3.—(1) Before the beginning of each year in which it intends to supply water for regulation 4(1) purposes, a water authority shall designate the names and areas within its area of supply that are to be its water supply zones for that year.
(2) A water supply zone may not comprise an area whose population immediately before the beginning of the year in question is estimated by the water authority to exceed 100,000.
(3) A water authority may not vary a designation under paragraph (1) after the beginning of the year in relation to which the designation has effect.