eli5 if deserts used to be wet where did all the water go?

Question

and why is there still a little bit in oases

In: 8

Answer 1

Water is constantly leaving via run-off, river flow, soaking into the ground, and evaporation.

In places that have lots of water, the amount of water falling from the sky as rain or snow is roughly equal to the amount of water flowing/soaking/evaporating. Sometimes there are short period of time where there is too much water falling too quickly and the water builds up. We call that flooding.

In deserts, the water stopped falling due to changes in climate and so all the water flowed/soaked/evaporated away.

However, the water that soaked into the ground may have been stopped from soaking down further by solid rocks. In places where the ground dips down below the level of this underground water that has soaked in, it’s available to us at the surface.

In deserts, we call them oases that water is generally not replenished and will eventually run out.