How much water to carry?

[eightpot]

I've just done the maths on the all in vehicle weight for my next trip, and I need to go on a diet!
We're crossing one of the Libyan sand seas in December, and will be away from water sources for five days. We've been planning to take 80 litres of water or two of us, on top of 120 litres of fuel (in a LR90) - but doing some quick sums, I think we could probably get away with less water, especially if we re-use cooking water and don't wash

Was thinking about 3 litres a day for drinking water and to cook with per person, as we'll be taking some cans of pop and juice as well - is this a ridiculous amount??