-consumptive uses where water is used without major diversions from its natural path and without changing its quality.When water is used in manufacturing industry, for example, this is a consumptive use; the use of water for recreation, such as sailing on a river, is non-consumptive.

Question 1

Which of the following uses of water would you classify as consumptive, and which as non-consumptive? Domestic water supply; irrigation and other agricultural use; industrial manufacturing; cooling (industrial and electricity generation); hydroelectric power generation; transportation; recreation.

Answer

The first four are consumptive uses. Cooling is 'borderline', but in fact about 1% of cooling water is lost by evaporation and the rest is generally returned to the water source at a slightly higher temperature — its quality has changed. It is also diverted from its natural path. The last three are non-consumptive uses.

Where water resources are limited, rapid reuse may be possible. Power stations using cooling towers recycle water after cooling, with only a small amount lost through evaporation. Such a cooling system uses less water (about 1/50th) than a power station without cooling towers, which return warmer water to its source without cooling it. Industry can also reuse water: steel production, for example, can use from 5 to 200 litres of water per kilogram of steel (a difference of 40 times the amount of water), depending on the degree of reuse.

Substitution is possible for some uses of water, but many uses do not have acceptable substitutes.

Question 2

Consider the uses of water listed in the question above, and for each use comment on whether a substitute is possible, and what the substitute could be. What do you think is the major disadvantage of using substitutes for water?

Answer

Domestic water supply

Most of the domestic uses are non-substitutable, such as drinking, cooking and washing. A major use that could be substitutable, although adding complexity, is the use of composting toilets rather than flushing WCs.

Irrigation and other agricultural use

Irrigation is non-substitutable and so are many other agricultural uses such as watering stock. However, the use of water for cleaning - for example, washing out milking sheds - could be substituted by the use of straw, or air or mechanical cleaning.

Industrial manufacturing

Water is used mainly for cleaning, transporting, waste disposal and cooling. Many of these uses may be substitutable by air or mechanical processes.

Cooling

Another liquid, air or gas may be used as substitutes.

Hydroelectric power generation

No substitute (apart from substituting another form of energy).

Transport

River and coastal shipping may be substituted by land transport. Marine shipping could be substituted by air transport.

Recreation

No substitute (for this type of recreation).

The major disadvantage of substituting water is the greater cost of substitutes. Water is often used not because it is the best washing agent, conveying agent or coolant but because it is the cheapest and also harmless.

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