It depends on the desert. Some of the dryest desert areas,
at least in the U.S. southwest, still provide enough annual rainfall
to keep an earthship and its occupants supplied with water. This assumes that
the occupant is mindful of his usage and maintains an awareness of his supply levels.
The number and capacity of
cisterns should be planned in advance according to the number and usage
requirements of the occupants who will depend on it.
In the case of prolonged
drought and the exhaustion of the water supply, water can be trucked in and the cisterns