I can take a shower and wash my (long) hair in two gallons of water. It's a lot easier than it sounds. Fill two one gallon containers with warm water then stand in the tub or shower. Pour enough water over your head to wet your hair, put down the container, shampoo and pour enough water to rinse. You'll be thoroughly wet by this time, so lather up all over. Pour the remaining gallon or so over yourself to rinse and you're done.