Tex. Nat. Res. Code § 21.073
For the purpose of using a system, the state is divided into five zones:
Acts 1977, 65th Leg., p. 2357, ch. 871, art. I, Sec. 1, eff. Sept. 1, 1977.
Amended by Acts 1987, 70th Leg., ch. 616, Sec. 5, eff. Sept. 1, 1987.