Extending water resources
Extending water resources

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Extending water resources

6 Conservation

An alternative approach to extending water resources is by water conservation. One way of doing this is to use less water, more efficiently. In countries where water is available at the turn of a tap, water is wasted in many ways — by water-inefficient appliances and unaware users. Industry may also use water inefficiently, and so does most irrigation, where water is transported to fields through unlined, uncovered canals, and used in surface furrow irrigation (Figure 9). Changes in technology can be used to reduce inefficient water use.

Figure 9 Surface furrow irrigation, in Jordan. Water flows in channels under gravity, infiltrating the soil. This is a very inefficient form of irrigation, as much of the water is wasted through evaporation or infiltration into the soil out of reach of plant roots.

Recycling water can also extend the available water resource. For example, power stations are often major users of water to cool the steam that has driven the turbines. This steam is produced from very expensive demineralised water, so recycling it makes economic sense. Power stations cool the steam in two main ways: direct cooling, where large quantities of cooling water are pumped through the cooling system and returned to its source (used mainly when there is a plentiful supply of water on lakes or a coastline), or indirect cooling, where the cooling water itself is cooled and recycled through cooling towers (Figure 10). Not all the cooling water can be recycled; some is evaporated by this process.

Figure 10 Cooling towers of the 2GW Cottam coal-fired power station, Nottinghamshire. This uses water from the River Trent, recycling most of it, except for that evaporated in the water droplet plumes above the cooling towers.

Another method of water conservation is substitution: using alternatives instead of water.

On a global scale this will make little difference, as most water (70%) is used for agriculture, mainly for irrigation, which is a non-substitutable use. However, on a local scale, particularly for many industrial purposes, substitution is possible. Unfortunately, alternatives to water are often more expensive and/or less efficient. For example, power stations can use a 'dry' cooling tower instead of the 'wet' cooling tower described above; dry cooling works like a car radiator, using air as a coolant instead of water (if you see a working cooling tower without the plume of evaporating water over it, it is a dry cooling tower). However, this substitution comes at a cost; wet towers cost about US$20 million each, whereas dry towers cost about US$35 million, and a major power station usually has between six and eight of these towers.

The last, and most fundamental, conservation technique is changing practice: to change from a water-consuming practice to one that uses less or no water. On a small scale, for example, homes in areas of low rainfall could use only desert plants in their gardens, requiring no watering. On a larger scale, a change for industrialised countries in their consumption of food, to a diet including more cereals and vegetables and less meat, may reduce the water used for agriculture, as it requires far more water per kilogram of meat protein produced than it does for cereal or vegetable protein. The need for irrigation water could be reduced by growing more crops that need a large amount of water in areas that already have sufficient natural rainfall. For example, many salad vegetables in UK supermarkets are imported from hotter countries where they are grown using scarce groundwater for irrigation. At the same time in the UK, agricultural land where similar crops could be rain-fed is designated as 'set aside' and not used.

Imported salad vegetables are an example of importing virtual water, which is the water used to produce the goods that a country imports. The virtual water concept is often used in a discussion of arid countries such as those in the Middle East. If, for example, it needs 1000 tonnes of water to produce one tonne of wheat then an arid but relatively rich country like Saudi Arabia can choose whether to use 1000 tonnes of water and grow the wheat itself or simply import the tonne of wheat. If it does the latter then it has saved the water; in effect it has imported 1000 tonnes of 'virtual' water. So a wealthy country does not need to be self-sufficient in producing food or manufactured articles. It becomes more relevant when considering the importation of food from places like Africa into countries such as the UK. Here we can grow a great many crops using rain-fed agriculture; when we import vegetables or cereals we are in effect importing virtual water into what is a relatively water-rich country. The situation becomes more absurd if the imports come from relatively arid countries and are grown using scarce water resources, for example lettuces from Spain or asparagus from Israel.

In most countries, the response to water shortages is to augment supplies, and in the short-term manage demand by prohibiting use (e.g. hosepipes), cutting off supplies for part of the day or using standpipes, and exhorting the public to use less water. These are often effective in the short-term, but are costly and inconvenient to users, and do not take into account the relative value of water to different consumers. Where water is provided to users at a price less than the supply cost — the situation common in most parts of the world— there is little incentive for conservation; price is a tool that can be used to make users value their water supply.

All the methods of conservation considered in this section are possible, but the extent to which they are used depends on the price of water; raising the selling price will encourage more efficiency in water use, more recycling, more substitution and changing practice. Their use will also be consequent on the method chosen and the ability of people to pay for it. In developed countries conservation will depend on our determination, or not, to change our lifestyles to achieve sustainability of water resources. Will it happen? Would you, for example, be willing to give up your dishwasher or lawn, install a water butt, pay more for many manufactured products, or eat less meat?


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