The Big Question: Why do we have water shortages in a country with as much rain as Britain?

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Why the sudden concern?

Because the Government has just granted a water company south of London (the Sutton and East Surrey company) the first "drought order" in Britain for more than 10 years, illustrating how potentially serious is the looming water shortage this summer, at least in the South-east. A drought order is a radical move - a considerable step up from the domestic hosepipe bans already in place over much of the region - because it gives the company draconian powers to ban what is considered non-essential water use, and this can range from filling swimming pools and watering cricket pitches to using car washes.

The inconvenience will be quickly noticeable: the shine will soon start to come off those shimmering blue pools behind Surrey's millionaires' mansions, and grimy cars and withered sports fields will be the norm. Businesses may go bust. It's not far from this to getting your supply with a bucket from a standpipe in the street.

So how bad is the shortage?

Over much of South-east England, it is already severe, according to the Environment Agency. Do not be misled by appearances: a lot of surface reservoirs may look fairly full, and river flows have not yet been reduced to tragic trickles. But they're not that healthy, and more importantly, the shortage is centred on the aquifers. These are the underground water-bearing layers of permeable rock that hold vastly more water than reservoirs do (accessed through boreholes).

In the aquifers of the South-east the groundwater level is very low because of two successive extremely dry winters. From October 2004 to April 2006, most of southern England has had less than 85 per cent of average rainfall, and some parts of Kent and Sussex have had less than 75 per cent.

Such a long sequence of dry weather with two dry winters is not only unusual, it is damaging for groundwater supplies, because it is through winter rain that the aquifers are recharged. In the summer, rainwater is caught on leaves and foliage and evaporates; it is in winter that it sinks into the ground. The heavy rain due in much of Britain this week is unlikely to get down to the aquifers. Even with average rainfall between now and the autumn, groundwater levels will continue to fall.

Is this just a problem for the South-east?

No. The shortage is most severe in south-east England currently, but most of England and Wales has had a dry winter, with only northern England experiencing average rainfall. The drought is beginning to affect Norfolk and Suffolk, and water companies there have been urged by the Environment Agency to monitor the situation carefully. If we get an extended period of hot, dry weather this summer, the drought will spread into other areas with the east of England and the south Midlands the most vulnerable. Reservoir levels in Wales and the South-west could also drop quickly. But the South-east is in most danger; the Environment Agency said this week that with a hot, dry summer, the region may face the worst drought in 100 years.

What will happen if there's a hot summer?

If the summer really is a scorcher, with say, only 60 per cent of average rainfall - and note that there is no forecast yet saying it will be - reservoirs serving London would by the beginning of autumn be close to the point where supply would be interrupted and standpipes in the street would be necessary. The Environment Agency says it "should be possible" for Thames Water, the biggest of the companies and the one serving the capital, to avoid the standpipe scenario "but much depends on the effectiveness of demand management measures and leakage control." (Those words may send a chill down some spines. Thames is the company with the worst leakage record - it loses a third of all its supplied water through leaks, and has missed its leakage improvement target in each of the last two years.)

Really severe shortages like this would also have a considerable effect on the environment. Very low river flows would lead to mass deaths of fish and other aquatic life and algal blooms would be likely on both rivers and stillwaters.

Is this anything to do with climate change?

It's hard to say. It may just be a part of natural climatic variability, for there have other cases of two successive dry winters over the last century, such as 1932-34 and 1974-76. Predictions by supercomputer models of how the climate may change over Britain in the coming century actually show us experiencing wetter winters. But they do show hotter, drier summers, and in particular, episodes of extreme heat, almost certainly of a level that has never been seen before in Britain.

These have already started. People have blithely forgotten that three years ago - on 10 August 2003 - Britain's record temperature took an enormous leap upwards on one day, exceeding 100 degrees Fahrenheit for the first time. This was a side-effect of the unprecedented two-week heatwave in France, which led to more than 14,000 excess deaths and which climate scientists now attribute directly to global warming. Extreme events like this, occurring with any regularity, will put a huge strain on water supplies.

So what can we do?

It helps to think of our water resource problems in terms of supply and of demand. In supply terms, the biggest difficulty is the UK's "rainfall gradient" - we get a lot of rain in the North-west, where it is least needed, and its volume rapidly diminishes towards the South-east, where it is needed most.

One solution that has been proposed for this is a national water grid, like the grid that distributes electricity, so that water could be moved around the country from where it is plentiful to where it is needed, perhaps via the canal system. But at present that's a long way off. In demand terms, it seems inevitable that every home will eventually have to have a water meter. In spite of concerns that meters may have a disproportionate impact on poor families with children, experience has shown they are the most effective way of cutting consumption back. If your bill goes up with the amount you use, you invariably use less.

Are we likely to have more water supply problems in the future?

Yes...

* If future hot summers due to climate change make drought in the UK much more frequent

* If water consumption continues to rise and no major effort is made to restrict demand

* If major water companies such as Thames do not bring their leakage rates down

No...

* If the recent two-winter drought turns out to be a temporary phenomenon

* If we can bring rising water consumption under control, perhaps by water meters

* If we can revitalise our water supply by building a national water grid

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