Twenty litres of water looks like quite a lot when it's laid out in front of you in 10 bottles. But it is considered by development experts to be the minimum amount a person needs each day for the basics – drinking, cooking and washing – in countries where water is most scarce. At this rate, those places may one day include Britain, where ministers this week hosted a drought summit and warned of a repeat of the thirsty days of 1976, when water was rationed and forests were so dry many burned down. Caroline Spelman, the Environment Secretary, urged us all to start checking our consumption, "so we can mitigate the impact in the future".
How much water do you use in a day? I had no idea. I'm a bad conservationist and – shoot me now – will admit to leaving the tap running while I brush my teeth. At the start of a day in which I prepare to test the 20-litre limit, I turn on my shower and offer it a 2l mixing bowl. It fills in just 14 seconds, and my shower isn't even the most powerful (the real Niagara models knock out water at twice that rate).
I then clamber in and start a stop watch. I wash, I hum, I daydream. Seven minutes and 35 seconds later, I cut the tap and reach for a calculator. Barely minutes into my day, I have used 65l of the world's most precious resource.
The flow does not stop. I brush my teeth, tap on (6l). I have breakfast, boil a kettle (1l), shove my dishes into the washer, which is almost full so I turn it on (20l). I put on my laundry (40l) and go to work. I drink about 3l during the day, consume water used to produce my lunch and dinner (a small bag of salad, for example, can "cost" 50l of water). I flush the loo (6l a time) and wash my hands. I don't have a garden (hoses can draw 15l a minute) but I do have a basil plant that will probably be dead in a few days, but I give it a glass of water anyway. By the end of an average day like mine, the average person in Britain consumes not 20l a day – but 155l.
It would be facile to restrict myself to 20l, I decide. British consumption is actually quite good by Western standards (Americans use 370l every day) and I would have to make significant hygienic sacrifices to have any hope of hitting my target. But I can do better. I start with that shower. I'm already better off than if I washed horizontally; a bath uses about 80l of water and, anyway, my flatmates turn up their noses when I offer the government's advice in 1976 to "save water, bath with a friend". I cut out the humming, turn on the tap half way, and off, unless I'm rinsing. In 2009, television adverts in Brazil urged people to pee in the shower, to save flushes. I don't do this.
I discover "eco" buttons on my washing machine and dishwasher, which promise to cut consumption by half. I invest in a Hippo, a plastic bag that takes up space in my loo cistern to slash the flush volume. At work, accessing the cistern would require a hard hat and a risk assessment, I decide that if it's possible not to flush without leaving evidence, I won't. This means drinking more water, but is, on balance, more economical.
I resolve to stop buying water-intensive foods, which can be tricky; even producing a pint of milk requires more than 2,000 pints of water on its journey from teat to table. I decide I will wash my car even less frequently – it's not smart enough to impress anyone even when clean.
I console myself with the thought that I'll probably never be able to afford to have a garden anyway (but persevere with the basil). But what I do above all things for, to my shame, the first time, is to think about a resource that we may all be forced to consider when our taps start to splutter. And with these small steps, I calculate I have cut my average consumption in half, to more like 80l a day. And if saving the environment weren't motivation enough, I look forward to receiving my next water bill.