Cal. Water Code § 43004

In the event that the volume of water under the control of any district is in any season so diminished below normal, by reason of water shortage or otherwise, as to make it probable that all the land cannot receive the full amount of water which it may need and to which it would otherwise be entitled, the deficiency shall be borne ratably by all the land, except insofar as priorities in the right to water as between different lands may prevent. The board may make rules and regulations to provide for distributing the burden of the deficiency and for the most economical and efficient use of the water which is or probably will be available.