Probably not very practical or feasible unless you're camping during monsoon season or in a tropical rain forest!
The best way to conserve water is to learn the Navy shower...from Wikipedia:
A navy shower
(also known as a "combat shower", "military shower", "sea shower" or "staggered shower") is a method of showering
that allows for significant conservation
of water and energy by turning off water during the "middle" portion. An initial thirty seconds or so are used to get wet, followed by soap and lather without running water, which is then rinsed off in a minute or less. The total time for the water being on is typically under two minutes.