The ozone layer depleted beyond reasonable levels and life on the surface has become impossible and life migrated underground in giant caves connected together by tunnels.
Some of those caves have access to underground rivers or aquifers but some don't. That's the case of one of the biggest cave (size depends on what fits your answer) holding the principal city and many fields. In order to maintain this city and fields water is necessary hence the need for a water cycle.
What should the characteristics of my cave be in order to sustain a water cycle ?
- The cave has access to sunlight with day/night cycle