California Code, Water Code - WAT § 7001

When two or more persons are associated by agreement in the use of a conduit, well, or pumping plant, for the conveyance, obtaining or disposing of water, or are using such conduit, well, or pumping plant, or any part thereof, for any lawful purpose, to the construction of which they or their grantors have contributed, each is liable, in the absence of any agreement to the contrary, to the others for the reasonable expenses of maintaining and repairing the same proportionately to the use actually made of such conduit, well or pumping plant whether used in connection with irrigation or drainage.