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Worldbuilding

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  • $\begingroup$ You still need to explain where all the water comes from. Evaporation will occur in the scenario you describe (at least during daylight hours). It might be at a very, very low rate but it will occur and the cumulative effect would see moisture from the valley carried away in the local winds and dispersed planet wide. Where does replacement water come from? The obvious answer is underground springs fed by subterranean basins. But again on a desert world where did that water in the basins come from? $\endgroup$
    – Mon
    Commented Jul 5 at 12:11
  • $\begingroup$ My first thought was a Mars like world to BTW. But that sought of leads to the oasis being the last vestigial location of life on the planet. This is because you have to assume complex life developed and evolved more widely on the surface & slowly ended up retreating to the valley as the surface dried out, leaving a rudimentary ecology of tough lifeforms on the valley floor. (The valley itself not being large enough to sustain the evolution of large complex ecology's like we see on Earth.) And there's still not going to be enough surface area down there to maintain Earth like oxygen levels. $\endgroup$
    – Mon
    Commented Jul 5 at 12:26
  • $\begingroup$ @Mon I don't see a problem with water. You might have a shallow sea at the bottom of the valley. The air at the valley edge will be cold and have very little water vapour. If the wind blows along the valley, the cold air would descend, and be very dry. The warm, wet air would drop its moisture as it rose at the other end, giving rivers or glaciers that bring the water back into the valley. Think of the weather in Chile: the winds may blow from the Pacific, but very little moisture makes it to the Atacama desert. $\endgroup$
    – Richard Kirk
    Commented Jul 5 at 14:17
  • $\begingroup$ Little is not none (re the Atacama desert), There has to be a replacement source of water for the depression/valley. The problem remains how the question is framed i.e. a more detailed description of the world is required that allows for some kind of a water cycle which leaves most of the planet dry(ish) most of the time is needed e.g cold desert biomes with large icecaps and seasonal weather? An impact crater or something on a formerly wet/verdant world that underwent catastrophic environmental change after life evolved with simple remnant lifeforms hanging on in the valley? $\endgroup$
    – Mon
    Commented Jul 6 at 1:22