My group is thinking of creating a customised setting for future campaigns, and we were thinking of having a bit of a Dark Sun vibe. With that in mind I was thinking about how to model magically draining the life from the land.
I was thinking of making it akin to sacrifices - a form of magical research that grants divine power (or something like it) in exchange for permanently lowering the land value of a hex.
Based on the Limits of Growth section on page 129, the maximum number of peasant families per 6-mile hex is 780. Assuming that represents complete exploitation of the land, draining 1 point of land value would represent a sacrifice of 780 GP per month, which using the standard 33x multiplier to convert a monthly value into a lump sum would produce a total Divine Power of 25,740 Divine Power. I would imagine it would take time to draw the life from an entire hex so it I was thinking the power would take a season to be drawn out from the land, for 8580 Divine Power a month for 3 months.
this sounds pretty reasonable to me. The only question is: how strong do you have to be in order to draw power from a single hex? Is there a limit to how many hexes you can draw from before you need underlings to help you? Based on my loose recollection of darksun this doesn't seem like the case, since it was implied the sorcerer kings were personally powerful, and then bestowed a fraction of their power onto their templars.
Do you have any plans to model Defiling vs. Preserving on an individual caster level?
For range I was thinking of line of sight (which from your sanctum's tallest tower may be quite a way), with range beign extendable with some kind of structure. Perhaps you build some kind of Dread Obelisk and that lets you draw the life from the hex it is placed in.
I wasn't really thinking about preserving or defiling for regular magic, just for the big stuff. Preserving would be a reverse operation - expending magical power (divine power or reageants as well as gold) to increase the land value.