Come to think of it, growing up I used to see my grandfather wash and rinse his hair with about a liter of water. Grandma would pour the rest over a basin when he needed a rinse. And then it was used to water the plants! Course, they lived through a depression and war, so they never wasted a thing. A liter of water once a week should be doable. If water supplies are too tight for that, washing your hair would probably be the least of your worries. Good topic BTW.