Remember, fat stores some water. And those guidelines are for baseline survival. More would be better, and needs to be adjusted for environment as well as health.
On a non-active day sitting in front of the TV/computer I probably drink a gallon and a half of water minimum per day (factoring in ICE Tea. I never include soft drinks in that total and try to keep them out for the most part). I could probably do okay with a gallon, especially if I was doing VERY little but sitting around and the temperature was moderate. All of my planning centers around 1.5Gal. (6liters) per day for me. I figure about 1L per ~20kg per day should be plenty, even enough for at least a spritz to the face and light washing of hands. That seems about right to me, but it's a rough estimate.
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Experience is a hard teacher because she gives the test first, the lesson afterwards.