Cal. Water Code § 75170

As an additional and alternative method of creating improvement districts of parts of the area within districts two-thirds of the holders of title or evidence of title of any tract or contiguous tracts of land, situated within any district, susceptible of conservation of water, or capable of development separate and apart from the main project or the development of the water resources of the entire district, may petition the board to form an improvement district within the district for the purpose of conserving and developing the water resources of such improvement district and for the levying of assessments to pay the costs thereof.