Water Code § 4253
ARTICLE 3. Apportionment of Expense
In all cases where rights exist to store or divert water for use for more than one purpose, the amount to be paid by the owner of such rights shall be based upon the total amount of water he is entitled to store or divert, and he shall not be required to pay more than one amount on the same water so stored or diverted notwithstanding use for more than one purpose.