Maybe the reason they DID settle there is because the ancient technology was already in place and rediscovered... or, here is a thought for you: evapotransporation. In english: plant sweat. You could have a micro climate inside the bowl that feeds itself. This is a common occurrence in jungles: banana farms can cause their own daily thunderstorms... thus watering the area.
Problem: how did the plants get here in the first place? No one remembers or knows... we just found it this way.