Tex. Natural Resources Code § 21.073: DIVISION OF STATE INTO ZONES.

For the purpose of using a system, the state is divided into five zones:

(1) the North Zone;

(2) the North Central Zone;

(3) the Central Zone;

(4) the South Central Zone; and

(5) the South Zone.

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.