My guess would be that the "desirable terrains" are hardcoded to some extent. But if relying on the AI changing the terrain doesn't work, you could put in the desired terrain, when the AI wishes to do irrigation or mining (this it usually will always do when/if possible) -say, grassland will change to red weed before one can do irrigation. This will affect settlertype units too, but you can easily make the time needed totally unreasonable for settler type units, so that in reality only engineers will be able to do it without getting killed.Originally posted by ravagon
One aspect I very much want to model is the spread of the Martian "Red Weed" as it was known and I've pretty much decided on making grassland/plain terrain transformable into this by the Martian engineer unit (Humans will not have engineers so will not be able to do this. Furthermore as it was extremely toxic to humans, no human city will be able to use red weed terrain for resources - house rule. Too hard to implement otherwise).
I did think of using flags to randomly change the terrains around Martian-held cities but this is rather inimical to any human/martian units on said terrain at the time.
Has anybody done anything similar with engineers before, "transforming" grassland/plain? Is it just a matter of making grs/pln less attractive than the red weed or do I have to exchange the positions of the grs/pln terrains for desert/jungle or somesuch for it to work. As this would involve fairly major work to the map its something I want to resolve before beginning placing cities, etc.