That's a shortage.
The Jacqueline Kennedy Onassis Reservoir in Central Park has a capacity of more than 1 billion US gallons.
If that was all the available / accessible water on Earth (even if we had a population of only, say, 50 million)...
Regards,
It was stored in order to keep it away from the worms to which it was a deadly poison, and the worm’s juvenile form, ‘sand trout’, which would attempt to encase the water with their own bodies and seal it off. The Fremen used predatory fish, like piranhas, to kill the sand trout. Fremen children would hunt sand trout for fun, and add their water to the tribes horde.
Dune was once a lush tropical planet with oceans, lakes and rivers, before the introduction of the Sand Worms from another planet................