Water – the Numbers
UK domestic use of water is, on average, about 148 litres per person per day[1]. Average annual rainfall in the UK is 890mm, of which about half evaporates again, leaving 465mm.
148 litres per person per day equates to about 54,000 litres per year – 54 cubic metres. Divide that by 465mm and you get about 120 square metres. That’s the area of land you need to collect the rain the average person consumes in an average year.
The area of the UK is 243,610 km2 and its population is 62.26 million.[2] That’s 3,913 square metres per person – more than thirty times the area needed to collect the water that comes out of our taps.
Of course the rain doesn’t fall equally all over the country, and people aren’t distributed uniformly, either. Let’s do the same calculations for Cambridgeshire, where I live – one of the driest parts of the country. Let’s do it for the driest year on record as well – 2011, when only 380mm of rain fell in the whole twelve months. What I’ve no information on is evaporation rates for Cambridgeshire in 2011, but let’s assume that 50% figure, so that’s 190mm of effective rainfall. Cambridgeshire’s population is 605,400[3], and its area is 3,004 km2.
Doing the sums again: 148 l/day / 190mm/year is about 285 m2; and 3,004 km2/605,400 is about 5,000 m2 – still seventeen times the area needed. But then, Cambridgeshire is rather sparsely populated, especially since Peterborough was taken out of it. It’s an agricultural county.
Which leads us on to another important point. Agriculture needs far more water than households do, but it doesn’t have to be purified for it, and most of its water doesn’t come from the public water supply. Mostly it falls directly onto the fields as rain, and what doesn’t is mostly pumped directly from rivers or aquifers. Most water used by households ends up in sewers which lead to sewage processing facilities which return the water to the rivers, so domestic users’ relatively tiny consumption has almost no effect on river water levels or aquifers[4].
Interestingly, this is a point made in the Environment Agency’s document[1] (p.5) in relation to water use in electricity generation – but they fail to mention it in relation to domestic use. I wonder why...
Finally, let’s do the numbers for Greater London. 148 l/day / 250mm/year is about 216 m2; and 1,570 km2/8,174,100[2] is about 200 m2. So okay, London needs to import water from surrounding areas. Is that a problem? Those surrounding areas generally have considerably lower population densities and significantly higher rainfall.
One other thing I’ve not mentioned, that does get a bit of an airing (but no comments) in the Environment Agency’s document is that per capita consumption also varies from one area to another, and according to whether or not properties have a meter (p.11). The geographical pattern is interesting – how much is down to wealth, and how much down to watering gardens more in less rainy places? – but the differences are not enough to affect the argument.
[1] Except where otherwise noted, numbers are from the Environment Agency (via the Wayback Machine) (Water resources in England and Wales). This is a 2008 document, but the numbers probably haven’t changed much – and if per capita consumption has increased, is that increase really necessary?
[2] Wikipedia.
[3] Cambridgeshire County Council’s Research Group
[4] I say ‘almost’ because the water usually enters the river further downstream than it otherwise would, depriving the midsection of the river of water, and possibly reducing the percolation to aquifers.