Cal. Water Code § 4999

The Legislature finds and declares that by reason of the combination of light rainfall, concentrated population, the transition of considerable areas of land from agricultural use to urban use, and a similar dependence on ground water supplies which prevails in the Counties of Riverside, San Bernardino, Los Angeles, and Ventura, together with the fact that most such underground water supplies are overdrawn, it is necessary that the provisions of this part apply to said counties only.