For those who might have no water a little longer than that (or those who are camping and don't want to go wandering in the night to find a privy) might want to invest in a bucket of clumping cat litter. When nature calls, take the lid off and use the litter in exactly the way your cat does. Dispose of the used litter as needed.
Got this from an SCA member who was heavily pregnant during the Pennsic War, and didn't want to make the hike to the privy 2 or 3 times a night.