See, they didn't ask me. Granted, those scrub brush to the west of *my line* are the beginnings of the more arid area, but I suppose I was being too provincial in my POV - considering they mean for the *whole* US.
Over at the 100th, the scrub is even scarcer. The Plains Indians, as opposed to the Civilized Ones, chose to stay just west of that 97.4 line for some reason.