Cal. Water Code § 75887

The board may require, as a condition precedent to the granting of the petition for inclusion of lands, that the petitioners severally pay to the district such respective sums, as nearly as they can be estimated (the several amounts to be determined by the board), as the petitioners or their predecessors in interest would have been required to pay to the district as assessments had the lands been included in the district at the time the district was originally formed.