Even not for us waterspoiled consumers ! Having one bath is wasting drinking water for one week in the cabin.
What u`ll need:
1.) A small, higher Bowl (Grey one in pic)
So that you can easily wash your hands in it, with a liter-ish water.
2.) A bigger, flat bowl which contains the small, high bowl.
(Transparent one in pic)
It's just to catch the splattering water while washing hands.
But the second use is to use for washing your hair.
3.) Biodegradable soap
Furthermore a kind of a recycling mindset is mandatory. When we are washing our hair, we use the water with the bio soap afterwards for handwashing, If it's too dirty, we fill the water in the outdoor handwash bowl, so you can clean your hands after working in the garden, making firewood etc.
Plain: Just to get the coarse dirt away...