My water bill arrived in the post the other day. According to the government's Water Supplies Department, in the four months between mid-January and mid-May, I used 25 cubic metres of the stuff, which because of the remorseless logic of the metric system, equals 25 tonnes of water by weight. That means I used an average of 194 litres of water a day, or enough to fill a bathtub to the brim every morning. To me that sounds like quite a lot, especially for someone who prefers a shower to a bath and is so lazy he drops his washing off at the laundry down the road rather than do it himself at home. But I'm not bothered, because the bill for using all that water came to just HK$51.10. In other words, I got all the water I wanted over a four-month period for only a touch more than the price of a couple of cups of coffee. That's extremely cheap. Assuming consumption of 100 tonnes of water a year, the International Water Association (IWA) calculates the average Hong Kong household can expect an annual water bill of just HK$357. As the first chart below shows, that's about the lowest water tariff of any major world city. Beijingers can expect to pay 27 per cent more than we do for using the same volume of water. Parisians are charged almost ten times as much, and the unfortunate inhabitants of Copenhagen have to pay over 20 times more for their water than we do. That would mean a bill of HK$7,453 a year. As a result, Hongkongers are relatively profligate in their water use. My consumption might seem a lot, but it actually works out to about a bucket a day less than the average for Hong Kong households, which according to the IWA amounts to 220 litres per person per day. Admittedly, that's low compared to Denver or Los Angeles, where millions of sprinklers keep the residents' lawns green, despite near desert conditions. But Hongkongers use twice as much water per head as the inhabitants of Brussels or Barcelona, and consume considerably more than the world average. Unfortunately, we will not be able to go on using water with such gay abandon. Hong Kong has few water resources of its own. Although the government embarked on a major programme of reservoir construction in the 1970s, earmarking almost a third of the territory's land area for water catchment, today Hong Kong is able to meet only about a quarter of its demand for water from its own resources. The rest, around 700 million tonnes a year, is piped in from the Dongjiang, or East River, 83 kilometres away in eastern Guangdong province. We pay for it, of course; at a rate of around 5 yuan a tonne for this year. In return, the Guangdong government guarantees us a clean and stable supply. The trouble is that demands on the water resources of the Dongjiang Basin are fast reaching unsustainable levels. At the moment, the Dongjiang fills the water needs of some 40 million people in Guangdong and Hong Kong, as well as demand from one of the greatest industrial concentrations on the planet. We extract 11 billion tonnes of water a year from the river. But as the second chart below shows, over the course of this decade, demand from urban centres -including Shenzhen and Dongguan, which also rely on its waters - is forecast to grow by almost 40 per cent. Yet even as we take more water out of the Dongjiang we are dumping more untreated waste into it. As today's Lai See column describes, a recent investigation by the Civic Exchange think tank found disturbing evidence of increasing water pollution in the Dongjiang Basin, as new heavy-industrial parks spring up along the river's banks, property companies throw up developments around the shores of its reservoirs, and illegal rare-earth mines proliferate among its headwaters. As a result, the quality of the water we extract is deteriorating even as we demand more of it. 'The Dongjiang is overloaded,' warns Liu Su, Greater China policy researcher at Civic Exchange. Ultimately, this overloading means Hongkongers will have to pay more for their water. Either the city will have to invest, along with the Guangdong government, in cleaning up the Dongjiang's water, or we will have to pay for ever more intensive treatment of the water we extract. Alternatively, we could beef up our domestic supplies, either by investing in desalinisation plants to purify seawater, or by recycling our own waste water as Singapore does. Whichever course we choose will be expensive. So whether we like it or not, we will have to get used to paying much bigger water bills in the future.