Kiril, that is a nice graphic, but it won't wash. The eastern third of that map gets way to MUCH natural rain to promote quality buffalo grass. Even NE TX is going to be marginal and above the 24" of natural rainfall. Also, the further north you get, the longer that dormant season gets and the more trouble you may have with invasive weeds. Start a line through the middle of KS and move south. Move the Eastern edge of the zone west to eliminate MO and AR and your on to something. I don't think where I am, here in NW NM gets hot enough to get the summer growth. My season isn't long enough for bermuda, even the newer varieties either. I've seen some really nice results in SW KS with the newer varieties of seed propagated bermudas too. They have "better" color than buffalo and while they need slightly more water, they are a much better choice than KBG or TTF.