The average annual rainfall is 1,090mm; multiply that by the land area and it works out at 250 thousand billion litres a year, or 12,600 litres of water per person per day. Why, then, are we still subject to hosepipe bans?
The problems arise from the way water is distributed. Draw a line from Teesside in the North- east down to the Isle of Portland in Dorset. The majority of the population - and the major agricultural users of water - lives to the south-east of that line, where there are few suitable sites for reservoirs, land is expensive and rainfall is low. Most of the rain falls to the north-west of the line.
These factors are mainly governed by geology. Much of north- west Britain is made up of old, hard rocks that form high ground and give rise to thin, poor soils. The weather systems that sweep in from the Atlantic deposit their moisture as the air is forced up over the hills, and the deep valleys in between provide ideal reservoir sites to catch the water that runs off the impermeable rock. In the South-east, most rock is younger and softer, forming lower ground, which receives much less rainfall, and often giving rise to fertile soil. Much of this rock - notably chalk - forms porous and permeable layers called aquifers; these absorb what rainfall there is and hold it in store, releasing it slowly to rivers, wells or boreholes. Chalky ground supplies only about one-sixth of Britain's water, but some towns and cities in the South and East are dependent on this source.
The distribution of rainfall is even more significant if we look at a map that distinguishes between the winter months of October to March, and the summer period from April to September. Then we see a great swathe down eastern England, where rainfall in winter is much lower than in summer. Because plants need more water in their growing season, between April and September, they dry out the soil. Any rain that falls is greedily absorbed and does not replenish aquifers. The amount of drying out that occurs is expressed as a 'soil-moisture deficit' (SMD), which is the amount of rain needed to wet the soil to the point where it stops absorbing water.
The SMD cannot increase indefinitely. As the soil dries and it becomes harder for plants to extract water, they begin to wilt and die. Typically, SMDs in the South- east begin to increase in April, rise to a little over 100mm in July and then decline through the autumn, usually being eliminated in November. Winter rainfall then recharges aquifers or fills reservoirs.
The last few years have not been typical. The Atlantic depressions that bring rain have tended to bypass Britain. The west-east decline in rainfall has been accentuated by this, and by the scarcity of the thunderstorms that usually bring rain to the East in summer. The result has been that the SMDs have lasted in the South-east through to the early winter and, in some cases, to the following spring. Last summer's SMDs persisted in much of the South-east until this spring, and began to develop again almost immediately.
The heavy rainfall in early July has helped the situation. It will have reduced demand from gardeners and for irrigation, and on clay soils and in urban areas it caused some run-off to rivers that water companies have been able to pump into reservoirs. But in the ensuing hot weather, the soils have been drying out again.
So what is the answer? To transfer water from the reservoirs in the North-west? Before opting for this expensive solution, we should look back to the droughts of 1975-76 and 1984. The first affected all of southern Britain, but the West, relying on reservoirs, ran into more trouble than the East, with its aquifers.
The 1984 drought, which lasted only a few months, affected the usually wet North and West, as well as the South and East, where it was hardly noticed. From full capacity after a wet January, storage in many of the major reservoirs of Scotland, Wales and northern England was down to about 30 per cent by September, when significant rainfall occurred. Reservoirs, which are expensive to build, hold less water than aquifers and need to be replenished regularly. Even after the present drought, there is probably more usable water in the chalk than in all the reservoirs in England and Wales combined. But those who use it to supply the public are vilified by many for causing the disappearance of wetlands.
As the climate gets wetter, there is also a tendency for rainfall to increase in winter and decrease in summer. These trends are in line with, though do not necessarily confirm, the predictions of global warming. While water resources will benefit generally, the country will be increasingly vulnerable to the rogue years that defy the statistics, leaving us with an exceptionally dry winter in between the drier summers.
For the present, the water situation in south-east England remains rather like the economy - severely depressed, and in serious danger of getting worse before it gets better.
Michael Price is a Senior Lecturer in Hydrogeology at the University of Reading.
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