I have no idea where the water would evaporate too. It's not in the atmosphere.
It may have once been in the atmosphere, and accumulated on surface as snow or something, then eventually got mixed up with all the loess and sand
etc. Maybe it is very high up in the atmosphere because of increased pressure near the surface? According to this link:
there is very little atmospheric pressure, and very little water in the atmosphere.
That whole area looks like part of a large drainage basin that has dried up over time. Perhaps it once was glacial, or a glacial run-off collection
That whole valley looks to be fairly recent. There are hardly any impact craters within it, although you an see a few small ones, the medium size ones
seem very weather in comparison to the landscape, and of course, the numbers are a lot less than the surroundings.
The dark area may have originally been an impact crater, a very old one. You would think that maybes hydrological or aeolian weathering to have
smoothed it out, but if that was the case, the bottom of it would be pretty flat, because of the in-fill, not conical! It looks more like a natural
feature than an impact site.
Even weirder is those two channels that lead into the depression from the south-west, and the small one from directly south - they aren't dendritic,
like normal hydrological drainage patterns. They have arced valley walls. Perhaps they were small rifts that have collapsed and the loess moved over
time from the aeolian systems? The center of the channels seem to have deposition marks that run down the center though. Same goes for that channel in
the very far bottom right hand corner, too.
The annoted image:
claims ice flow, which could account for the arcing valley wall collapses.
Still a very intriguing area of Mars though.