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Water is the key to every form of life, so it is worth looking to see how changing technologies influence the availability and quality of the water needed for agriculture (and humans). Where water exists, plants can grow and animals graze. The effects are often spectacular: to see a beautiful green golf course in the middle of a desert is an instant reminder of what it can achieve.

Free delivery by rainfall, in the appropriate amounts and correct timing, is a farmer’s dream, but more typically it is variable, and irrigation must be provided. This requires construction of dams, aqueducts, canals, and wells or containers, so is costly and labour-intensive. Once delivered, water will soak into the soil, run off with the fertilizers, evaporate, or be absorbed into the plants and shipped away with them. In each case, the net effect is a loss of water that needs replacement. We are now using water from ancient underground stores at rates that mean it cannot be replenished at the speed we extract it. So the water we are now drinking may have come from rainfall a few thousand years ago, but the volume available is shrinking.

Water extraction also causes the land to sink. In some areas this is quite obvious with examples in, say, California, where there is extensive water pumping technology, and in some isolated areas the ground level is dropping at rates as high as a metre per year. The problem is widespread; over the last century, entire areas have dropped as much as 10 metres (~30 feet).

Water is also used in mining, oil extraction, a vast range of technological processes, plus home usage and sewage systems. The pattern in each case is that we may have started with a pure water source, but the processing always degrades it. We may choose to live with the contaminated water and reuse it, but perhaps a disquieting thought is that the water drunk from the taps in London is claimed to have already passed through seven other people on the way there. They have added to the flavour with traces of their food, chemicals, and drugs. Sometimes we are convinced we can taste the history of the water, but generally these are just from the chemicals used in the attempts at purification.

Nevertheless, modern analysis, which can achieve sensitivity down to parts per billion, will definitely reveal traces of drugs (of all types). My earlier example was drug detection in the Italian river Po, but this is universal, and London can currently claim to have the dubious position of having the highest cocaine content in the water supply of any major city. Our concern is that water supplies inevitably contain detectable traces of drugs, impurities, and a wide range of biological compounds linked to diseases. Switching to wine can dull our concerns, but it does not solve the problem, as wines also have the chemical signatures of the water from their local regions.

For the high water volumes needed in agriculture, not all used water can be adequately recycled; therefore large quantities become unsuitable both for human consumption and for irrigation. The story does not end there, as polluted water heads down the rivers and out to sea where it interferes with, and contaminates, marine creatures and fish. They may even detect the pollutants better than we do because as I already mentioned, many species, such as salmon and turtles, return to their breeding grounds by the ‘chemical smell’ of their home rivers or beaches, so they certainly are actively sensitive to the parts per billion of contaminants.

My historic examples of collapses of civilizations from drought should keep us aware of our dependence on water, and force us to treat it with care and respect. It may fall from the heavens, but it is not guaranteed. Anyone can contaminate it, but purification is difficult and very expensive.

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