California Code, Water Code - WAT § 20569

The value of any land and the holders of title to the land are conclusively determined, for the purpose of any petition provided for in this division, by the district assessment book last equalized at the time of the presentation of the petition.

When no assessment book of the district has been equalized, the county assessment roll of the county in which the land is situated last equalized at the time of the presentation of the petition is conclusive evidence of the value and title.

However, for the purposes of a petition to form a district and so far as land outside a district is concerned for the purposes of a petition to form an improvement district under Chapter 2A of Part 7   1 the county assessment roll of the county in which the land is situated, last equalized at the time of the first publication of the petition is conclusive evidence of the value and title.