[mrfusion]

Nice analysis! Three things to consider:

I don’t think you can just look at insolation over a given area. Wind and even the cooling itself will cause the air over the area to redistribute with warmer dryer air that can continue picking up water.

Besides that it’s more important to consider how saturated the air in the region is. If it can hold more water it will.

Other things to consider

Some of the water would seep into the ground instead of waiting around to evaporate. Maybe there are ways to speed that up too.

Finally Another option is the spay the water into the air to speed up evaporation a bit.

[philipkglass]

I considered mentioning wind from warmer neighboring regions but at this scale you are basically remaking geography, so I wouldn't count too much on said winds reaching the middle of your evaporation ponds. ("Manmade shallow hypersaline lakes", rather than ponds, at this scale.)

Seepage into the ground -- good point. If there is existing groundwater, seepage would taint it. If there is no existing groundwater, but ground is permeable, that helps.

Spray water into air? That costs more energy. Just waiting for natural evaporation is nice because it doesn't require any more human-added energy.

Of course you'd want to analyze more than the One Big Factor if you were pondering implementing this in practice. But I think that the one-factor approach illustrates why this hasn't been an "obvious" disposal solution for waste brine.