Wow you are going into some serious detail here, and it's turning out beautifully I think!

1. I think you could make the dunes significantly sparser and still have it read as dry. Honestly I think colour is the best way to convey dryness, but I don't think your style here is very open to that possibility, so perhaps consider some sort of sparse hardy plants and rock brush, or some sparse grass brushes?

2. I think the farmland looks great. Except for the greenness of it. Apparently I'm on a crusade against green this week.

3. Those didn't read as terraces to me, and they still don't really. They are not going to read that way without something to indicate depth, which can be done wither with lines indicating the drop or shadow. Not sure how well line indications would work at that scale though.

4. I really like that area.