Cal. Wat. Code § 75165.1
A petition signed by holders of title or evidence of title of land representing at least fifteen percent (15%) of the assessed valuation of taxable land in the special improvement district may be presented to the board of directors requesting that the special improvement district be governed by a board of trustees to be elected by qualified electors in the special improvement district in lieu of the board of directors. The last equalized county assessment roll of the county in which the greater portion of the lands within the special improvement district are situated shall be prima facie evidence of ownership for purposes of this section, and a finding by the board of directors as to the sufficiency of the signatures based on certification by the secretary of the district shall be conclusive.
Ca. Water Code § 75165.1