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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
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  • $\begingroup$ You can't eliminate influences from the surface for this since the path the water takes is likely to be on the surface at some point. What's the reason folks fled underground? $\endgroup$ – Hyfnae Jul 17 '17 at 11:36
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    $\begingroup$ The ozone layer became too thin and most of the surface is an irradiated wasteland. Some caves' roof are full of crystals that filter most of the UBV and let light go through. $\endgroup$ – Liquid Same Jul 17 '17 at 11:41
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    $\begingroup$ Accepting an answer in less than an hour is a bit hasty. This site has members globally, Many of whom will be discouraged from answering when they find an answer has already been accepted. Waiting longer ensures your question gets more and, hopefully, better answers. This is not to detract from the accepted answered. It is a courtesy to give everybody a chance to answer. $\endgroup$ – a4android Jul 17 '17 at 13:33
  • $\begingroup$ I see. I'll try to wait some more the next time $\endgroup$ – Liquid Same Jul 17 '17 at 13:44
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    $\begingroup$ Why would the fields need to be underground? Do crops dislike UV? I would think they'd consider it food. $\endgroup$ – Harper Jul 17 '17 at 18:47
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Intertwined water cycle.

You don't need a separate water cycle for the insides of the caves and outside of the caves. You don't even need the caves to be waterproof. The increased UV-radiation could even be used to decontaminate water to consumption.

Water that seeps through from the surface will have undergone filtration naturally by seeping through sand, rock and every sediment in between. So biologically the water that comes in from the outside will be as safe, perhaps even safer than nowadays.

However lifestock and any kind of biology that can't deal with increased UV radiation has to remain inside, the increased radiation can actually be used for some purposes, clean water is one of these things.

As for your crystals that filter UV-radiation, you do realize that glass actually functions as a filter for UV light and one could safely live behind glass while still having full benefit from the sun's light without getting a deadly dose of UV light even if the ozone layer is shot.

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  • $\begingroup$ That's even more than I asked. Thanks a lot ! $\endgroup$ – Liquid Same Jul 17 '17 at 12:57
  • $\begingroup$ Regular glass stops UVB (the one that gives you sunburns), put UVA penetrates unless the glass is specifically treated to prevent it. That would still be a long term risk $\endgroup$ – Carl Kevinson Jul 17 '17 at 13:33
  • $\begingroup$ @CarlKevinson It's not a far fetch to assume people in such a setting would use glass treated to deal with both UV-A and B if it poses trouble otherwise. Even if it's only after trouble arose from not properly shielding against UV-A. $\endgroup$ – Hyfnae Jul 17 '17 at 14:03
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    $\begingroup$ @Hyfnae, you're certainly right there, it's just something OP might want to mention in their story to patch a hole that glass nerds might notice. $\endgroup$ – Carl Kevinson Jul 17 '17 at 14:27
  • $\begingroup$ I'll keep that in mind. Thx to the both of you $\endgroup$ – Liquid Same Jul 18 '17 at 8:44
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If Earth's surface had too much UV for life, it would still have a water cycle. There would be rain, and rivers.

You could divert water from topside streams or rivers to come down and water your fields. Just because you cannot farm up there does not mean people with wide-brimmed hats cannot go up and dig irrigation canals. Or better - just dig a reservoir or use an existing one, like a like. Put in a water-filled pipe leading to your cave and water will siphon down to you. Bonus: the water will taste great because it has been UV sterilized.

Your scheme reminds me of the Forestiere underground gardens in Fresno. http://www.undergroundgardens.com/forestiere-underground-gardens-news/2017/6/15/baldassare-forestiere-was-a-california-dreamer enter image description here

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  • $\begingroup$ You often have the best ideas, but a tiny, small nitpick: If there has been an "apocalypse" and humans have left the surface, that means their stuff is still there. This isn't much of a problem below some nice village, but if you have a large city in a cave you most likely had a large city above the ground - and that city might have had some nasty chemicals lying around for their industry - and no one maintains anything anymore. Something did destroy the ozon layer, who knows what other regulations this world didn't have for their industry that we do. I would not recommend drinking the water. $\endgroup$ – Raditz_35 Jul 17 '17 at 12:27
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This sort of caves actually exist for real. For example, China has the Er Wang Dong, and Vietnam has the Son Doong cave. These caves are so big they have their own weather systems.

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  • $\begingroup$ They have their own weather system but do they have their own water cycle ? I'm not sure one implies the other. Still that is not something I knew of and that looks really cool $\endgroup$ – Liquid Same Jul 18 '17 at 8:47
  • $\begingroup$ They got their own jungles/forests, so I'd say it's close enough. $\endgroup$ – Raf Jul 18 '17 at 8:49
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Water cycle on surface requires few kilometers in the up direction (from sea level to the full extent of troposhere) and few thousands kilometers on the surface (water evaporated on the ocean falls as rain/snow on land and from there returns to the ocean).

Unless you want to call the planet a "cave with transparent ceiling" this cannot apply to a cave.

The closest you can get is an equilibrium between evaporating water (from water free surface) and condensing water (on surfaces) through the air humidity. This will be particularly relevant when during the day your cave can become warmer and more humid and then overnight the moisture can condense on colder rocks.

But this is just a smaller step in the cycle listed at the beginning of my answer.

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The cave doesn't need a natural water cycle of any kind, it's a site of major habitation so water will be brought in to accommodate local necessity. Groundwater systems will probably be unaffected by the loss of surface biomes although human activity may be an issue in some areas, pollutants, interruptions to natural lines of flow because of mines etc... and water quality be adversely effected by lose of topsoil. Water will be available for rerouting to sites where it's needed, L.A. survives only because of water piped in from elsewhere, so does Las Vegas, it's a huge engineering task but not insurmountable.

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